Transformations of Monoterpenes with the p-Menthane Skeleton in the Enzymatic System of Bacteria, Fungi and Insects

The main objective of this article was to present the possibilities of using the enzymatic system of microorganisms and insects to transform small molecules, such as monoterpenes. The most important advantage of this type of reaction is the possibility of obtaining derivatives that are not possible to obtain with standard methods of organic synthesis or are very expensive to obtain. The interest of industrial centers focuses mainly on obtaining particles of high optical purity, which have the desired biological properties. The cost of obtaining such a compound and the elimination of toxic or undesirable chemical waste is important. Enzymatic reactions based on enzymes alone or whole microorganisms enable obtaining products with a specific structure and purity in accordance with the rules of Green Chemistry.


Introduction
The study of the properties and composition of plant secondary metabolites is of interest to many scientists. Among them, terpenes are a significant and very interesting group due to their comprehensive action in plants and their use in food, pharmaceutical, cosmetic or agricultural products [1]. Terpenes are isoprene oligomers (2-methylbuta-1,3-diene) usually connected together according to the isoprene rule-head-to-tail". Monoterpenes as isoprene dimers are the simplest terpenes [2]. These compounds are the main components of essential oils found in plants of different climate zones. In plants they are located mainly in specialized types of storage tissues like glandular capitate trichomes [3][4][5]. It is surprising that their role in plants is not fully explained. So far, we know that essential oils are responsible for the fragrance of the raw material and they can attract pollinating insects [6]. Scientists have confirmed their physiological roles include chemical defenses against abiotic and biotic stresses [7,8], e.g., it protects against bacterial or fungal infection, insect or plant-eating animals. The emission of most stress-induced volatile compounds is thought to be mediated by the expression of the genes encoding the responsible proteins, such as TPS and CYPs, as well as by a burst of volatiles from storage organs when some of this organs are damaged [3]. The volatiles support the plants in the competition for habitat between different plant species (allelopathy).
Despite this designed operation, many microorganisms, insects and animals have developed a selective ability to deal with plant defense mechanisms. As a result of changes taking place in these organisms, toxic compounds are usually transformed into less toxic derivatives. These derivatives can also be useful to the organism, sometimes even more than the parent compound. For example, Scheme 1. Biotransformation of (+)-limonene.

