Oxidation-reduction properties of chloroplast thioredoxins, Ferredoxin : Thioredoxin reductase, and thioredoxin f-regulated enzymes

Oxidation-reduction midpoint potentials were determined, as a function of p H, for the disulfide/dithiol couples of spinach and pea thioredoxins f, for spinach and Chlamydomonas reinhardtii thioredoxins m, for spinach ferredox in:thioredoxin reductase (FTR), and for two enzymes regulated by thioredoxi n f, spinach phosphoribulokinase (PRK)and the fiuctose-1,6-bisphosphatases (FBPase) from pea and spinach. Midpoint oxidation-reduction potential (E-m) values at pH 7.0 of -290 mV for both spinach and pea thioredoxin f, -300 m V for both C. reinhardtii and spinach thioredoxin m, -320 mV for spinach FT R, -290 mV for spinach PRK, -315 mV for pea FBPase, and -330 mV for spinach FBPase were obtained. With the exception of spinach FBPase, titrations sho wed a single two-electron component at all pH values tested. Spinach FBPase exhibited a more complicated behavior, with a single two-electron componen t being observed at pH values greater than or equal to 7.0, but with two co mponents being present at pH values <7.0. The slopes of plots of E-m versus pH were close to the -60 mV/pH unit value expected for a process that invo lves the uptake of two protons per two electrons (i.e., the reduction of a disulfide to two fully protonated thiols) for thioredoxins f and m, for FTR , and for pea FBPase;The slope of the E-m versus pH profile for PRK shows t hree regions, consistent with the presence of pK(a) values for the two regu latory cysteines in the region between pH 7.5 and 9.0.