S-(4-bromo-2,3-dioxobutyl)-CoA labels two distinct sites on citrate synthase

The chemical nature of the inactivation of citrate synthase by S-(4-bromo-2,3-dioxobutyl)-CoA, an active site-directed irreversible inhibitor, has been investigated. Active site-directed inactivation leads to derivatization of either Lys22 by epsilon-amino Schiff base formation or Glu363 by apparent alkylation of the gamma-carboxyl group, respectively. Lys22 is labeled in the tight (catalytic) form of the enzyme while Glu363 is labeled in the open (product release) form. Glu363 and Lys22 are both located at or near the entrance to an active site in the crystal structure of citrate synthase (Remington, S., Wiegand, G., and Huber, R. (1982) J. Mol. Biol. 158, 111-152). Glu363 is in the sequence of the protomer forming the active site while Lys22 is in the sequence of the other polypeptide in the homodimer. Labeling in this region appears to inactivate the enzyme by preventing access of substrates to the active site. A distinct and separate labeling process involves derivatization of Asn192 in the tight (catalytic) form and Ser198 and/or Ser199 in the open (product release) form at a locus far removed from the active site. Labeling at the second site may simply identify chemically reactive residues, or it may identify the binding site for long chain acyl-CoA, which has been identified as a possible allosteric negative effector of citrate synthase (Caggiano, A. V., and Powell, G. L. (1979) J. Biol. Chem. 254, 2800-2806). This second labeling process apparently inactivates the enzyme by interfering with catalytically essential conformational changes.