Mechanism of Rate-Limiting Motions in Enzyme Function

The ability to use conformational flexibility is a hallmark of enzyme function. Here we show that protein motions and catalytic activity in a RNase are coupled and display identical solvent isotope effects. Solution NMR relaxation experiments identify a cluster of residues, some distant from the active site, that are integral to this motion. These studies implicate a single residue, histidine-48, as the key modulator in coupling protein motion with enzyme function. Mutation of H48 to alanine results in loss of protein motion in the isotope-sensitive region of the enzyme. In addition, k cat decreases for this mutant and the kinetic solvent isotope effect on k cat, which was 2.0 in WT, is near unity in H48A. Despite being located 18 Å from the enzyme active site, H48 is essential in coordinating the motions involved in the rate-limiting enzymatic step. These studies have identified, of ≈160 potential exchangeable protons, a single site that is integral in the rate-limiting step in RNase A enzyme function