Small weak acids reactivate proton transfer in reaction centers from Rhodobacter sphaeroides mutated at AspL210 and AspM17.

In reaction centers of Rhodobacter sphaeroides, site-directed mutagenesis has implicated several acidic residues in the delivery of protons to the secondary quinone (Q(B)) during reduction to quinol. In a double mutant (Asp(L210) --> Asn + Asp(M17) --> Asn) that is severely impaired in proton transfer capability over a wide pH range, proton transfer was "rescued" by added weak acids. For low pK(a) acids the total concentration of salt required near neutral pH was high. The ionic strength effect of added salts stimulated the rate of proton-coupled electron transfer at pH < 7, but decreased it at pH > 7.5, indicating an effective isoelectric point between these limits. In this region, a substantial rate enhancement by weak acids was clearly evident. A Brønsted plot of activity versus pK(a) of the rescuing acids was linear, with a slope of -1, and extrapolated to a diffusion-limited rate at pK(a)(app) approximately 1. However, the maximum rate at saturating concentrations of acid did not correlate with pK(a), indicating that the acid and anion species compete for binding, both with weak affinity. This model predicts that pK(a)(app) corresponds to a true pK(a) = 4-5, similar to that for a carboxylic acid or Q(B)(-), itself. Only rather small, neutral acids were active, indicating a need to access a small internal volume, suggested to be a proton channel to the Q(B) domain. However, the on-rates were near the diffusion limit. The implications for intraprotein proton transfer pathway design are discussed.