Biotransformation of Oleanane and Ursane Triterpenic Acids

Oleanane and ursane pentacyclic triterpenoids are secondary metabolites of plants found in various climatic zones and regions. This group of compounds is highly attractive due to their diverse biological properties and possible use as intermediates in the synthesis of new pharmacologically promising substances. By now, their antiviral, anti-inflammatory, antimicrobial, antitumor, and other activities have been confirmed. In the last decade, methods of microbial synthesis of these compounds and their further biotransformation using microorganisms are gaining much popularity. The present review provides clear evidence that industrial microbiology can be a promising way to obtain valuable pharmacologically active compounds in environmentally friendly conditions without processing huge amounts of plant biomass and using hazardous and expensive chemicals. This review summarizes data on distribution, microbial synthesis, and biological activities of native oleanane and ursane triterpenoids. Much emphasis is put on the processes of microbial transformation of selected oleanane and ursane pentacyclic triterpenoids and on the bioactivity assessment of the obtained derivatives.


Introduction
Drugs derived from secondary plant metabolites make up about 25% of the global pharmaceutical market [1]. Secondary metabolites of plants are several groups of compounds; the most numerous (about 25,000 representatives) and diverse group is terpenic hydrocarbons and their oxygen-containing derivatives (terpenoids). Depending on the number of isoprene units (C 5 H 8 ) in their structure, they contain a certain number of carbon atoms and are classified into mono-(C 10 ), sesqui-(C 15 ), di-(C 20 ), triterpenoids (C 30 ), etc.
Naturally occurring triterpenoids are represented by more than 100 various types of skeletons [2]. Native triterpenoids, in particular, oleanane and ursane representatives, are of interest for researchers due to their availability and multiple biological activities, including antimicrobial, anti-inflammatory, antitumor, cytotoxic, hepatoprotective, and other activities [3][4][5][6][7]. Triterpenic molecules, however, are highly hydrophobic which significantly limits their use as effective pharmacological agents. At present, one of the most common ways to increase the effectiveness and bioavailability of triterpenoids is by chemical modification. This usually requires high temperature and pH, use of expensive reagents, and introduction of protective groups of molecule reactive centers [8][9][10][11]. An alternative way to obtain valuable derivatives is by biotransformation under normal and environmentally friendly conditions employing the catalytic activity of microorganisms with high regio-and stereoselectivity in one [8][9][10][11]. An alternative way to obtain valuable derivatives is by biotransformation under normal and environmentally friendly conditions employing the catalytic activity of microorganisms with high regio-and stereoselectivity in one technological stage. Furthermore, microbial conversion ensures specific modifications of triterpenic molecule sites that are either not modified or poorly modified by synthetic transformations [12]. Note that, among the known microbial biocatalysts, members of mycelial fungi are the most studied [13][14][15] whereas bacterial catalysts are only represented by a few gram-positive species [16][17][18][19][20]. The first papers on microbial transformation of triterpenoids were published in the 1960s [21]. The earliest information related to bioconversion processes of oleanane derivatives catalyzed by fungi, such as Curvularia lunata ATCC 13432, Trichotecium roseum ATCC 8685, Cunninghamella sp. ATCC 3229, Mucor griseo-cyanus ATCC 1207-a, Helicostylum piriforme ATCC 8992, Fusarium lini, and Cunninghamella blakesleana dates back to about the same time [22][23][24][25]. The data on bacterial transformation of oleanane triterpenoids by Streptomyces sp. G-20 and Chainia antibiotica IFO 12,246 were reported in the second half of the 1980s [26,27]. As for microbial transformations of ursane pentacyclic triterpenoids by both fungal (Mucor plumbeus ATCC 4740 [28]) and bacterial (Nocardia sp. NRRL 5646 [29]) strains, those studies were initiated only in the 2000s. Henceforth, the interest in the topic discussed has been increasing and the Active Triterpenoid Biocatalysts List has been expanded every year, as is the number of various bioactive triterpenic derivatives formed via biotransformations [30][31][32]. Now, preparation of biologically active compounds based on pentacyclic triterpenoids is an actual research discussed in plenty of experimental and review publications [8,12,[32][33][34]. However, the reviews are overwhelmingly focused on chemical transformations or describe specific types of biological activities of triterpenoids. Less frequently, they deal with biological transformations. The latest of the few reviews on microbial transformations of pentacyclic triterpenoids include literature data up to 2016 [12,32,33,35]. Our review summarizes the data from 2013 to the present on distribution, microbial biosynthesis, biological activity, and mainly biotransformation of oleanane and ursane pentacyclic triterpenoids to obtain promising biologically active compounds or intermediates of their synthesis.
