THE OXIDATION-REDUCTION PROPERTIES OF SPINACH THIOREDOXIN-F AND THIOREDOXIN-M AND OF FERREDOXIN-THIOREDOXIN REDUCTASE

Oxidation-reduction midpoint potentials have been determined, using cy clic voltammetry, for the active-site disulfide/dithiol couples of spi nach thioredoxins f and m and of spinach ferredoxin:thioredoxin reduct ase (FTR) and for a component likely to be the [4Fe-4S] cluster of FTR . Values for the midpoint potentials (n = 2) of -210 +/- 10 mV were de termined for both thioredoxins f and m. Two redox centers were detecte d in FTR, with midpoint potential values of -230 +/- 10 mV (n = 2) and +340 +/- 30 mV, respectively. Alkylation of the active-site cysteines of FTR by treatment of the enzyme with N-ethylmaleimide (NEM) elimina tes the component with the -230 mV midpoint potential, allowing one to assign this value to the active site disulfide/dithiol couple. Inasmu ch as the only other electron-carrying center known to be present in F TR is the [4Fe-4S] cluster, it appears likely that the high-potential component can be attributed to this redox moiety. The midpoint potenti al value of the high-potential feature shifts slightly, to +380 +/- 20 mV, in the NEM-treated enzyme.