THE MECHANISM OF REDOX REGULATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE TURNOVER - A HYPOTHESIS

The chloroplastic CO2-fixing enzyme ribulose-1,5-bisphosphsite carboxy lase/oxygenase (EC 4.1.1.39; Rubisco) is known to be rapidly and selec tively degraded during natural and stress-induced senescence. Differen t lines of evidence (derived from both in vivo and in vitro experiment s) suggest that the turnover of Rubisco may be regulated by redox modi fication of sulfhydryl groups from critical cysteine residues whose ox idation renders the enzyme sensitive to proteases. Since the critical sulfhydryls are not particularly oxidizable, this proposal may seem im plausible in a reductive: compartment such as the chloroplast. However , a hypothetical mechanism is presented here by which the oxidation of the Rubisco may be driven under a mildly reducing environment (as gov erned by an intrachloroplastic glutathione disulfide/reduced glutathio ne ratio below unity), provided that the oxidation of the critical sul fhydryls progresses only to mixed disulfides and the bonding of these residues to glutathione is kinetically impeded. It is proposed that th e redox equilibrium of the critical residues with oxidized/reduced glu tathione is established by means of an intermediate disulfide/thiol pa ir having a standard reduction potential similar to cystamine/cysteami ne. Thus, it is shown that the redox state of the Rubisco can be effec tively modulated by changing the concentration of the intermediate pai r. This provides a mechanism for regulation of Rubisco turnover which is loosely dependent on the ambient redox potential.