Controlling Active Site Loop Dynamics in the (β/α)8 Barrel Enzyme Indole-3-Glycerol Phosphate Synthase

The beta 1 alpha 1 loop in the tryptophan biosynthetic enzyme indole-3-glycerol phosphate synthase (IGPS) is important for substrate binding, product release and chemical catalysis. IGPS catalyzes the ring closure of the substrate 1-(0-carboxyphenylamine)-1-dexoyribulose 5-phosphate to form indole-3-glycerol phosphate, involving distinct decarboxylation and dehydration steps. The ring closure step is rate-determining in the thermophilic Sulfolobus sulfataricus enzyme (ssIGPS) at high temperatures. The beta 1 alpha 1 loop is especially important in the dehydration step as it houses the general acid Lys53. We propose that loop dynamics are governed by competing interactions on the N- and C-terminal sides of the loop. We had previously shown that disrupting interactions with the N-terminal side of the loop through the N90A substitution decreases catalytic efficiency, slows down the dehydration step and quenches loop dynamics on the picosecond to millisecond timescales. Here, we show that disrupting interactions on the C-terminal side of the loop through the R64A/D65A substitutions likewise decreases catalytic efficiency, slows down the dehydration step and quenches loop dynamics. Interestingly, the triple substitution R64A/D65A/N90A leads to new mu s-ms timescale loop dynamics and makes the ring-closure step rate-determining once again. These results are consistent with a model in which the beta 1 alpha 1 loop is maintained in a structurally dynamic state by these competing interactions, which is important for the dehydration step of catalysis. Competing interactions in other enzymes may likewise keep their loops and other structural elements appropriately mobile.