Bistability in Coupled Open Substrate Cycles: Numerical and Experimental Approaches

Abstract

The dynamic and steady-state behaviors of two open substrate cycles sharing a common interconversion enzyme are investigated in a homogeneous 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 the presence of the oxidized form of 2-(hydroxymethyl)-6-methoxy-1,4-benzoquinone (Q), lactate is reoxidized into pyruvate (+ Q_red) by flavocytochrome b₂ (FCytb₂). When operating under thermodynamically open conditions by a continuous supply of pyruvate, quinone, NADH, and formate, 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 pyruvate. The numerical predictions of a simple mathematical model, taking into account the coupling between the actual enzyme rate equations and mass transfers, agree both quantitatively and qualitatively with the observed experiments.