The phosphoenolpyruvate‐dependent fructose‐specific phosphotransferase system in Rhodopseudomonas sphaeroides

Abstract

Redox titrations of the fructose-specific carrier protein, .**GRAPHIC**. 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 .**GRAPHIC**. is slower than that of .**GRAPHIC**. whereas the opposite is true for the oxidation rate. Consequently the midpoint potential of the redox centre is more negative on .**GRAPHIC**. than .**GRAPHIC**. 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.