Channeling Behavior and Activity Models for Escherichia coli K-12 Acetohydroxy Acid Synthases at Physiological Substrate Levels

The channeling behavior of acetohydroxy acid synthases I and III (EC 4.1.3.18; AHAS) was studied by computer simulation of activities over a wide range of concentrations for the substrates pyruvate and 2-ketobutyrate. The ratios of reaction rates for both channels and three-dimensional plots of single-channel reaction rates versus substrate concentrations were introduced to compare the substrate channeling properties of the isozymes. Substrate ranges were identified in which AHAS I and III operated both channels, and in which they used only one. Kinetic constants were varied to simulate whether and how AHAS might be made channel-specific. Our study suggests that AHAS might be made channel-specific for acetolactate but not for acetohydroxybutyrate. We postulate specific physiological roles for AHAS I and III to support cell growth under conditions that vary the levels and balance of substrates.