Protein differentiation: a comparison of aspartate transcarbamoylase and ornithine transcarbamoylase from Escherichia coli K-12.

The amino acid sequence of aspartate transcarbamoylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) has been compared with that of ornithine transcarbamoylase (carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3). The primary sequence homology is 25-40%, depending upon the alignment of homologous residues. The homologies are incorporated into discrete clusters and are interrupted by regions of length polymorphism. The most striking homologies correspond to regions putatively involved in the binding of the common substrate, carbamoyl phosphate. Chou-Fasman predictive analysis [Chou, P. Y. & Fasman, G. D. (1974) Biochemistry 13, 211-222; 222-245] indicates substantial conservation of secondary structural elements within the two enzymes, even in regions whose primary sequence is quite divergent. The results reported herein demonstrate that the two enzymes, aspartate transcarbamoylase and ornithine transcarbamoylase, share a common evolutionary origin and appear to have retained similar structural conformations throughout their evolutionary development.