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Open AccessArticle

From Cycling Between Coupled Reactions to the Cross-Bridge Cycle: Mechanical Power Output as an Integral Part of Energy Metabolism

Marschweg 10, D-29690 Schwarmstedt, Germany
Metabolites 2012, 2(4), 667-700; https://doi.org/10.3390/metabo2040667
Received: 20 July 2012 / Revised: 7 September 2012 / Accepted: 24 September 2012 / Published: 8 October 2012
(This article belongs to the Special Issue Metabolic Network Models)
ATP delivery and its usage are achieved by cycling of respective intermediates through interconnected coupled reactions. At steady state, cycling between coupled reactions always occurs at zero resistance of the whole cycle without dissipation of free energy. The cross-bridge cycle can also be described by a system of coupled reactions: one energising reaction, which energises myosin heads by coupled ATP splitting, and one de-energising reaction, which transduces free energy from myosin heads to coupled actin movement. The whole cycle of myosin heads via cross-bridge formation and dissociation proceeds at zero resistance. Dissipation of free energy from coupled reactions occurs whenever the input potential overcomes the counteracting output potential. In addition, dissipation is produced by uncoupling. This is brought about by a load dependent shortening of the cross-bridge stroke to zero, which allows isometric force generation without mechanical power output. The occurrence of maximal efficiency is caused by uncoupling. Under coupled conditions, Hill’s equation (velocity as a function of load) is fulfilled. In addition, force and shortening velocity both depend on [Ca2+]. Muscular fatigue is triggered when ATP consumption overcomes ATP delivery. As a result, the substrate of the cycle, [MgATP2−], is reduced. This leads to a switch off of cycling and ATP consumption, so that a recovery of [ATP] is possible. In this way a potentially harmful, persistent low energy state of the cell can be avoided. View Full-Text
Keywords: energetic coupling; zero resistance; cross-bridge cycle; muscular efficiency; muscular fatigue energetic coupling; zero resistance; cross-bridge cycle; muscular efficiency; muscular fatigue
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Diederichs, F. From Cycling Between Coupled Reactions to the Cross-Bridge Cycle: Mechanical Power Output as an Integral Part of Energy Metabolism. Metabolites 2012, 2, 667-700.

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