The essential sulfhydryl group of ornithine transcarbamylases. Reaction with anionic, aromatic disulfides and properties of its cyano derivative.

The essential sulfhydryl group of the ornithine transcarbamylases (ornithine carbamoyltrasferase, 2.1.3.3) from bovine liver and Streptococcus faecalis reacts slowly with aromatic disulfides at alkaline pH. But at pH 4.5, the apparent second-order rate constant for the reaction of this group in the S. faecalis enzyme with 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2) is 40-fold that for the reaction of 2-mercaptoethanol. This enhanced reactivity at acid pH, because it occurs only with anionic, aromatic disulfides and results in rates greater than those for low molecular weight thiols, must be due to a specific interaction with these reagents. The Nbs derivatives of both enzymes are inactive; the cyano derivatives prepared from them by cyanolysis are active but with greatly increased Kmorn. The slow rates of cyanolysis and thiolysis suggest that access to the Nbs residue is limited. Also, the red shift of the spectrum of the Nbs residue in the enzymes from that of Nbs2 implies that its microenvironment is different from that of the bulk medium. Deprotonation of a residue in the S. faecalis enzyme causes a further red shift. Since even the small, uncharged cyano group interferes with the binding of ornithine to both enzymes, the essential sulfhydryl group may actually be a part of the binding site for ornithine.