BISTABILITY IN COUPLED OPEN SUBSTRATE CYCLES - NUMERICAL AND EXPERIMENTAL APPROACHES

The dynamic and steady-state behaviors of two open substrate cycles sh aring a common interconversion enzyme are investigated in a homogeneou s flow-through reactor. Lactate dehydrogenase (LDH) converts pyruvate and NADH into lactate and NAD, respectively. In turn, NAD (+ formate) is recycled into NADH (+ CO2) by formate dehydrogenase (FDH), and in t he presence of the oxidized form of 2-(hydroxymethyl)6-methoxy-1,4-ben zoquinone (Q), lactate is reoxidized into pyruvate (+Q(red)) by flavoc ytochrome b(2) (FCytb(2)). When operating under thermodynamically open conditions by a continuous supply of pyruvate, quinone, NADH, and for mate, this multienzyme system can exhibit multiple steady states under the form of dynamic hysteresis when using, among others, the pyruvate input concentration as the control parameter. This nonlinear behavior results from the strong inhibition of LDH exerted by its substrate py ruvate. The numerical predictions of a simple mathematical model, taki ng into account the coupling between the actual enzyme rate equations and mass transfers, agree both quantitatively and qualitatively with t he observed experiments.