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

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Abstract
Two interrelated sites have been detected on the fructose carrier in Rhodopseudomonas sphaeroides: an activity‐linked dithiol and a Zn2+‐binding site. Binding of Zn2+ brings EFruII into a new conformation that to some extent mimics the conformation of phosphorylated EFruII, an essential intermediate in the turnover of the enzyme. Binding of zinc to EFruII or phosphorylating the enzyme protects it against trypsin inactivation relative to the dephos‐phorylated zinc‐free enzyme. A dithiol is essential for activity. Interchanges between the redox states of the enzyme can be brought about by dithiothreitol and ferricyanide, but not, or very slowly, by molecular oxygen. The dithiol is protected, in the EFruII‐Zn2+ complex, against (a) alkylation by MalNEt, (b) reversible oxidation by Fe(CN)3‐6 and Cu2+, (c) irreversible oxidation by Cu2+. The pK value of the activity linked thiol is 7.8. Protection experiments show that the dithiol is not located in any of the substrate‐binding sites. The redox state of the enzyme does not influence the rate of inactivation of EFruII by trypsin.

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