FACTORS LIMITING ADENOSINE-TRIPHOSPHATASE FUNCTION DURING HIGH-INTENSITY EXERCISE - THERMODYNAMIC AND REGULATORY CONSIDERATIONS

It is widely accepted that a structural organisation favouring interaction between functionally-related enzymes is required for the economy and efficiency of metabolic reactions. Many functionary-related enzymes have been shown to be reversibly bound to cellular structures and to other enzymes at the sites where they are required. Resulting from this binding, close structural proximity and concentration of enzymes, a microenvironment is generated where the product of one enzyme is the substrate of the other. This reduces the diffusion distance for the substrate, saturates binding sites with maximal speed and, as a final outcome, increases the efficiency and economy of function behind these metabolic reactions. Available data indicate that the above-described association between adenosine triphosphatase (ATPase) and enzymes regenerating ATP has an important role in the regulation of ATPase function. A general consensus exists among published studies that the concentration of ATP ([ATP]) is not significantly decreased in fatigued muscle, even in those with severely diminished power output. However, in studies with isolated perfused hearts it has been possible to significantly reduce [ATP] in muscle cells without compromising mechanical activity. An explanation for this discrepancy is connected with local ATP regeneration in the vicinity of ATPase. Furthermore, when ATP regeneration is unable to balance ATP consumption a critical drop in the free energy of ATP hydrolysis is avoided by down-regulation of ATP consumption. The main function of local ATP regeneration is to maintain a low concentration of adenosine diphosphate ([ADP]), and the ADP/ATP ratio in the vicinity of the ATP-binding site of ATPase that is a prerequisite for high thermodynamic efficiency of ATP hydrolysis. Close proximity of creatine kinase and glycolytic enzymes to ATPase and high-affinity binding of substrates generate an ATPase microenvironment, where ADP and ATP are not in free equilibrium with those adenine nucleotides in the surrounding medium. In the physiological range of operation for important cellular ATPases (free energy change of 55 to 60 KJ/mol ATP) only a small fraction of energy, available in ATP, can be utilised, provided that no ATP regeneration takes place. However, ATP regeneration allows utilisation of most of the regenerating capacity, before ATP hydrolysis drops below the critical 55 kJ/mol. The importance of local ATP regeneration increases in parallel with an increase in the rate of ATPase turnover. Furthermore, the time during which cells can maintain high rates of ATP hydrolysis depends on the capacity of mechanisms for local ATP regeneration, provided that ATPase is not inhibited by factors other than the reaction products. When this capacity is exhausted and the ADP/ATP ratio starts to increase, a further decrease in the free energy of ATP hydrolysis is avoided by down-regulation of ATP consumption. Because cellular ATPases, and especially sarcoplasmic reticulum Ca++-ATPase, require substantial amounts of the free energy available from ATP hydrolysis, this down-regulation of ATP hydrolysis is directed to maintain that level of structural organisation (primarily ionic gradients) which is essential for living cells. Consequently, the biological significance of this down-regulation is to prevent an increase in entropy and irreversible structural changes.