METABOLIC MODELING OF POLYHYDROXYBUTYRATE BIOSYNTHESIS

A mathematical model describing intracellular polyhydroxybutyrate (PHB ) synthesis in Alcaligenes eutrophus has been constructed. The model a llows investigation of issues such as the existence of rate-limiting e nzymatic steps, possible regulatory mechanisms in PHB synthesis, and t he effects different types of rate expressions have on model behavior. Simulations with the model indicate that activities of all PHB pathwa y enzymes influence overall PHB flux and that no single enzymatic step can easily be identified as rate limiting. Simulations also support r egulatory roles for both thiolase and reductase, mediated through AcCo A/CoASH and NADPH/NADP+ ratios, respectively. To make the model more r ealistic, complex rate expressions for enzyme-catalyzed reactions were used which reflect both the reversibility of the reactions and the re action mechanisms. Use of the complex kinetic expressions dramatically changed the behavior of the system compared to a simple model contain ing only Michaelis-Menten kinetic expressions; the more complicated mo del displayed different responses to changes in enzyme activities as w ell as inhibition of flux by the reaction products CoASH and NADP+. Th ese effects can be attributed to reversible rate expressions, which al low prediction of reaction rates under conditions both near and far fr om equilibrium. (C) 1998 John Wiley & Sons, Inc.