Stereoisomer
Organism Products Reference
Molecules 2020, 25, x FOR PEER REVIEW 5 of 25 On the other hand, the Mycobacterium sp. HXN-1500 strain carries out the oxidation of limonene to perillyl alcohol through a multi-component electron transfer chain consisting of cytochrome P450, ferredoxin and ferredoxin reductase. It was found that the ability of Mycobacterium sp. to hydroxylate limonene was due to its ability to hydroxylate alkanes [36]. The P450-dependent alkane monooxygenase system is responsible for the hydroxylation of alkanes. The next stage involves the action of alcohol oxidizing enzymes (alcohol oxidases or alcohol dehydrogenases) and aldehyde dehydrogenase, resulting in the formation of the appropriate acid [41]. Scheme 3, Table 1. The yeast, in particular the strain Yarrowia lipolytica ATCC 18942, was able to oxidize limonene to perillic acid. The authors suggest that this process is probably initiated by monooxygenases associated with cytochrome P-450. This supposition was confirmed by an experiment in which perillyl alcohol was used as a substrate. Yeast Y. lipolytica was able to oxidize perillyl alcohol to perillyl aldehyde and then to perillic acid. Another experiment was carried out in the absence of oxygen, incubating the reaction mixture in a screwed-up flask in a nitrogen atmosphere. At that time, perillyl aldehyde was reduced to peril alcohol. This suggests that the oxidation of limonene to perillyl alcohol occurs in stages and through a multi-enzymatic oxidation pathway [37]. Scheme 3, Table 1.
The psychrotrophic fungus Mortierella minutissima 01 is also capable of oxidizing C-7 carbon in the (+)-limonene molecule. As a result of the reaction carried out for 48 h at 15 °C with a substrate at 6.7 g/L, perillyl alcohol (71.5 mg/L) and perillyl aldehyde (7.9 mg/L) were obtained as main products. For additional mycelium aeration, 1% H2O2 was added to the reaction mixture. Due to the fact that the fungus Mortierella minutissima 01 is able to decompose H2O2 into oxygen and water via catalase, the addition of H2O2 had a positive effect on the biotransformation process, increasing its efficiency. The reaction carried out under the same conditions as described above, but with the addition of 1% hydrogen peroxide, yielded 104.9 mg/L of perillyl alcohol and 9.8 g/L of perillyl aldehyde [38]. Scheme 2, Table 1.
Common products of limonene biotransformation are trans-1,2-diol and α-terpineol. These compounds are formed by the use of both yeasts and filamentous fungi.
Yeast of the genus Trichosporon sp. UOFS Y-2041 transformed (R)-(+)-limonene into two products, resulting from the hydroxylation of the double bond 1,2 and the oxidation of carbon C-3. The yeast, in particular the strain Yarrowia lipolytica ATCC 18942, was able to oxidize limonene to perillic acid. The authors suggest that this process is probably initiated by monooxygenases associated with cytochrome P-450. This supposition was confirmed by an experiment in which perillyl alcohol was used as a substrate. Yeast Y. lipolytica was able to oxidize perillyl alcohol to perillyl aldehyde and then to perillic acid. Another experiment was carried out in the absence of oxygen, incubating the reaction mixture in a screwed-up flask in a nitrogen atmosphere. At that time, perillyl aldehyde was reduced to peril alcohol. This suggests that the oxidation of limonene to perillyl alcohol occurs in stages and through a multi-enzymatic oxidation pathway [37]. Scheme 3, Table 1.
The psychrotrophic fungus Mortierella minutissima 01 is also capable of oxidizing C-7 carbon in the (+)-limonene molecule. As a result of the reaction carried out for 48 h at 15 • C with a substrate at 6.7 g/L, perillyl alcohol (71.5 mg/L) and perillyl aldehyde (7.9 mg/L) were obtained as main products. For additional mycelium aeration, 1% H 2 O 2 was added to the reaction mixture. Due to the fact that the fungus Mortierella minutissima 01 is able to decompose H 2 O 2 into oxygen and water via catalase, the addition of H 2 O 2 had a positive effect on the biotransformation process, increasing its efficiency. The reaction carried out under the same conditions as described above, but with the addition of 1% hydrogen peroxide, yielded 104.9 mg/L of perillyl alcohol and 9.8 g/L of perillyl aldehyde [38]. Scheme 2, Table 1.
Common products of limonene biotransformation are trans-1,2-diol and α-terpineol. These compounds are formed by the use of both yeasts and filamentous fungi.
(R)-(+)-Limonene was subjected to solid-state fermentation with the endophytic fungus Diaporthe sp. Dried and ground orange waste consisting of peelings and pomace was used as substrate. The substrate was chosen due to the high content of limonene (5.08%) in peel and orange pomace. As a result of the 7-day biotransformation, limonene-1,2-diol was obtained in the amount of 2.66 g/kg of substrate. Other compounds produced in significant amounts were α-terpineol, trans-carveol and cis-carveol [28]. Scheme 2, Table 1.
Repeated tests have shown that in biotransformation of both limonene isomers it is possible to obtain α-terpineol and limonene-trans-diol. Various microorganisms are used for biotransformation, e.g., filamentous fungi of the genus Fusarium oxysporum 152B. Observations made by the authors confirmed the hypothesis that F. oxysporum 152B uses two parallel metabolic pathways for limonene isomers, depending on the substrate. In the case of (S)-(−)-limonene, the 1,2 double bond is epoxidized and the oxirane ring is opened to form the diol by an oxidation process leading to energy production. (R)-(+)-Limonene is converted to α-terpineol, in a process independent of oxygen, which can be seen as a process of medium detoxification [29]. Schemes 1 and 2, Table 1.
In order to optimize the biotransformation process of (R)-(+)-limonene to limonene-1,2-diol, the fungal strain Colletotrichum nymphaeae CBMAI 0864 was used. It was found that oxygenation of the medium plays an important role in biotransformation. Carrying out biotransformation in anaerobic conditions did not allow obtaining any products. The best result was obtained after addition of 15 g/L to the substrate to the mycelium growth. After 8 days of biotransformation, 4.19 g/L of limonene-1,2-diol was obtained. Scheme 2, Table 1.
The authors investigated the mechanism of diol formation. From previous studies it is known that in case of Rhodococcus erythropolis bacteria, in the first stage FAD-and NADH-dependent limonene-1,2-monooxygenase acts, which oxidizes limonene to limonene-1,2-epoxide, which in turn is opened to form the diol by co-factor-independent epoxide hydrolyase. Colletotrichum nymphaeae CBMAI 0864 strain was found to be free of these enzymes. Similarities to Grosmannia claviger strain were observed. In both these strains FAD-binding monooxygenase (Acc No F0 × 7A8) is responsible for the process of epoxy formation, whereas epoxide hydrolyase (Acc No F0 × 7A7) is responsible for opening the epoxide ring to form the diol [23,31]. Scheme 3, Table 1.
The use of the fungal strains Penicillium digitatum and Corynespora casssicola for biotransformation of both limonene enantiomers allowed to obtain α-terpineol and limonen-1,2-diol, respectively, as main products. The complete overreaction of limonene to α-terpineol was observed already after 8 h, while the transformation of the substrate to diol took much longer, i.e., 5 days [39]. Schemes 1 and 2, Table 1.
Orange essential oil, containing mainly (R)-(+)-limonene (94%), was biotransformed using strains of Penicillium sp. 2025, Aspergillus sp. 2038, Fusarium oxysporum 152B. In the biotransformations manioc meal (manipueira) was used as a medium. Mycelium was cultured in the manioc medium and then after growth it was transferred to the mineral medium. Orange oil was added in three portions after three, four and five days. The biotransformation was carried out for 7 days. The strain leading the process was F. oxysporum 152B, and the main product obtained in the amount of 450 mg/L was α-terpineol [32]. Scheme 2, Table 1. The Sphingobium sp. strain was used to optimize the biotransformation of R-(+)-limonene to α-terpineol. The authors investigated a number of parameters such as pH, biocatalyst and substrate concentration, temperature, time and agitation. The best result, taking into account the concentration of the product, was obtained when a reaction medium was used as a mixture of water with pH = 7 and soybean oil in the proportion of 1:3, with biomass concentration of 2.8 g/L, and limonene concentration of 350 g/L. The cultivation was carried out for 96 h at 28 • C and agitation speed of 200 rpm. Under such conditions, 240 g/L of α-terpineol was obtained, which was about 20 g of a-terpineol per 1 g of biomass.
On the other hand, using a mixture of water and oil in the proportion of 1:1 led obtaining α-terpineol in a lower concentration (182 g/L), but with higher yield, 65 g of α-terpineol per 1 g of biomass [30]. Scheme 2, Table 1. Limonene can also be used to make carveols. Rhodococcus opacus PWD4 strain, capable of degrading toluene, carried out hydroxylation of (R)-(+)-limonene only in the 6-position, allowing to obtain enantiomerically pure trans-carveol with a 97% yield. Such a result was obtained by culturing the bacteria on a mineral medium with the addition of toluene as the only carbon source. On the other hand, the bacteria grown on the medium in which the carbon source was glucose did not transform (R)-(+)-limonene at all. This suggests that one of the enzymes in the toluene degradation pathway, which is toluene 2,3-dioxygenase, is responsible for the biotransformation [33]. Scheme 2, Table 1.
The use of Pleurotus sapidus P 226-1 as a fungal biocatalyst made it possible to obtain, from (R)-(+)-limonene, a mixture of cisand trans-carveols in the proportion of 2:3 and racemic carvone resulting from the oxidation of carveols [34]. Scheme 1, Table 1.