A B  Representatives of various higher plant families are active producers of both oleanane and ursane triterpenoids (Table 1). Frequently, OA and UA are simultaneously detected in the same plant sources. OA and UA contents in Meconopsis henrici, Dracocephalum tanguticum, Comastoma pulmonaria, Corydalis impatiens, and Swertia racemosa-traditionally used in Chinese medicine-can reach 0.96 ± 0.01 mg/g and 0.64 ± 0.01 mg/g dry weight, respectively [38]. Flowers and leaves of the shrubs Ocimum tenuiflorum and Syzygium aromaticum and the herbs Origanum vulgare, Rosmarus officinalis, and Salvia officinalis used as condiments contain up to 15.3 mg/g OA and up to 26.2 mg/g UA (wet weight) [39]. The main source of OA is considered to be the fruits and leaves of Olea europaea. The acid content in olive leaves can reach 27.16 mg/g wet weight and 25.09 ± 0.72 mg/g dry weight [39,40]. GA is commonly extracted from herbaceous plants of the genus Glycyrrhiza [41][42][43]; the acid content in their roots can reach 10.2 ± 1.7 mg/g wet weight [44].
The amount of pentacyclic triterpenoids in plants is not constant and can significantly vary depending on the activity of enzyme systems and external factors [45]. Thus, the fruits and leaves of olive (Olea europaea) of various varieties contained OA from 0.4 ± 0.1 mg/g to 0.81 ± 0.16 mg/g dry weight and from 29.2 ± 1.8 mg/g to 34.5 ± 3.1 mg/g dry weight, respectively [46,47]. The OA content decreased by 70-80% during olive fruit ripening [40]. The same tendency was observed when grapes (Vitis vinifera) ripen [48]. Changes in pentacyclic triterpenoid concentrations in plant sources may be related to specific climate, season, landscape, and cultivation strategies [47]. Representatives of various higher plant families are active producers of both oleanane and ursane triterpenoids (Table 1). Frequently, OA and UA are simultaneously detected in the same plant sources. OA and UA contents in Meconopsis henrici, Dracocephalum tanguticum, Comastoma pulmonaria, Corydalis impatiens, and Swertia racemosa-traditionally used in Chinese medicine-can reach 0.96 ± 0.01 mg/g and 0.64 ± 0.01 mg/g dry weight, respectively [38]. Flowers and leaves of the shrubs Ocimum tenuiflorum and Syzygium aromaticum and the herbs Origanum vulgare, Rosmarus officinalis, and Salvia officinalis used as condiments contain up to 15.3 mg/g OA and up to 26.2 mg/g UA (wet weight) [39]. The main source of OA is considered to be the fruits and leaves of Olea europaea. The acid content in olive leaves can reach 27.16 mg/g wet weight and 25.09 ± 0.72 mg/g dry weight [39,40]. GA is commonly extracted from herbaceous plants of the genus Glycyrrhiza [41][42][43]; the acid content in their roots can reach 10.2 ± 1.7 mg/g wet weight [44].
The amount of pentacyclic triterpenoids in plants is not constant and can significantly vary depending on the activity of enzyme systems and external factors [45]. Thus, the fruits and leaves of olive (Olea europaea) of various varieties contained OA from 0.4 ± 0.1 mg/g to 0.81 ± 0.16 mg/g dry weight and from 29.2 ± 1.8 mg/g to 34.5 ± 3.1 mg/g dry weight, respectively [46,47]. The OA content decreased by 70-80% during olive fruit ripening [40]. The same tendency was observed when grapes (Vitis vinifera) ripen [48]. Changes in pentacyclic triterpenoid concentrations in plant sources may be related to specific climate, season, landscape, and cultivation strategies [47]. Vitaceae Juss., nom. cons.

