Picornaviruses are small pathogen RNA viruses, like poliovirus, hepatitis A virus, rhinovirus, and others. They produce a large polyprotein, which is cleaved by virally encoded cysteine peptidases, picornains 2A and 3C. Picornain 3C represents an intermediate between the serine peptidase chymotrypsin and the cysteine peptidase papain. Its steric structure resembles chymotrypsin, but its nucleophile is a thiol instead of the hydroxyl group. The histidine is a general base catalyst in chymotrypsin but forms a thiolate-imidazolium ion pair in papain. The third member of the catalytic triad is an acid (Glu71) as in chymotrypsin rather than an amide found in papain. Transformation of poliovirus 3C peptidase into a serine peptidase results in lower activity by a factor of 430, but the activity extends toward higher pH with the more basic hydroxyl group. The decrease in activity is caused by the less ordered active site, as supported by the unfavorable entropy of activation. At 25 °C the specificity rate constant for the thiol enzyme approaches k1, the rate constant for the formation of the enzyme - substrate complex, but k2, the acylation constant, becomes predominant with the increase in temperature. In contrast, for the serine peptidase the specificity constant is less than k1 over the entire temperature range, and the transition state is controlled by both k1 and k2. The acidic component of the catalytic triad is essential for activity, but its negative charge does not influence the ionization of the thiol group.
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