Enzymes from thermophilic organisms often are barely active at low temperatures. To obtain a better understanding of this sluggishness, we used DNA shuffling to mutagenize the trpC gene, which encodes indoleglycerol phosphate synthase, from the hyperthermophile Sulfolobus solfataricus. Mutants producing more active protein variants were selected by genetic complementation of an Escherichia coli mutant bearing a trpC deletion. Single amino acid changes and combinations of these changes improved growth appreciably. Five singly and doubly altered protein variants with changes at the N- and C-termini, or at the phosphate binding site, were purified and characterized with regard to their kinetics of enzymatic catalysis, product binding, cleavage by trypsin, and inactivation by heat. Turnover numbers of the purified variant proteins correlated with the corresponding growth rates, showing that the turnover number was the selected trait. Although the affinities for both the substrate and the product decreased appreciably in most protein variants, these defects were offset by the accumulation of high levels of the enzyme's substrate. Rapid mixing of the product indoleglycerol phosphate with the parental enzyme revealed that the enzyme's turnover number at low temperatures is limited by the dissociation of the enzyme-product complex. In contrast, representative protein variants bind and release the product far more rapidly, shifting the bottleneck to the preceding chemical step. The turnover number of the parental enzyme increases with temperature, suggesting that its structural rigidity is responsible for its poor catalytic activity at low temperatures. In support of this interpretation, the rate of trypsinolysis or of thermal denaturation is accelerated significantly in the activated protein variants.

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