Imidazolium groups

Figure 14-11 Schematic diagram of the active site of the pyruvoyl enzyme histidine decarboxylase showing key polar interactions between the pyruvoyl group and groups of the inhibitor O-methylhistidine and surrounding enzyme groups. Aspartate 63 appears to form an ion pair with the imidazolium group of the substrate.268 Hydrogen bonds are indicated by dotted lines. See Gallagher et al.269...

Imidazolium groups of histidine are essential for the transfer of a hydrogen ion to the displaced fructosyl moiety [12], The reprotonation of the imidazole takes place by abstracting a proton from the C6 hydroxyl moiety and facilitating the formation of the glycosidic linkage. 

Note We have found that it is of no particular importance whether the phosphino group or the imidazolium group is introduced last into the phosphino functionalised imidazolium framework, but it pays to generate the carbene after the phosphino groups are in place. 

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. 

Figure 4 The enzyme ribonuclease A cleaves RNA using the imidazole group of histidine 12 as a base and the imidazolium group of histidine 119 as an acid in a simultaneous two-proton transfer process. Studies discussed in this article indicate that the process is not as simple as shown.

The above considerations apply, also, to the calf-intestine enzyme, where the carboxylate ion (—COO ) will act as a nucleophilic center and the imidazolium group as an electrophile. According to Scheme 5, the intermediate enzyme-/S-D-galactoside compound will possess the nature of an ester. Evidently, Scheme 6 can apply equally well. It is not known, as yet, whether the large carbohydrate content of the most highly purified, calf-intestine /3-galactosidase is a part of the enzyme molecule or not. Nothing... 

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