Serine 948 and threonine 1042 are crucial residues for allosteric regulation of Escherichia coli carbamoylphosphate synthetase and illustrate coupling effects of activation and inhibition pathways

Escherichia coli carbamoylphosphate synthetase (CPSase) is a key enzyme in the pyrimidine nucleotides and arginine biosynthetic pathways. The enzyme harbors a complex regulation, being activated by ornithine and inosine 5'-monophosphate (IMP), and inhibited by UMP. CPSase mutants obtained by in vivo mutagenesis and selected on the basis of particular phenotypes have been characterized kinetically. Two residues, serine 948 and threonine 1042, appear crucial for allosteric regulation of CPSase. When threonine 1042 is replaced by an isoleucine residue, the enzyme displays a greatly reduced activation by ornithine. The T1042I mutated enzyme is still sensitive to UMP and IMP, although the effects of both regulators are reduced. When serine 948 is replaced by phenylalanine, the enzyme becomes insensitive to UMP and IMP, but is still activated by ornithine, although to a reduced extent. When correlating these observations to the structural data recently reported, it becomes clear that both mutations, which are located in spatially distinct regions corresponding respectively to the ornithine and the UMP/IMP binding sites, have coupled effects on the enzyme regulation. These results provide an illustration that coupling of regulatory pathways occurs within the allosteric subunit of E. coli CPSase. In addition, other mutants have been characterized, which display altered affinities for the different CPSase substrates and also slightly modified properties towards the allosteric effecters: P165S, P170L, A182V, P360L, S743N, T800F and G824D. Kinetic properties of these modified enzymes are also presented here and correlated to the crystal structure of E. coli CPSase and to the phenotype of the mutants. (C) 1999 Academic Press.