Biosynthesis of Pentacyclic Triterpenic Acids Using Microorganisms
Today, pentacyclic triterpenoids and their natural derivatives are mainly obtained by extraction from plant sources. However, the extraction and separation of these compounds (often with organic solvents) are extremely labor-intensive, and energy-and time-consuming. Besides, most of pentacyclic triterpenoids are found in relatively low concentrations in plants, entailing the use of huge amounts of plant raw materials and the formation of waste biomass in large volumes [62]. An alternative source of pentacyclic triterpenoids seems to be highly efficient cell factories, increasingly popular in the last decade. They allow to obtain valuable biologically active compounds of plant origin in environmentally friendly conditions using available compounds as the sole source of carbon (glucose, galactose, and ethanol) [63]. Cell factories are usually yeast cells-natural catalysts of the mevalonate (MVA) pathway-with plant genes responsible for pentacyclic triterpenoid synthesis introduced into their genome. The MVA pathway includes formation of mevalonate involving 3-hydroxy-3-methylglutaryl-CoA reductases (HMG1). The mevalonate formed is further transformed into isopentenyl diphosphate and dimethylallyl diphosphate, being converted to farnesyl diphosphate by a farnesyl phosphate synthase (ERG20). This pathway provides natural synthesis of squalene (4)-a common precursor of triterpenoids-by the squalene synthase (ERG9) based on two molecules of farnesyl diphosphate and its further transformation into 2,3-oxidosqualene (5) by squalene epoxidase (ERG1) [64]. Subsequent synthesis of pentacyclic triterpenoids involves plant genes encoding amyrin synthase, CYP450, and CYP450 reductase (Scheme 1).

Biosynthesis of Pentacyclic Triterpenic Acids Using Microorganisms
Today, pentacyclic triterpenoids and their natural derivatives are mainly obtained by extraction from plant sources. However, the extraction and separation of these compounds (often with organic solvents) are extremely labor-intensive, and energy-and time-consuming. Besides, most of pentacyclic triterpenoids are found in relatively low concentrations in plants, entailing the use of huge amounts of plant raw materials and the formation of waste biomass in large volumes [62]. An alternative source of pentacyclic triterpenoids seems to be highly efficient cell factories, increasingly popular in the last decade. They allow to obtain valuable biologically active compounds of plant origin in environmentally friendly conditions using available compounds as the sole source of carbon (glucose, galactose, and ethanol) [63]. Cell factories are usually yeast cells-natural catalysts of the mevalonate (MVA) pathway-with plant genes responsible for pentacyclic triterpenoid synthesis introduced into their genome. The MVA pathway includes formation of mevalonate involving 3hydroxy-3-methylglutaryl-CoA reductases (HMG1). The mevalonate formed is further transformed into isopentenyl diphosphate and dimethylallyl diphosphate, being converted to farnesyl diphosphate by a farnesyl phosphate synthase (ERG20). This pathway provides natural synthesis of squalene (4)-a common precursor of triterpenoids-by the squalene synthase (ERG9) based on two molecules of farnesyl diphosphate and its further transformation into 2,3-oxidosqualene (5) by squalene epoxidase (ERG1) [64]. Subsequent synthesis of pentacyclic triterpenoids involves plant genes encoding amyrin synthase, CYP450, and CYP450 reductase (Scheme 1).    Insertion of βAS (G. glabra) β-Amyrin (6, 4.16 mg/L) [73] Insertion of βAS (G. glabra) and ERG1 (Candida albicans) β-Amyrin (6, 24.50 mg/L) Insertion of βAS (G. glabra), ERG1 (C. albicans), IDI (Escherichia coli) Overexpression of ERG9 and   strain expressing CYP716Y1 and CYP716A12 to obtain a self-processing polyprotein with two enzymes bound via oligopeptide 2A catalyzed the formation of β-amyrin (6), erythrodiol (8), OA, oleanolic aldehyde (9), and 16α-hydroxy-oleanolic aldehyde (10), while the second strain produced two self-processing polyproteins, one consisting of CYP716Y1 and CYP716A12 and the other consisting of AtATR1 and UDP-dependent glycosyl transferase UGT73C11, and catalyzed the formation of 3-O-Glc-echinocystic acid (11) and 3-O-Glc-OA (12).

