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-ketobu tyrate. The ratios of reaction rates for both channels and three-dimen sional plots of single-channel reaction rates versus substrate concent rations 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 c hannel-specific. Our study suggests that AHAS might be made channel-sp ecific for acetolactate but not for acetohydroxybutyrate. We postulate specific physiological roles for AHAS I and III to support cell growt h under conditions that vary the levels and balance of substrates. (C) 1995 Academic Press, Inc.