The phosphoenolpyruvate-dependent fructose-specific phosphotransferase system in Rhodopseudomonas sphaeroides. Evidence for a shift in the midpoint potential of the dithiol redox center during turnover of the carrier.

Redox titrations of the fructose-specific carrier protein, EFruII, in Rhodopseudomonas sphaeroides show that only the reduced form of the enzyme is active. The oxidized form of the enzyme can still be phosphorylated but is unable to transfer the phosphoryl group to fructose. The redox properties of the enzyme change upon phosphorylation. The reduction rate of EFruII is slower than that of EFruII-P, whereas the opposite is true for the oxidation rate. Consequently the midpoint potential of the redox centre is more negative on EFruII than EFruII-P, most likely due to an upwards pK shift of the thiols upon phosphorylation. The measurements indicate that the phosphotransferase system is regulated by the redox potential in a way that is dependent on the substrate concentrations. We propose that the change in the midpoint potential during turnover could be a mechanism for an electron transport function of the carrier. The binding of Zn2+ protects the carrier dithiol against oxidation but the presence of Zn2+ does not stimulate the reduction of the oxidized carrier.