Biological Activities of Triterpenic Acids and Their Native Derivatives
Extracts obtained from plant sources using various solvents and containing the pentacyclic triterpenoids reviewed in this paper usually exhibit a wide range of biological properties. Methanolic OA-containing extracts from various parts of Betula pendula exhibited antibacterial activity against test cultures Staphylococcus aureus and Bacillus subtilis [3]. Methanolic extracts from the aerial part of the tropical plant Baccharis uncinella containing OA and UA showed antiparasitic activity by limiting the growth of promastigote and amastigote forms of Leishmania amazonensis and enhanced the immune response in infected mice [52]. The ethyl acetate fraction of Glycyrrhiza uralensis root extract inhibited TNF-α-induced activation of NF-κB in HepG2 cells [42]. The ethyl acetate fraction of Potentilla fulgens root extract containing ursane triterpenoids showed antioxidant activity by inhibiting the production of free radicals [58]. An alcohol extract and ethyl acetate fraction of Fragaria ananassa perianth containing various triterpenoids had a pronounced cytotoxic effect on B16-F10 melanoma cells and inhibited their melanogenesis by 79.1% and 80.2%, respectively [4]. The allelopathic effect (inhibition of seed maturation and root growth of nearby plants) of Alstonia scholaris was also due to high UA content (2.5 ± 0.6 mg/g dry weight of leaves) [49].
Isolation of every triterpenic acid individually allows studying their bioactive properties in detail and explaining the pharmacological properties of some plants. The antibacterial property of birch bark was determined by a high content of pentacyclic triterpenoids, in particular, OA, which exhibits a pronounced antibacterial activity against S. aureus (minimal inhibitory concentration (MIC) 1.25%) and B. subtilis (MIC 0.625%) [3]. Antimicrobial activity of GA was manifested as the ability to reduce the motility of Pseudomonas aeruginosa cells and the level of biofilm formation. This can make a significant contribution to the development of effective antibiotic-free therapy for Pseudomonas infections [74]. UA was able to inhibit both the growth of Mycobacterium tuberculosis in vitro [75] and the replication of rotavirus in a dose-dependent manner [76]. In addition, OA and UA were shown to inhibit the COVID-19 (SARS-CoV-2) main protease, a key enzyme of the virus replication, through in silico studies [77,78].
OA, as an antitumor agent, increased the sensitivity of sarcoma cells to chemotherapeutic drugs in human soft tissues [79]. UA had similar properties, significantly increased the effectiveness of colorectal cancer chemotherapy, and reduced its side effects in vitro and in vivo [80]. The cytotoxic effect of the extract of Fragaria ananassa perianth on B16-F10 melanoma cells was determined partly by the presence of cytotoxic UA that suppressed the melanin production by 40.2% [4]. Additionally, UA reduced the spread of human myeloma cells by inhibiting the deubiquitinating protease USP7 [81] and caused apoptosis of gastric cancer cells by activating the caspases poly (ADP-ribose) polymerase and by inducing the release of reactive oxygen species [6].
The OA hepatoprotective activity was shown to be related to its inhibitory effect against carboxylesterase (therapeutic target for hypertriglyceridemia) and the hepatitis C virus (HCV) [5,82]. GA exhibited the hepatoprotective effect by inhibiting NO formation in rat hepatocytes, iNOS suppression, and COX-2 expression and by decreasing the activity of NF-κB transcription factor in HepG2 cells [42]. The ability of GA to stimulate a neuroprotective property of microglia and to suppress the MAPK signaling pathway of the central nervous system caused a decrease in the severity of experimental autoimmune encephalomyelitis in mice [83]. UA could be an effective antidiabetic agent due to its ability to inhibit α-glucosidase activity [60]. OA and UA exhibited their inhibitory effects against lypopolysaccharide (LPS)-induced NO production in RAW 264.7 cells that determined their anti-inflammatory activity [7].

