An immunological study of the interaction of ligands with pyruvate kinase of Neurospora crassa

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Abstract
Antibodies against pyruvate kinase of Neurospora crassa, induced in rabbits, were used to monitor the interaction of ligands with this enzyme. This technique of microcomplement fixation was employed to probe for conformational alterations elicited by binding of substrates (phosphoenolpyruvate (PEP) and ADP), the allosteric activator (fructose 1,6-diphosphate [FDP]), and the inhibitor (valine). On binding of PEP and valine to pyruvate kinase, a pronounced reduction in the extent of complement fixation was observed. The second substrate, ADP, had no effect while FDP elicited a moderate suppression of complement fixation. As a consequence of conformational changes induced by PEP and valine, some antigenic determinants on the surface of pyruvate kinase are probably rendered inaccessible to the antibodies.