Imidazole groups chemical modifications

The presence of imidazole groups in the active site region of human carbonic anhydrase B has, in fact, been demonstrated by chemical modification. Thus, bromoacetate reacts specifically with the 3 -N of a histidine residue to give a partially active monocarboxymethyl enzyme (65). The reaction depends on the initial combination of the bromoacetate ion with the anion binding site (65,83). In a detailed study, Bradbury (83) has shown that the irreversible reaction at saturation with iodoacetate... 

Chymotrypm, probably the most thorou y studied enzyme because of its stability and availability, primarily catalyzes the hydrolysis of amide bonds of proteins and peptides adjacent to the carbonyl group of the aromatic L-amino acid residues of tryptophan, tyro e, and phenylalanine. Therefore, the hydrophobic interaction between the active site and substrate molecules is believed to make a mryor contribution to the stabflity of enzyme-substrate complexes. In fact, the X-ray data showed that the a-chymotryfsin molecule is an elipsoid (51x40x40 A) and that the active site is a hydrophobic cavity of 10—12 Ax5.5 —6.5 Ax3.5 —4.5 A (5). In chemical modification experiments, histidyl imidazole 2, and seryl hydroxyl 1 groups were found to be directly involved in the catal) ic process. The formation of acyl enzyme intermediates at the seryl residue was demonstrated by phyacal and chemical means. 

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