Properties of the histidine residues of indole-chymotrypsin. Implications for the activation process and catalytic mechanism.
The use of a linear free-energy relationship shows that both histidine residues of alpha-chymotrypsin and chymotrypsinogen are super-reactive toward 1-fluoro-2,4-dinitrobenzene. The binding of indole to the specificity site of alpha-chymotrypsin causes both histidine residues to become less reactive. On the basis of these results and those from X-ray-crystallographic studies, the following conclusions are made. (1) The super-reactivity of the catalytic-site histidine-57 is due to charge transfer from aspartic acid-102 by means of hydrogen bonding. (2) The aspartic acid-102-histidine-57-serine-195 'charge-relay' system is not complete in the zymogen or native enzyme and only on binding of a suitable substrate or ligand to the specificity site of the enzyme is the charge transfer to serine-195 completed. (3) The lack of substantial enzymic activity in the zymogen is due to the absence of a completed specificity site, and therefore it cannot bind suitable substrates or ligands to induce completion of the charge-relay system.
D001665:Binding Sites; D001667:Binding, Competitive; D002384:Catalysis; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002918:Chymotrypsin / Q000378:metabolism; D004789:Enzyme Activation; D006639:Histidine; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles; D007211:Indoles; D008024:Ligands; D011485:Protein Binding
D007093:Imidazoles; D007211:Indoles; D008024:Ligands; D006639:Histidine; D002918:Chymotrypsin