How can a catalytic lesion be offset? The energetics of two pseudorevertant triosephosphate isomerases

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Abstract
The reaction energetics of four triosephosphate isomerase mutants are compared with those of the wild-type enzyme. The two primary mutants, E165D and H95N, contain site-specific alterations of active site residues. In one case the active site base has been altered (E165D), and in the other, an active site electrophile has been removed (H95N), yet the major effect in each case is the relative destabilization of the transition states for the two chemical (enolization) steps that constitute the catalytic reaction. When the genes encoding each of these sluggish mutant isomerases were subjected to random mutagenesis using chemical reagents and a selection for isomerases of increased catalytic potency was performed, pseudorevertant enzymes with dramatic increases in activity were found. Remarkably, the same second-site suppressor locus partially corrects each lesion. The E165D,S96P pseudorevertant is a 20-fold better catalyst than the E165D mutant from which it is derived, and the H95N,S96P pseudorevertant is about 60 times more active than its H95N parent. The S96P substitution thus increases the catalytic activity in each of two different contexts, H95N and E165D. The energetic consequences of the S96P change are surprisingly similar in each pseudorevertant. The H95N,S96P enzyme is more effective than H95N at stabilizing the intermediate enediol(ate) phosphate and its flanking transition states. The E165D,S96P enzyme likewise stabilizes the transition states for enolization better than E165D, and this pseudorevertant also forms a tighter enzyme-dihydroxyacetone phosphate complex than its parent. These data show how, in these two cases, the catalytic potency of sluggish mutant enzymes can be improved by second-site changes. The results thus provide the beginnings of a detailed understanding of the kinetic refinement of enzyme catalysts.