The Biotransformation of Limonene by Insects
In experiments comparing the metabolism of both enantiomers in the enzyme system of Spodoptera litura larvae, it has been shown that there are no great differences in the products produced and their amounts. The larvae converted both enantiomers to the corresponding limonene-8,9-diol and perillic acid (Scheme 4). These products were formed as a result of the dihydroxylation of the double bond at the 8,9 position or the oxidation of the C-7 carbon. It should be emphasized that the dihydroxylation of the 8,9 double bond is unique for insects. During the experiment, no formation of any intermediate products (alcohol, aldehyde, epoxide) was observed. Moreover, it was found that 8,9-diols arise as a mixture of diastereoisomers. On this basis, it can be concluded that the larvae do not recognize the difference between the (+)-and (−)-limonene forms [40]. Scheme 4, Table 1.
Molecules 2020, 25, x FOR PEER REVIEW 7 of 25 added in three portions after three, four and five days. The biotransformation was carried out for 7 days. The strain leading the process was F. oxysporum 152B, and the main product obtained in the amount of 450 mg/L was α-terpineol [32]. Scheme 2, Table 1. The Sphingobium sp. strain was used to optimize the biotransformation of R-(+)-limonene to α-terpineol. The authors investigated a number of parameters such as pH, biocatalyst and substrate concentration, temperature, time and agitation. The best result, taking into account the concentration of the product, was obtained when a reaction medium was used as a mixture of water with pH = 7 and soybean oil in the proportion of 1:3, with biomass concentration of 2.8 g/L, and limonene concentration of 350 g/L. The cultivation was carried out for 96 h at 28 °C and agitation speed of 200 rpm. Under such conditions, 240 g/L of α-terpineol was obtained, which was about 20 g of a-terpineol per 1 g of biomass. On the other hand, using a mixture of water and oil in the proportion of 1:1 led obtaining α-terpineol in a lower concentration (182 g/L), but with higher yield, 65 g of α-terpineol per 1 g of biomass [30]. Scheme 2, Table 1. Limonene can also be used to make carveols. Rhodococcus opacus PWD4 strain, capable of degrading toluene, carried out hydroxylation of (R)-(+)-limonene only in the 6-position, allowing to obtain enantiomerically pure trans-carveol with a 97% yield. Such a result was obtained by culturing the bacteria on a mineral medium with the addition of toluene as the only carbon source. On the other hand, the bacteria grown on the medium in which the carbon source was glucose did not transform (R)-(+)-limonene at all. This suggests that one of the enzymes in the toluene degradation pathway, which is toluene 2,3-dioxygenase, is responsible for the biotransformation [33]. Scheme 2, Table 1.
The use of Pleurotus sapidus P 226-1 as a fungal biocatalyst made it possible to obtain, from (R)-(+)-limonene, a mixture of cis-and trans-carveols in the proportion of 2:3 and racemic carvone resulting from the oxidation of carveols [34]. Scheme 1, Table 1.

The Biotransformation of Limonene by Insects
In experiments comparing the metabolism of both enantiomers in the enzyme system of Spodoptera litura larvae, it has been shown that there are no great differences in the products produced and their amounts. The larvae converted both enantiomers to the corresponding limonene-8,9-diol and perillic acid (Scheme 4). These products were formed as a result of the dihydroxylation of the double bond at the 8,9 position or the oxidation of the C-7 carbon. It should be emphasized that the dihydroxylation of the 8,9 double bond is unique for insects. During the experiment, no formation of any intermediate products (alcohol, aldehyde, epoxide) was observed. Moreover, it was found that 8,9-diols arise as a mixture of diastereoisomers. On this basis, it can be concluded that the larvae do not recognize the difference between the (+)-and (−)-limonene forms [40]. Scheme 4, Table 1.

