Isocitrate lyase from flax : terminal residues, composition, active site, and catalysis

The cleavage of D_s-isocitrate catalyzed by isocitrate lyase from Linum usitatissimum results in the ordered release of succinate and glyoxylate. The glyoxylate analog 3-bromopyruvate irreversibly inactivates the flax enzyme in a process exhibiting saturation kinetics and protection by glyoxylate or isocitrate or the competitive inhibitor l-tartrate. Succinate provides considerably less protection. Results with 3-bromopyruvate suggest that this reagent modifies plant and prokaryotic isocitrate lyases differently. Treatment of the tetrameric 264,000-dalton flax enzyme with carboxypeptidase A results in a release of one histidine/subunit which is concordant with loss of activity. The only N-terminal residue is methionine. Treatment of flax enzyme with diethylpyrocarbonate at pH 6.5 selectively modifies two histidines per 67,000-dalton subunit. The reaction of one histidine residue is abolished by the binding of l-tartrate and the modification of one is coincident with inactivation. The carboxy-terminal and active-site modifications establish that one histidine residue/monomer is essential in the flax enzyme and considerably extend information heretofore available only for fungal and bacterial isocitrate lyase.