Biological Transformation
Taking into account the relative availability of the discussed triterpenic acids in natural sources and their high bioactivity, it is interesting to assess the possibility of directed transformations of these compounds to expand the range of biologically active compounds and to increase their bioavailability. Chemical methods are currently the most tested and used to transform acids 1-3. However, chemical methods often require extreme acidity and temperature values, expensive catalysts, or protective groups of molecule reactive centers [8][9][10]106]. In contrast, biological transformation processes do not use aggressive reagents and can occur under normal eco-friendly conditions. Moreover, microorganisms are able to catalyze a wide range of regio-and stereoselective reactions that are difficult to perform chemically [12].
One of the most promising ways to highlight the pharmacological potential of native pentacyclic triterpenoids is the functionalization of their molecules by polyhydroxylation. Such functionalized derivatives hydroxylated by plant P450-dependent monooxygenases [107] are widespread in nature but are usually found in trace amounts or as part of a difficult-to-separate mixture. Enzymatic activity of microorganisms used for transformation of pentacyclic triterpenoids allows for obtaining hydroxylated derivatives with high yield and regioselectivity. Moreover, microbial hydroxylation occurs not only in the A ring but also at hard-to-reach positions on the B, D, and E rings. In addition to hydroxylation, microbial functionalization of pentacyclic triterpenoids can occur by less frequent reactions of carboxylation, glycosylation, lactone formation, and others.

Fungal Transformation
The described biotransformation processes of the compounds discussed in this review often occur using mycelial fungi of various species from the phyla Ascomycota (orders Glomerellales, Hypocreales) and Mucoromycota (order Mucorales). Fungal conversions of these compounds are accompanied by the formation of derivatives with hydroxyl groups at C1, C7, C15, C21, C24, or C30; oxo groups at C3, C7, or C21; glucopyranoside groups at C3, C28, or C30; lactone groups at C28/C13 or C3/C4, etc. as well as by the A ring fragmentation. The acid concentration used in biotransformation experiments usually ranges from 0.02 g/L to 1.0 g/L. The yield of transformation products (1.0% to 77.5%) and the duration of the processes (2 to 20 days) vary depending on the fungal catalyst characteristics (Table 3).

Bacterial Transformation
The literature describes a few cases of pentacyclic triterpenoid bioconversion using grampositive bacteria of the genera Bacillus, Nocardia, and Streptomyces and accompanied by the formation of С1, С2, С7, С11, С21, С24, or С29 hydroxylated derivatives, derivatives with a methyl ester group at C28; oxogroup at C3; additional carboxyl groups at С29 or С30; glucopyranoside groups at С3, С28, or С30; lactone group at С28/С13; and derivatives with a fragmented A ring. In biotransformation experiments, the compounds are usually used in concentrations ranging from 0.04 g/L to 0.3 g/L, and the yield of derivatives ranges from 5.0% to 60.0%. The duration of bioconversion is 3 to 5 days; only in the case of using Nocardia, it reached 13 days ( Table 3).