γ-Terpinene
γ-Terpinene is isolated from e.g., Eucalyptus dives oil or the terpeneol variety of marjoram oil. The scent of this compound is described as herbaceous and citrus [45].

γ-Terpinene
γ-Terpinene is isolated from e.g., Eucalyptus dives oil or the terpeneol variety of marjoram oil. The scent of this compound is described as herbaceous and citrus [45].
As a result of administration of γ-terpinene at a concentration of 1 mg/g of food for S. litura larvae, two main products were obtained. These were p-mentha-1,4-dien-7-oic acid (46%) and p-cymen-7-oic acid (48%). Similarly, as in the case of α-terpineol, oxidation of C-7 carbon was observed here. However, unlike then, intestinal bacteria were not involved in the above process. This difference may be due to a slightly different substrate structure and the use of a different substrate concentration in the insect diet [47]. Scheme 6, Table 3. Scheme 6. Biotransformation of racemic γ-terpinene. Table 3. Biotransformation of γ-terpinene.
As a result of administration of γ-terpinene at a concentration of 1 mg/g of food for S. litura larvae, two main products were obtained. These were p-mentha-1,4-dien-7-oic acid (46%) and p-cymen-7-oic acid (48%). Similarly, as in the case of α-terpineol, oxidation of C-7 carbon was observed here. However, unlike then, intestinal bacteria were not involved in the above process. This difference may be due to a slightly different substrate structure and the use of a different substrate concentration in the insect diet [47]. Scheme 6, Table 3.

Terpinen-4-ol
Both enantiomers and racemic terpinen-4-ol are found in many essential oils, such as lavender, eucalyptus or pine. The smell is described as spicy, nut-mag, wood-earthy with a distinct lilac-like note. This compound is used in perfumery when creating tea and lavender notes.
(R)-Terpinen-4-ol and (S)-terpinen-4-ol were subjected to biotransformation in the enzymatic system of S. litura larvae. These compounds were given to insects at a concentration of 1 mg/g of food.
Each of the enantiomers was converted to one metabolite, respectively (R)-p-menth-1-ene-4,7-diol (71%) and (S)-p-menth-1-ene-4,7-diol (72%). In both substrates C-7 carbon was hydroxylated. Moreover, the larvae did not differentiate the form of (R) and (S) of the substrate, i.e., the asymmetric carbon atom at C-4 did not affect the course of the reaction. It was also found that intestinal bacteria did not participate in the observed transformations [47]. Scheme 7, Table 4.

Terpinen-4-ol
Both enantiomers and racemic terpinen-4-ol are found in many essential oils, such as lavender, eucalyptus or pine. The smell is described as spicy, nut-mag, wood-earthy with a distinct lilac-like note. This compound is used in perfumery when creating tea and lavender notes.
(R)-Terpinen-4-ol and (S)-terpinen-4-ol were subjected to biotransformation in the enzymatic system of S. litura larvae. These compounds were given to insects at a concentration of 1 mg/g of food. Each of the enantiomers was converted to one metabolite, respectively (R)-p-menth-1-ene-4,7-diol (71%) and (S)-p-menth-1-ene-4,7-diol (72%). In both substrates C-7 carbon was hydroxylated. Moreover, the larvae did not differentiate the form of (R) and (S) of the substrate, i.e., the asymmetric carbon atom at C-4 did not affect the course of the reaction. It was also found that intestinal bacteria did not participate in the observed transformations [47]. Scheme 7, Table 4.

α-Terpineol
This compound exists as a colorless crystalline solid. The scent is determined as lilac-like. Due to its fragrance properties, it is used as a fragrance.

α-Terpineol
This compound exists as a colorless crystalline solid. The scent is determined as lilac-like. Due to its fragrance properties, it is used as a fragrance.

Terpinen-4-ol
Both enantiomers and racemic terpinen-4-ol are found in many essential oils, such as lavender, eucalyptus or pine. The smell is described as spicy, nut-mag, wood-earthy with a distinct lilac-like note. This compound is used in perfumery when creating tea and lavender notes.
(R)-Terpinen-4-ol and (S)-terpinen-4-ol were subjected to biotransformation in the enzymatic system of S. litura larvae. These compounds were given to insects at a concentration of 1 mg/g of food. Each of the enantiomers was converted to one metabolite, respectively (R)-p-menth-1-ene-4,7-diol (71%) and (S)-p-menth-1-ene-4,7-diol (72%). In both substrates C-7 carbon was hydroxylated. Moreover, the larvae did not differentiate the form of (R) and (S) of the substrate, i.e., the asymmetric carbon atom at C-4 did not affect the course of the reaction. It was also found that intestinal bacteria did not participate in the observed transformations [47]. Scheme 7, Table 4.

