Lolkema J S, Robillard G T
Eur J Biochem. 1986 Aug 15;159(1):141-7. doi: 10.1111/j.1432-1033.1986.tb09844.x.
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.
对球形红假单胞菌中果糖特异性载体蛋白EFruII进行的氧化还原滴定表明，只有该酶的还原形式具有活性。该酶的氧化形式仍可被磷酸化，但无法将磷酰基转移至果糖。酶的氧化还原性质在磷酸化后会发生变化。EFruII的还原速率比EFruII-P慢，而氧化速率则相反。因此，氧化还原中心的中点电位在EFruII上比在EFruII-P上更负，这很可能是由于磷酸化后硫醇的pK值上移所致。测量结果表明，磷酸转移酶系统受氧化还原电位的调节，且这种调节方式取决于底物浓度。我们认为，周转过程中中点电位的变化可能是载体发挥电子传递功能的一种机制。Zn2+的结合可保护载体二硫醇不被氧化，但Zn2+的存在不会刺激氧化型载体的还原。
