1. The pH-dependence of the second-order rate constant (k) for the reaction of actinidin (EC 220.127.116.11) with 2-(N'-acetyl-L-phenylalanylamino)ethyl 2'-pyridyl disulphide was determined and the contributions to k of various hydronic states were evaluated. 2. The data were used to assess the consequences for transition-state geometry of providing P2/S2 hydrophobic contacts in addition to hydrogen-bonding opportunities in the S1-S2 intersubsite region. 3. The P2/S2 contacts (a) substantially improve enzyme-ligand binding, (b) greatly enhance the contribution to reactivity of the hydronic state bounded by pKa 3 (the pKa characteristic of the formation of catalytic-site-S-/-ImH+ state) and pKa 5 (a relatively minor contributor in reactions that lack the P2/S2 contacts), such that the major rate optimum occurs at pH 4 instead of at pH 2.8-2.9, and (c) reveal the kinetic influence of a pKa approx. 6.3 not hitherto observed in reactions of actinidin. 4. Possibilities for the interplay of electrostatic effects and binding interactions in both actinidin and papain (EC 18.104.22.168) are discussed.
The interplay of electrostatic fields and binding interactions determining catalytic-site reactivity in actinidin. A possible origin of differences in the behaviour of actinidin and papain
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D Kowlessur, M O'Driscoll, C M Topham, W Templeton, E W Thomas, K Brocklehurst; The interplay of electrostatic fields and binding interactions determining catalytic-site reactivity in actinidin. A possible origin of differences in the behaviour of actinidin and papain. Biochem J 15 April 1989; 259 (2): 443–452. doi: https://doi.org/10.1042/bj2590443
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