α-Terpineol
This compound exists as a colorless crystalline solid. The scent is determined as lilac-like. Due to its fragrance properties, it is used as a fragrance.
S. litura larvae were administered (±)-α-terpineol in an amount of 10 mg per g of insect body weight on three consecutive days. As a result of biotransformation, two products were obtained, i.e., p-menth-1-ene-7,8-diol (26.7%) and 8-hydroxy-p-menth-1-en-7-oic acid (57.6%). Analysis of the results showed that the C-7 carbon was hydroxylated in the first step, followed by the primary alcohol oxidation to acid in the next step. Additionally, the conducted studies showed that intestinal bacteria were not involved in these transformations [49,50]. Scheme 8, Table 5.

Menthol
Menthol has there chiral centers; therefore, four pairs of enantiomers are known. Only (−)-menthol has a pure mint scent and it is found in many essential oils, mainly of the Mentha genus.

Microbiological Biotransformation of Menthol
The use of fungi of the genus Aspergillus niger for the biotransformation of (1R,3S,4R)-(−)-and (1S,3R,4S)-(+)-menthol allowed obtaining several different hydroxyl derivatives. Biotransformations were carried out in static and shaking culture for 3 days, which allowed for the complete reactivation of the substrates. For (−)-menthol the preferred sites for hydroxylation were carbons C-8 and C-9, while for (+)-menthol it was carbon C-7. Moreover, in both cases formed were (in small amounts) hydroxylation products at C-6 and C-1 positions. Schemes 10 and 11, Table 6. Scheme 9. Biotransformation of (S)-α-terpineol.
S. litura larvae were administered (±)-α-terpineol in an amount of 10 mg per g of insect body weight on three consecutive days. As a result of biotransformation, two products were obtained, i.e., p-menth-1-ene-7,8-diol (26.7%) and 8-hydroxy-p-menth-1-en-7-oic acid (57.6%). Analysis of the results showed that the C-7 carbon was hydroxylated in the first step, followed by the primary alcohol oxidation to acid in the next step. Additionally, the conducted studies showed that intestinal bacteria were not involved in these transformations [49,50]. Scheme 8, Table 5.

Menthol
Menthol has there chiral centers; therefore, four pairs of enantiomers are known. Only (−)-menthol has a pure mint scent and it is found in many essential oils, mainly of the Mentha genus.

Microbiological Biotransformation of Menthol
The use of fungi of the genus Aspergillus niger for the biotransformation of (1R,3S,4R)-(−)-and (1S,3R,4S)-(+)-menthol allowed obtaining several different hydroxyl derivatives. Biotransformations were carried out in static and shaking culture for 3 days, which allowed for the complete reactivation of the substrates. For (−)-menthol the preferred sites for hydroxylation were carbons C-8 and C-9, while for (+)-menthol it was carbon C-7. Moreover, in both cases formed were (in small amounts) hydroxylation products at C-6 and C-1 positions. Schemes 10 and 11, Table 6. For (−)-menthol biotransformation 12 isolates from Rhizoctonia solani, a plant pathogen commonly found in soil, were used. These isolates were derived from infected rice, Kentucky blue grass, white clover, European pear, sugar beet and coffee. Three of them, which came from sugar beet, were able to transform the substrate into products with a yield of 89.7-99.9% within 5 days. The metabolic pathways of (−)-menthol biotransformation by Rhizoctonia solani were studied. It was found that in the first stage there was stereoselective hydroxylation in C-1 or C-6 position. In the second stage (−)-hydroxymenthol was hydroxylated in C-8 position [53]. Scheme 10, Table 6.
Incubation of the pathogenic fungus Macrophomina phaseolin with (+)-menthol within 12 days allowed obtaining products resulting from hydroxylation of C-1, C-6, C-8 and C-9 carbon. These compounds were then subjected to another hydroxylation giving further products, with C-8 carbon being the preferred hydroxylation position [55]. Scheme 11, Table 6. Scheme 11. Microbiological biotransformations of (+)-menthol. For (−)-menthol biotransformation 12 isolates from Rhizoctonia solani, a plant pathogen commonly found in soil, were used. These isolates were derived from infected rice, Kentucky blue grass, white clover, European pear, sugar beet and coffee. Three of them, which came from sugar beet, were able to transform the substrate into products with a yield of 89.7-99.9% within 5 days. The metabolic pathways of (−)-menthol biotransformation by Rhizoctonia solani were studied. It was found that in the first stage there was stereoselective hydroxylation in C-1 or C-6 position. In the second stage (−)-hydroxymenthol was hydroxylated in C-8 position [53]. Scheme 10, Table 6.
The fungi of the genus Cephalosporium aphidicola were used for the biotransformation of (−)-menthol. As a result of the 12-day incubation, several dihydroxy derivatives were obtained. These fungi preferred primarily the hydroxylation of C-7 and C-9, and, to a lesser extent, also C-6 and C-8 [52]. Scheme 10, Table 6.
For (−)-menthol biotransformation 12 isolates from Rhizoctonia solani, a plant pathogen commonly found in soil, were used. These isolates were derived from infected rice, Kentucky blue grass, white clover, European pear, sugar beet and coffee. Three of them, which came from sugar beet, were able to transform the substrate into products with a yield of 89.7-99.9% within 5 days. The metabolic pathways of (−)-menthol biotransformation by Rhizoctonia solani were studied. It was found that in the first stage there was stereoselective hydroxylation in C-1 or C-6 position. In the second stage (−)-hydroxymenthol was hydroxylated in C-8 position [53]. Scheme 10, Table 6.
Incubation of the pathogenic fungus Macrophomina phaseolin with (+)-menthol within 12 days allowed obtaining products resulting from hydroxylation of C-1, C-6, C-8 and C-9 carbon. These compounds were then subjected to another hydroxylation giving further products, with C-8 carbon being the preferred hydroxylation position [55]. Scheme 11, Table 6.
The use of Chlorella vulgaris microalgae for the bioconversion of menthol resulted in obtaining various products that appeared as the reaction progressed. After 72 h of biotransformation, the appearance of dihydroterpineol (48.8%) and isomenthol (20.2%) was observed. After 92 h, the major product in the reaction mixture was isomenthol (92.3%). After 120 h of experimentation, the appearance of cis-p-menth-1-en-3-ol (46.0%) and dihydroterpineol (49.2%) was observed. The optimal pH value for this process was 5.5. The authors explained the formation of dihydroterpineol from menthol by the formation of carbocation as an intermediate product, and then its rearrangement [56]. Scheme 12, Table 6. The use of Chlorella vulgaris microalgae for the bioconversion of menthol resulted in obtaining various products that appeared as the reaction progressed. After 72 h of biotransformation, the appearance of dihydroterpineol (48.8%) and isomenthol (20.2%) was observed. After 92 h, the major product in the reaction mixture was isomenthol (92.3%). After 120 h of experimentation, the appearance of cis-p-menth-1-en-3-ol (46.0%) and dihydroterpineol (49.2%) was observed. The optimal pH value for this process was 5.5. The authors explained the formation of dihydroterpineol from menthol by the formation of carbocation as an intermediate product, and then its rearrangement [56]. Scheme 12, Table 6. Scheme 12. Biotransformation of (−)-menthol by microalgae.

