Sulfhydryl groups, metal binding active site

Inhibition by a variety of metal-binding agents competitive with respect to phosphoryl substrates (118-120) has suggested that an enzyme-bound divalent cation (other than Mg2+) may participate also in the binding of phosphate substrates. Observed inhibition by p-chloro-mercuriphenyl sulfonate and iodoacetate suggests the possibility that sulfhydryl groups may also be involved at, or near, the active enzymic site (119, 120). 

In noncompetitive inhibition, the inhibitor is presumed not to bind to an active site on the enzyme, but rather to bind at some other site. This complex formation may involve some change in the conformation of the enzyme, which makes it impossible for the substrate to bind at the active site. The inhibition of urease by Ag+, Pb2+, or Hg2+ is believed to be the result of these metal ions binding to the sulfhydryl (—SH) groups on the enzyme. For this type of action, we can write the equilibria... 

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