Solvent-slaved protein motions accompany proton but not hydride tunneling in light-activated protochlorophyllide oxidoreductase

H(+) but not H(-): The reduction reaction of protochlorophyllide catalyzed by protochlorophyllide oxidoreductase features solvent-slaved motions that control the proton- but not the hydride-tunneling mechanism. These motions imply a long-range dynamic network from the solvent to the enzyme active site that facilitate proton transfer (see picture, left). Motions for hydride transfer are more localized and are not slaved by the solvent (see picture, right).

Barrier compression enhances an enzymatic hydrogen-transfer reaction

Putting the squeeze on: Hydrostatic pressure causes a shortening of the charge-transfer bond in the binary complex of morphinone reductase and NADH(4) (see diagram). Molecular dynamics simulations suggest that pressure reduces the average reaction barrier width by restricting the conformational space available to the flavin mononucleotide and NADH within the active site. The apparent rate of catalysis increases with pressure.

Solvent effects on environmentally coupled hydrogen tunnelling during catalysis by dihydrofolate reductase from Thermotoga maritima

Protein motions may be perturbed by altering the properties of the reaction medium. Here we show that dielectric constant, but not viscosity, affects the rate of the hydride-transfer reaction catalysed by dihydrofolate reductase from Thermotoga maritima (TmDHFR), in which quantum-mechanical tunnelling has previously been shown to be driven by protein motions. Neither dielectric constant nor viscosity directly alters the kinetic isotope effect of the reaction or the mechanism of coupling of protein motions to tunnelling. Glycerol and sucrose cause a significant increase in the rate of hydride transfer, but lead to a reduction in the magnitude of the kinetic isotope effect as well as an extension of the temperature range over which "passive" protein dynamics (rather than "active" gating motions) dominate the reaction. Our results are in agreement with the proposal that non-equilibrium dynamical processes (promoting motions) drive the hydride-transfer reaction in TmDHFR.

Solvent as a probe of active site motion and chemistry during the hydrogen tunnelling reaction in morphinone reductase

The reductive half-reaction of morphinone reductase involves a hydride transfer from enzyme-bound beta-nicotinamide adenine dinucleotide (NADH) to a flavin mononucleotide (FMN). We have previously demonstrated that this step proceeds via a quantum mechanical tunnelling mechanism. Herein, we probe the effect of the solvent on the active site chemistry. The pK(a) of the reduced FMN N1 is 7.4+/-0.7, based on the pH-dependence of the FMN midpoint potential. We rule out that protonation of the reduced FMN N1 is coupled to the preceding H-transfer as both the rate and temperature-dependence of the reaction are insensitive to changes in solution pH above and below this pK(a). Further, the solvent kinetic isotope effect is approximately 1.0 and both the 1 degrees and 2 degrees KIEs are insensitive to solution pH. The effect of the solvent's dielectric constant is investigated and the rate of H-transfer is found to be unaffected by changes in the dielectric constant between approximately 60 and 80. We suggest that, while there is crystallographic evidence for some water in the active site, the putative promoting motion involved in the H-tunnelling reaction is insensitive to such changes.