As a continuation to our studies on the importance of interloop interactions in the Escherichia coli DHFR catalytic cycle, we have investigated the role of the βG-βH loop in modulating the closed and occluded conformations of the Met20 loop during the DHFR catalytic cycle. Specifically, to assess the importance of the hydrogen bond formed between Ser148 in the βG-βH loop and the Met20 loop, Ser148 was independently substituted with aspartic acid, alanine, and lysine. Moreover, the βG-βH loop was deleted entirely to yield the Δ(146-148) DHFR mutant. Steady-state turnover rates for all mutants were at most 3-fold lower than the wild-type rate. Lack of an isotope effect on this rate indicated the chemistry step does not contribute to the steady-state turnover. Consistent with this finding, hydride transfer rates for the DHFR mutants were at least 10-fold greater than the observed steady-state rates. The values ranged from a 30% decrease (Ser148Ala and Ser148Lys) to a 50% increase (Ser148Asp) in rate relative to that of the wild type. Modifications of the βG-βH loop enhanced the affinity for the cofactor and decreased the affinity for pterin, as determined by the KD values of the mutant proteins. Further analysis of Ser148Ala and Δ(146-148) DHFRs indicated these effects were manifest mainly in ligand off rates, although in some cases the on rate was affected. The Ser148Asp and Δ(146-148) mutations perturbed the preferred catalytic cycle through the introduction of branching at key intermediates. Rather than following the single WT pathway which involves loss of NADP+ and rebinding of NADPH to precede loss of the product H4F (negative cooperativity), the mutants can reenter the catalytic cycle through different pathways. These findings suggest that the role of the interloop interaction between the βG-βH loop and the Met20 loop is to modulate ligand off rates allowing for proper cycling through the preferred kinetic pathway.
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