Actinobaceria of the genus Nocardia were capable of selective methylation of the C28-carboxylic group of pentacyclic triterpenoids [123]. The use of resting or immobilized cells of N. iowensis DSM 45197 as biocatalysts of the OA (approximately 0.3 g/L) transformation process for 13 days resulted in the formation of methyl OA (91) as the main bioconversion product (more than 60.0%), small amounts (≤5.0%) of methyl 3-oxo-olean-12-en-28-oat (92), and metabolite 93 unidentified by the authors [16]. 3-oxo-OA (92) was shown to have pronounced antimelanoma [124], antileishmanial, and antitrypanosomal effects [125]. Despite numerous successful examples to increase the efficiency of the biotransformation process by immobilizing microbial cells [126], the use of fixed Nocardia cells in alginate carriers led to a decrease in their catalytic activity, as confirmed by a 10-fold decrease in the formation of compound 91 and only a short-term occurrence of compound 93 in the culture medium [16]. The ability of Nocardia sp. to transform UA by methylation, by C3 oxidation, and by formation of the enone moiety in the A ring was previously shown. It was noted that the biotransformation process did not depend on the composition of the culture medium used, while the temperature increase (from 28 °С to 36 °С) for actinobacteria cultivation contributed to a 2-fold increase in the reaction rate [127].

Bacterial Transformation
The literature describes a few cases of pentacyclic triterpenoid bioconversion using gram-positive bacteria of the genera Bacillus, Nocardia, and Streptomyces and accompanied by the formation of C1, C2, C7, C11, C21, C24, or C29 hydroxylated derivatives, derivatives with a methyl ester group at C28; oxogroup at C3; additional carboxyl groups at C29 or C30; glucopyranoside groups at C3, C28, or C30; lactone group at C28/C13; and derivatives with a fragmented A ring. In biotransformation experiments, the compounds are usually used in concentrations ranging from 0.04 g/L to 0.3 g/L, and the yield of derivatives ranges from 5.0% to 60.0%. The duration of bioconversion is 3 to 5 days; only in the case of using Nocardia, it reached 13 days ( Table 3).
Actinobaceria of the genus Nocardia were capable of selective methylation of the C28-carboxylic group of pentacyclic triterpenoids [123]. The use of resting or immobilized cells of N. iowensis DSM 45197 as biocatalysts of the OA (approximately 0.3 g/L) transformation process for 13 days resulted in the formation of methyl OA (91) as the main bioconversion product (more than 60.0%), small amounts (≤5.0%) of methyl 3-oxo-olean-12-en-28-oat (92), and metabolite 93 unidentified by the authors [16]. 3-oxo-OA (92) was shown to have pronounced antimelanoma [124], antileishmanial, and antitrypanosomal effects [125]. Despite numerous successful examples to increase the efficiency of the biotransformation process by immobilizing microbial cells [126], the use of fixed Nocardia cells in alginate carriers led to a decrease in their catalytic activity, as confirmed by a 10-fold decrease in the formation of compound 91 and only a short-term occurrence of compound 93 in the culture medium [16]. The ability of Nocardia sp. to transform UA by methylation, by C3 oxidation, and by formation of the enone moiety in the A ring was previously shown. It was noted that the biotransformation process did not depend on the composition of the culture medium used, while the temperature increase (from 28 • C to 36 • C) for actinobacteria cultivation contributed to a 2-fold increase in the reaction rate [127]. The bacterial culture of Streptomyces griseus ATCC 13273 catalyzed hydroxylation and siteselective oxidation of the C29 methyl group of OA (0.04 g/L) to the carboxyl group within 5 days to form 3β-hydroxy-olean-12-ene-28,29-dioic acid (94, 21.9%), 3β,24-dihydroxy-olean-12-ene-28,29dioic acid (95, 32.7%), and 3β,21β,24-trihydroxy-olean-12-ene-28,29-dioic acid (96, 5.9%). Hydroxylation at C21 was shown to increase the anti-inflammatory activity of OA derivatives [128]. Using the same strain, biotransformation of OA (approximately 0.05 g/L) with the formation of derivatives 94 and 96 was previously described by Y. Zhu et al. [129].
Bioconversion of GA (0.2 g/L) using R. chinensis CICC 40335 occurred by selective oxidation with the formation of 7β-hydroxy-GA (61, 77.5%) on day 4 [34]. Note that the C7-hydroxylation process is typical for many cultures, for example, C. muscae AS 3.2695, Rhizopus arrhizus AS 3.2893 [115], and representatives of the genus Cunninghamella [116,119]. Further addition of GA or compound 61 in the culture medium of B. subtilis ATCC 6633 led to the formation of 30-O-β-D-glucopyranoside derivatives (111 (27.5%) and 68 (44.0%), respectively) previously obtained by B. Fan et al. [115]. Assessment of the neuroprotective potential of the obtained OA and GA derivatives revealed that glycosylation significantly contributed to a decrease in the neuroprotective activity of compounds while carboxylation led to a significant increase in the neuroprotective effect of OA derivatives [34].