The Biotransformation of Menthol by Insects
(1R,3S,4R)-(−)-Menthol and (1S,3R,4S)-(+) menthol were fed to Spodoptera litura larvae in the diet at a concentration of 1 mg of compound per gram of food. Carbon C-7 oxidation products were obtained from both menthol isomers. From (−)-menthol obtained was (−)-7-hydroxymenthol, and from (+)-menthol analogously (+)-7-hydroxymenthol was obtained. The percentage of substrate conversion in both cases was similar and amounted to 86-90%. The process of menthol biotransformation in an in vitro system in the culture of intestinal bacteria of Spodoptera litura larvae was also investigated. However, it turned out that they were not involved in the metabolism of this substrate [54]. Scheme 13, Table 6.

Menthone
Menthone is an example of a terpenoid that contains a carbonyl group. This compound accompanies menthol and limonene in peppermint oils. It is used in the food industry as a flavoring ingredient, as well as in fragrance compositions.

Microbiological Biotransformation of Menthone
The bacterial strains Acinetobacter NCIEI 9871 and Acinetobacter TD63 turned out to be able to transform menthone in a completely different way from the ones previously encountered. These Scheme 12. Biotransformation of (−)-menthol by microalgae.

The Biotransformation of Menthol by Insects
(1R,3S,4R)-(−)-Menthol and (1S,3R,4S)-(+) menthol were fed to Spodoptera litura larvae in the diet at a concentration of 1 mg of compound per gram of food. Carbon C-7 oxidation products were obtained from both menthol isomers. From (−)-menthol obtained was (−)-7-hydroxymenthol, and from (+)-menthol analogously (+)-7-hydroxymenthol was obtained. The percentage of substrate conversion in both cases was similar and amounted to 86-90%. The process of menthol biotransformation in an in vitro system in the culture of intestinal bacteria of Spodoptera litura larvae was also investigated. However, it turned out that they were not involved in the metabolism of this substrate [54]. Scheme 13, Table 6. The use of Chlorella vulgaris microalgae for the bioconversion of menthol resulted in obtaining various products that appeared as the reaction progressed. After 72 h of biotransformation, the appearance of dihydroterpineol (48.8%) and isomenthol (20.2%) was observed. After 92 h, the major product in the reaction mixture was isomenthol (92.3%). After 120 h of experimentation, the appearance of cis-p-menth-1-en-3-ol (46.0%) and dihydroterpineol (49.2%) was observed. The optimal pH value for this process was 5.5. The authors explained the formation of dihydroterpineol from menthol by the formation of carbocation as an intermediate product, and then its rearrangement [56]. Scheme 12, Table 6. Scheme 12. Biotransformation of (−)-menthol by microalgae.

The Biotransformation of Menthol by Insects
(1R,3S,4R)-(−)-Menthol and (1S,3R,4S)-(+) menthol were fed to Spodoptera litura larvae in the diet at a concentration of 1 mg of compound per gram of food. Carbon C-7 oxidation products were obtained from both menthol isomers. From (−)-menthol obtained was (−)-7-hydroxymenthol, and from (+)-menthol analogously (+)-7-hydroxymenthol was obtained. The percentage of substrate conversion in both cases was similar and amounted to 86-90%. The process of menthol biotransformation in an in vitro system in the culture of intestinal bacteria of Spodoptera litura larvae was also investigated. However, it turned out that they were not involved in the metabolism of this substrate [54]. Scheme 13, Table 6.

