ROLE OF ALLOSTERIC-ZINC INTERDOMAIN REGION OF THE REGULATORY SUBUNIT IN THE ALLOSTERIC REGULATION OF ASPARTATE TRANSCARBAMOYLASE FROM ESCHERICHIA-COLI

The hydrophobic interface between the allosteric and the zinc domains of the regulatory subunit of aspartate transcarbamoylase has previousl y been implicated in the heterotropic ATP activation of the enzyme. Th e present work shows that this :interface also affects CTP and CTP-UTP inhibition and proposes a structural explanation for the effects. Mut ant enzymes derived from nonselective mutagenesis of residues r101-r10 6 (residues that contribute part of the interface) displayed a variety of homotropic and heterotropic effects. The cooperative behavior of t he enzymes was affected, as indicated by reduced aspar tate S-0.5 valu es and apparent Hill coefficient values for V106L, V106L/N105S, and I1 03F/R102C, In addition, both ATP activation and CTP inhibition were si gnificantly reduced and CTP+UTP synergistic inhibition was decreased i n these mutants. The D104G mutant enzyme was subject to inhibition by CTP and CTP+UTP, but was not activated by ATP. Finally, the I103T muta nt enzyme had an increased S-0.5 value of 11.5 mM and displayed altere d effector responses: ATP acted as an inhibitor, and the CTP+UTP syner gistic inhibition was reduced. Most of these allosteric variations can be explained in terms of perturbations to the ''tongue and groove'' h ydrophobic interface between the allosteric and the zinc domains and a consequent impact on a second interface (''regl:cat4'') between regul atory and catalytic subunits, (C) 1998 Academic Press.