INHIBITION AND STIMULATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE BY GLYOXYLATE

Glyoxylate is a slowly reversible inhibitor of the CO2/Mg2+-activated form of ribulose-1,5-bisphosphate carboxylase/oxygenase from spinach leaves. Inactivation occurred with an apparent dissociation constant of 3.3 mM and a maximum pseudo-first-order rate constant of 7 .times. 10-3 s-1. The rate constant for reactivation was 1.2 .times. 10-2 s-1. Glyoxylate did not cause differential inhibition of ribulosebisphosphate carboxylase or oxygenase activities. 6-Phosphogluconate protected the enzyme from inactivation by glyoxylate. Glyoxylate was incorporated irreversibly into the large subunit of ribulosebisphosphate carboxylase after reduction with sodium borohydride. Activated enzyme incorporated 1.3 mol of glyoxylate per mol protomer, while enzyme treated with carboxyarabinitol 1,5-bisphosphate (CABP) to protect the active sites incorporated only 0.3 mol glyoxylate per mol protomer. The data sugest that glyoxylate forms a Schiff base with a lysyl residue in the region of the catalytic site. Glyoxylate stimulated the activity of the unactivated enzyme by .apprx. 2-fold. Pseudo-first-order inactivation also occurred with the unactivated enzyme after the initial stimulation by glyoxylate, although at a much slower rate than with the activated enzyme. Glyoxylate treatment of partially activated enzyme did not stimulate formation of the quaternary complex of enzyme.cntdot.CO2.cntdot.Mg2+.cntdot.CABP.