Abstract
Millisecond motions in the cyclophilin active site correlate with the overall enzymatic turnover, but the structural nature of the conformational change has not yet been defined. Stringent analysis of ultra-high resolution crystal structures of the free enzyme, together with NMR chemical shift information, suggested that a single side-chain may be responsible for the observed dynamics. A mutation designed to restrict this motion indeed severely reduces the rate of the active-site motions and, strikingly, catalysis by the same amount as measured by NMR relaxation dispersion experiments. The reduction in catalytic power by restricting active site motions is on the same order as mutating the active site Arg responsible for the chemical step. These results illustrate on an atomic level how both dynamics and chemistry contribute to catalytic efficiency.