Irreversible transitions in a model substrate cycle An experimental illustration

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Abstract
In a previous article [(1987) J. Theor. Biol, 127, 439-449], the dynamic behavior of a simple substrate cycle, bounded by moiety conservation, and in which one of the two antagonist enzymes is subjected to a destabilizing factor, was investigated. Depending upon the control parameter chosen, that is, the total interconverted substrate concentration and the ratio of the interconverting enzyme maximal activities, monostability, reversible (hysteresis) and/or irreversible transitions could be observed. In the present work, we report experiments dealing with the moiety ATP/ADP interconverted by enzymes phosphofructokinase (PFK) and pyruvate kinase (PK). The cycle operates under conditions where (1) PFK is inhibited by excess of its substrate, ATP, and (2) both enzymes are working under zero-order kinetics for their respective cosubstrates F6P and PEP. Under conditions where the PK maximal activity is lower than the PFK optimal activity, irreversible transitions from a high ATP (resp. low ADP) steady-state concentration to a lower (resp. higher) one, are observed when varying the total moiety (ATP + ADP) concentration. A graphical interpretation of the observed behavior is given. Plausible biochemical consequences of this phenomenon are also emphasized.

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