The metabolism and role of Rubisco inhibitors

Abstract

Rubisco activity is modulated in vivo either by reaction with CO2 and Mg++ to carbamylate a lysine residue in the catalytic site and render the enzyme active or by binding inhibitors within the catalytic site, preventing turnover or carbamylation. Both processes are reversible. At night, in many species, 2-carboxyarabinitol 1-phosphate (CA1P) is formed and binds tightly to Rubisco, inhibiting catalytic activity. In the present work, the contribution of Rubisco inhibitors to the activity of Rubisco in illuminated leaves will be considered. In tobacco plants the "total Rubisco activity" was positively correlated with leaf water content. However, "total Rubisco activity" in extracts from leaves with low water content markedly increased when steps were taken to remove tightly bound inhibitors and thus make more catalytic sites available. The results suggest that the decrease of Rubisco activity under drought stress is not primarily the result of changes in activation by CO2 and Mg++ but rather due to the presence of tight binding inhibitors. The biosynthesis and metabolism of these inhibitors will be discussed. Many of the properties of a tight binding inhibitor found in illuminated leaves (the so-called "daytime inhibitor") are similar to those of D-glycero-2,3-pentodiulose-1,5-bisphosphate, obtained by the oxidation of RuBP. We have established unequivocally that both CA1P and the daytime inhibitor are chloroplastic and that such inhibitors may protect Rubisco from degradation during stress.