The essential sulfhydryl group of ornithine transcarbamylases. pH dependence of the spectra of its 2-mercuri-4-nitrophenol derivative.

The essential sulfhydryl group of the ornithine transcarbamylases (ornithine carbamoyltransferase, 2.1.3.3) from bovine liver and Streptococcus faecalis reacts preferentially with 2-chloromercuri-4-nitrophenol. The spectra of this derivative between pH 4.4 AND 8.8 HAVE BEEN RESOLVED INto the spectrum of the nitrophenolate ion (III) and two species of phenol (I and II). The lambda max of I and II (both enzymes) and III (bovine) are red shifted from those of the comparable species in the same derivative of 2-mercaptoethanol. Deprotonation of a residue on the enzyme must be responsible for the transition from I to II. The pK values of the phenolic group are 7.1 (mercaptoethanol), 7.7 (bovine), and 8.8 (S. faecalis). The red shift in the lambda max of III and the modest increase in the pK of the phenolic group are consistent with a relatively hydrophobic environment for the nitrophenolate ion in the bovine enzyme. Since deprotonation of the residue in the bovine enzyme perturbs the pK of the phenolic group only slightly, its effect may be indirect. Interaction with a neighboring carboxyl group (pK 5.3) would account for the large increase in the pK of the phenolic group in the S. faecalis enzyme, which is not accompanied by an appreciable shift in the lambda max. Carbamyl-P increases the pK of the phenolic group in both enzymes, a result consistent with its binding site being close to the essential sulfhydryl group.