Menthone
Menthone is an example of a terpenoid that contains a carbonyl group. This compound accompanies menthol and limonene in peppermint oils. It is used in the food industry as a flavoring ingredient, as well as in fragrance compositions.

Microbiological Biotransformation of Menthone
The bacterial strains Acinetobacter NCIEI 9871 and Acinetobacter TD63 turned out to be able to transform menthone in a completely different way from the ones previously encountered. These

Menthone
Menthone is an example of a terpenoid that contains a carbonyl group. This compound accompanies menthol and limonene in peppermint oils. It is used in the food industry as a flavoring ingredient, as well as in fragrance compositions.

Microbiological Biotransformation of Menthone
The bacterial strains Acinetobacter NCIEI 9871 and Acinetobacter TD63 turned out to be able to transform menthone in a completely different way from the ones previously encountered. These bacteria, instead of reducing the carbonyl group, carried out the Baeyer-Villiger oxidation, yielding a lactone product. The biotransformation efficiency was 90% for the Acinetobacter NCIEI 9871 strain and 61% for the Acinetobacter TD63 strain. The mentioned bacterial strains showed high substrate specificity. They oxidized only the (+)-menthone enantiomer while the (−)-enantiomer remained intact in the reaction mixture. This substrate specificity is related to the Baeyer-Villiger reaction mechanism, which involves the nucleophilic attack of the ketone by hydroperoxyflavin, leading to the formation of an intermediate hydroxyperoxyflavine. According to the authors, the location of the intermediate compound in the active site of the enzyme, as well as certain stereoelectronic effects determine the regioselectivity, and thus the enantioselectivity of the reaction. The comparison of various peroxide forms that can arise as intermediate forms allowed stating that in order to match the active site of the enzyme, the isopropyl group must be in the equatorial position. The axial orientation of the propyl group makes it impossible to match the intermediate product to the active center of the enzyme due to steric hindrance [57]. Scheme 14, Table 7.
Molecules 2020, 25, x FOR PEER REVIEW 15 of 25 bacteria, instead of reducing the carbonyl group, carried out the Baeyer-Villiger oxidation, yielding a lactone product. The biotransformation efficiency was 90% for the Acinetobacter NCIEI 9871 strain and 61% for the Acinetobacter TD63 strain. The mentioned bacterial strains showed high substrate specificity. They oxidized only the (+)-menthone enantiomer while the (−)-enantiomer remained intact in the reaction mixture. This substrate specificity is related to the Baeyer-Villiger reaction mechanism, which involves the nucleophilic attack of the ketone by hydroperoxyflavin, leading to the formation of an intermediate hydroxyperoxyflavine. According to the authors, the location of the intermediate compound in the active site of the enzyme, as well as certain stereoelectronic effects determine the regioselectivity, and thus the enantioselectivity of the reaction. The comparison of various peroxide forms that can arise as intermediate forms allowed stating that in order to match the active site of the enzyme, the isopropyl group must be in the equatorial position. The axial orientation of the propyl group makes it impossible to match the intermediate product to the active center of the enzyme due to steric hindrance [57]. Scheme 14, Table 7.

Stereoisomer
Organism Products Reference
The (1R,4S)-(−)-menthone isomer was used as a substrate for the microalgae cultures of Chlorella minutissima, Nannochloris atomus, Dunaliella parva, Porphyridium purpureum and Isochrysis galbana. All algae showed the ability to carry out non-stereoselective reduction of the carbonyl group. After 5 days of biotransformation, only (1RS,3S,4R)-(−)-menthol was present in the post-reaction mixture in addition to the substrate. I. galbana algae showed the highest degree of transformation, in the culture of which 37% menthol was obtained [60]. Scheme 15, Table 7.
Molecules 2020, 25, x FOR PEER REVIEW 16 of 25 addition to the substrate. I. galbana algae showed the highest degree of transformation, in the culture of which 37% menthol was obtained [60]. Scheme 15, Table 7.
The same substrate, (1R, 4S)-(−)-menthone, was subjected to 24 h biotransformation in Oocystis pusilla microalgae culture. These algae is also able to reduce the carbonyl group, yielding (1R,3S,4R)-(−)-menthol as a product in a yield of 11% [61]. As a result of subjecting (1R,4S)-(−)-menthone to biotransformations in the culture of Chlorella vulgaris MCCS 012 algae, menthol was obtained in the form of a racemate with a yield of 43% [62]. A similar reaction was observed for the cyanobacteria Synechococcus PCC 6716. It was also found that the cyanobacteria strains Synechococcus 6911 and Anabaena oscillarioides were not able to reduce menthone [63]. Scheme 15, Table 7.

Carvone
The optical isomers of this compound differ in terms of fragrance properties. (+)-Carvone has herbaceous odor which is reminiscent to caraway and dill seeds and occurs in caraway and dill oil. (−)-Carvone has herbaceous odor with a note of spearmint and occurs in spearmint oil.

