J. Biol. Chem., Vol. 255, Issue 15, 7291-7295, Aug, 1980
The essential sulfhydryl group of ornithine transcarbamylases. Reaction with anionic, aromatic disulfides and properties of its cyano derivative
M Marshall and PP Cohen
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.
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