Cysteine imidazolium group

Studies of the variation of enzymic activity with pH have indicated that a-amino and imidazolium groups are important entities in the potato phosphorylase molecule, and this evidence has been further substantiated by the results of photo-oxidation and acetylation experiments. Rabbit-muscle phosphorylase a has been modified by photo-oxidation and alkylation, and the effect of both procedures was markedly decreased by the presence of D-glucosyl phosphate or glycogen this result indicates that the amino acid residues affected are at, or near, the active center. These acids may be histidine and cysteine, but other interpretations of the results are possible. 

The mechanism of hydrolysis of cysteine peptidases, in particular cysteine endopeptidases (EC 3.4.22), shows similarities and differences with that of serine peptidases [2] [3a] [55 - 59]. Cysteine peptidases also form a covalent, ac-ylated intermediate, but here the attacking nucleophile is the SH group of a cysteine residue, or, rather, the deprotonated thiolate group. Like in serine hydrolases, the imidazole ring of a histidine residue activates the nucleophile, but there is a major difference, since here proton abstraction does not appear to be concerted with nucleophilic substitution but with formation of the stable thiolate-imidazolium ion pair. Presumably as a result of this specific activation of the nucleophile, a H-bond acceptor group like Glu or Asp as found in serine hydrolases is seldom present to complete a catalytic triad. For this reason, cysteine endopeptidases are considered to possess a catalytic dyad (i.e., Cys-S plus H-His+). The active site also contains an oxyanion hole where the terminal NH2 group of a glutamine residue plays a major role. 

Probably the aspect of primary importance for the catalytic activity of cysteine proteases is the high nucleophilicity of the active-site thiol group. It is now generally accepted that the active form of papain and of cysteine proteases in general consists of a thiolate-imidazolium ion-pair, built from Cys25 and Hisl59. 

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