Conclusions
Triterpenoids are secondary metabolites of plants, fungi, marine invertebrates, and algae that are formed during cyclization of an acyclic triterpene squalene [132][133][134][135][136][137][138]. According to the number of cycles, triterpenes and triterpenoids are divided into several groups; the most numerous are pentacyclic triterpenic derivatives [139]. In nature, this group is most widely represented by compounds of oleanane (OA and GA) and ursane (UA) types, which in large quantities can accumulate in various parts of higher plants [39,41,46]. In addition, the biosynthesis of these compounds can be carried out in microbial cells able to catalyze the MVA pathway and to be genetically modified using plant genes [64,[68][69][70]. The main difficulties of microbial biosynthesis are generally considered to be complexity and long duration of processes of searching for terpenoid synthesis genes of plants, their isolation, and the preparation of genetically modified microorganisms. The rapid development of bioinformatics methods, sequencing techniques, and de novo DNA synthesis significantly simplified the abovementioned processes and gave a new impetus to research in this field [140]. With the close cooperation of biochemists, microbiologists, and genetic scientists, microbial biosynthesis can become a promising technology for obtaining valuable pentacyclic triterpenoids. The compounds discussed in the review exhibit antitumor, antiviral, hepatoprotective, neuroprotective, and other activities [5,7,42,60,74,82,83]. Despite the wide range of known biological properties, the use of pentacyclic triterpenoids in pharmacology and medicine is limited because of their high hydrophobicity. The solution to this problem might be the synthesis of triterpenic derivatives with increased bioactivity, solubility, and bioavailability [5,82,93,94,141].
Studies of the possibility of obtaining new OA, GA, and UA derivatives by directed biotransformations should be considered a promising area. Over 20 examples of biotransformations of these compounds using fungal and bacterial cultures most often catalyzing hydroxylation have been described since 2013. Less frequently, the literature describes processes of deeper oxidation of triterpenoids as well as their glycosylation, esterification, acetylation, or carboxylation. Biocatalytic formation of triterpenic lactones or their derivatives with fragmented C-C bond was reported only in a few cases using UA [13,121]. In the biotransformation processes employing fungi, the degree of triterpenic acid conversion usually ranges from 2.6% to 77.5%, with an initial concentration of 0.02 g/L to 1.0 g/L, whereas in bacterial transformations, the degree of conversion reaches 27.5-70.0%, with an initial concentration of 0.04-0.3 g/L. When analyzed, the data showed that the biotransformation of oleanane and ursane pentacyclic triterpenoids led to derivatives with antioxidant, anti-inflammatory, antiviral, antitumor, antiparasitic, antimicrobial, neuroprotective, and hepatoprotective properties ( Table 3). Provided more active development as an interdisciplinary tool, this method of obtaining biologically active compounds and their intermediates seems to be a promising strategy to design new medicinal agents against cancer and neurodegenerative diseases as well as potent antibacterial drugs against antibiotic-resistant pathogenic strains of microorganisms. By combining methods of microbial synthesis of native pentacyclic triterpenoids and their subsequent microbial transformations into bioavailable compounds, the industrial microbiology could provide a cycle of production of valuable biologically active substances. However, it should be noted that the described microbial catalysts have significant drawbacks. Fungi usually demonstrate mycelial growth type and form spores and mycotoxins, whereas few bacterial catalysts described are mainly represented by species, with their individual strains being pathogens. In this context, it is essential to conduct further in-depth studies of the processes of biological transformation of pentacyclic triterpenoids and to search for new nonpathogenic bacterial strains able to carry out highly effective synthesis of triterpenic derivatives with pronounced biological activities.

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