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Enzyme mechanisms of facultative anaerobiosis in molluscs: regulation of the phosphoenolpyruvate crossroads in the oyster Mustafa, Tariq


Catalytic and regulatory properties of pyruvate kinase (EC and phosphoenolpyruvate carboxykinase (EC from oyster adductor muscle were studied. Particular attention was given to those properties of the enzymes which could help to explain the "switch over" from aerobic to anaerobic glucose degradation between aero⇄ anaerobic conditions in a facultative anaerobe, such as the oyster. All the available data can be summarized as follows: Situated strategically at the primary branching point between aerobic and anaerobic metabolism are the two enzymes, pyruvate kinase (favoured during aerobiosis) and P-enolpyruvate carboxykinase (favoured during anaerobiosis). H⁺ ion plays a pivotal role in the channelling of P-enolpyruvate through this branching point. When O₂ is absent, the pH is known to drop because of the build-up of various acid products. In the absence of any other factors, this would lead to an effective activation of p-enolpyruvate carboxykinase due to (1) an increase in absolute activities, and (2) an increase in the affinity for p-enolpyruvate. At the same time pyruvate kinase would be inhibited by (1) a decrease in the absolute activity, and (2) a decrease in the affinity for p-enolpyruvate. Alanine and ATP inhibition of pyruvate kinase potentiate these effects particularly at low pH. In contrast, alanine effectively activates p-enolpyruvate carboxykinase by reversing ITP inhibition. The more acidic the anaerobic system becomes, the more avidly would p-enolpyruvate carboxykinase channel p-enolpyruvate towards oxaloacetate. Upon return to aerobic conditions, the pH would be expected to rise again and all of the above events would be reversed. The most important feature of such a regulatory system is that it is a kind of autocatalytic cascade. Once either pyruvate kinase activation orp-enolpyruvate carboxykinase activation is initiated, all the various regulatory interactions potentiate one another. There is little doubt that the specific control components at this point in the metabolism of molluscan facultative anaerobes are the outcome of selective tailoring of the 2 enzymes functioning at this point. What evolution seems to have done in molluscs was to arrange the control characteristics of pyruvate kinase and p-enolpyruvate carboxykinase in a reciprocal manner so that the two reactions cannot be fully active simultaneously.

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