A Twin-track Approach Has Optimized Proton and Hydride Transfer by Dynamically Coupled Tunneling during the Evolution of Protochlorophyllide Oxidoreductase

Protein dynamics are crucial for realizing the catalytic power of enzymes, but how enzymes have evolved to achieve catalysis is unknown. The light-activated enzyme protochlorophyllide oxidoreductase (POR) catalyzes sequential hydride and proton transfers in the photoexcited and ground states, respectively, and is an excellent system for relating the effects of motions to catalysis. Here, we have used the temperature dependence of isotope effects and solvent viscosity measurements to analyze the dynamics coupled to the hydride and proton transfer steps in three cyanobacterial PORs and a related plant enzyme. We have related the dynamic profiles of each enzyme to their evolutionary origin. Motions coupled to light-driven hydride transfer are conserved across all POR enzymes, but those linked to thermally activated proton transfer are variable. Cyanobacterial PORs require complex and solvent-coupled dynamic networks to optimize the proton donor-acceptor distance, but evolutionary pressures appear to have minimized such networks in plant PORs. POR from Gloeobacter violaceus has features of both the cyanobacterial and plant enzymes, suggesting that the dynamic properties have been optimized during the evolution of POR. We infer that the differing trajectories in optimizing a catalytic structure are related to the stringency of the chemistry catalyzed and define a functional adaptation in which active site chemistry is protected from the dynamic effects of distal mutations that might otherwise impact negatively on enzyme catalysis.