Microbiological Biotransformation of Carvone
In microbial biotransformations of a carvone, the most frequently observed reaction is the reduction of the double bond followed by the reduction of the carbonyl group [64]. In studies conducted by Verstegen-Haaksma [65] (4S)-(+)-carvone was reduced to both dihydrocarvones in the bacterial cultures. Scheme 16, Table 8.

Carvone
The optical isomers of this compound differ in terms of fragrance properties. (+)-Carvone has herbaceous odor which is reminiscent to caraway and dill seeds and occurs in caraway and dill oil. (−)-Carvone has herbaceous odor with a note of spearmint and occurs in spearmint oil.
In the culture of Diplogelasinospora grovesii IMI 171018 (4S)-(+)-carvone was reduced to a mixture of carveol with a predominance of 2R isomer, without breaking the double bond in the ring [67]. During the (4S)-(+)-carvone biotransformation, which was carried out in the Mucor circinelloides culture, 75% (1R,4S)-dihydrocarvone was formed after 4 h of transformation and in smaller quantities a second diastereoisomeric dihydrocarvone and dihydrocarveols were formed. As a result of the reduction of endocyclic double carvone bond catalyzed by enone reductase, both dihydrocarvone isomers were formed in a 9:1 ratio. The reduction of the carbonyl dihydrocarvone group was already without stereoselectivity. It is worth noting that the Mucor strain used was isolated from Pinus taeda, a plant that is a good producer of monoterpenoids, including carvone [68]. Scheme 16, Table 8.
In the same culture, M. circinelloides also underwent a transformation of (4R)-(−)-carvone and additional formation of trihydroxylated menthanetriols was observed by changing the extraction solvent from ethyl acetate to n-butanol. The first stage of biotransformation was a stereo-selective reduction of double bond and carbonyl groups catalyzed by enone reductase and carbonyl reductase respectively. The next step was dihydroxylation of the remaining double bond, leading to trihydroxylated mentanotriols. Since in the proposed pathway of biotransformation dihydrocarveol is a biosynthetic precursor of mentanotriols, stereogenic centers on C-1 and C-2 in dihydrocarveol are not involved in the biotransformation process. For this reason, both mentanotriols and dihydrocarveol must have the same absolute configuration. (1R,2S,4R,8S)-p-menthane-2,8,9-triol was also formed during biotransformation in the Lasiodiplodia theobromae BRF118 culture. The same authors also described the biotransformation of (4R)-(−)-carvone in the culture of Trichoderma harzianum BRF117, where the only isolated product was neodihydrocarveol [68]. Scheme 17, Table 8.
Baeyer-Villiger oxidation of the dihydrocarone was also observed in the culture of Acinetobacter sp. NCIB 9871 and Acinetobacter sp. TD63 [57,72]. (+)-Dihydrocarvone was converted to a lactone in which the carbonyl group is next to the methyl group due to the higher migration capacity of the tertiary carbon atom adjacent to the carbonyl group. On the other hand, with (−)-dihydrocarvone, a lactone was formed in both cultures, in which the oxygen atom is located between the methyl and carbonyl groups. The resulting lactone is an intermediate in the synthesis of (3S,6R)-3-methyl-6-(1-methylethenyl)-9-decen-1-yl acetate, which is an attractant for male Aonidiella Scheme 17. Biotransformations of (4R)-(−)-carvone.
Baeyer-Villiger oxidation of the dihydrocarone was also observed in the culture of Acinetobacter sp. NCIB 9871 and Acinetobacter sp. TD63 [57,72]. (+)-Dihydrocarvone was converted to a lactone in which the carbonyl group is next to the methyl group due to the higher migration capacity of the tertiary carbon atom adjacent to the carbonyl group. On the other hand, with (−)-dihydrocarvone, a lactone was formed in both cultures, in which the oxygen atom is located between the methyl and carbonyl groups. The resulting lactone is an intermediate in the synthesis of (3S,6R)-3-methyl-6-(1-methylethenyl)-9-decen-1-yl acetate, which is an attractant for male Aonidiella aurantii-a citrus pest and can be used in combination with other agents for controlling these pests [73]. Scheme 17, Table 8.

Conclusions
Research on the transformation of terpene compounds in the enzymatic systems of living organisms has been conducted since the mid-twentieth century. Bacteria, fungi and insects catalyze several specific reactions that make it possible to obtain derivatives that are very difficult to obtain with standard organic synthesis. The reactions that give products with high enantiomeric excess or with specific regioselectivity are worth mentioning here. The use of an enzyme bouquet of living organisms allows us to reduce the production costs of biologically active compounds and reduce possible environmental pollution.
The review indicated that monoterpenes with p-methane system are very interesting substrates for biotransformation with enzymatic system of microorganisms and insects. From the information we have gathered, it appears that the formation of oxidation products is mainly preferred. Such reaction products were described in manuscripts on transformations of both hydrocarbon, alcohol and ketone compounds. A variety of very interesting derivatives was obtained depending on the biocatalyst used. Some of them are very important for their application in perfumery, cosmetics, food and pharmaceutical industries e.g., menthol, terpinen-4-ol [74][75][76][77][78].
Author Contributions: Writing-draft preparation-A.K.Ż., writing-review and editing-K.W., W.M., M.G. supervision, K.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.