The direct energy source for muscle contraction is ATP. The contractile protein, actomyosin, hydrolyzes ATP to ADP + Pi, the used ATP is quickly restored via the Lohmann reaction during contraction (Fig. EN5a). The depleted store of PCr is restored from ATP when the muscle is returned to the resting state.
ATP is synthesized via glycolysis in the sarcoplasm and/or via oxidative phosphorylation in the mitochondria. Glycolysis may occur under anaerobic condition, the yield is 2 or 3 ATP/glucose. The lactate produced from glycogen is oxidized to yield 36 ATP/glucose. Also, dismutation of 2 ADP catalyzed by adenylate kinase yields 1 ATP + 1 AMP.
It is important to realize that the ATP concentration in muscle is quite low (5-8 ^.moles/g muscle), enough only for a few contractions. The used ATP is immediately resynthesized from PCr. However, the PCr concentration is also low (20-25 ^.moles/g), enough for some additional contractions. Glycogen offers a rapid but still limited energy store (endogenous glycogen concentration about 75 ^moles glucose units in glycogen/g muscle), whereas oxidative phosphorylation is the slowest and most efficient process for ATP production. For example, the glycogen store offers energy for a runner for about 1/2 hr and oxidative phosphorylation for an additional 2 hrs (Paul et al., 1993).
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