Changes in Small-Angle X-ray Scattering Parameters Observed upon Binding of Ligand to Rabbit Muscle Pyruvate Kinase Are Not Correlated with Allosteric Transitions

Protein fluorescence and small-angle X-ray scattering (SAXS) have been used to monitor effector affinity and conformational changes previously associated with allosteric regulation in rabbit muscle pyruvate kinase (M(1)-PYK). In the absence of substrate [phosphoenolpyruvate (PEP)], SAXS-monitored conformational changes in M(1)-PYK elicited by the binding of phenylalanine (an allosteric inhibitor that reduces the affinity of M(1)-PYK for PEP) are similar to those observed upon binding of alanine or 2-aminobutyric acid. Under our assay conditions, these small amino acids bind to the protein but elicit a minimal change in the affinity of the protein for PEP. Therefore, if changes in scattering signatures represent cleft closure via domain rotation as previously interpreted, we can conclude that these motions are not sufficient to elicit allosteric inhibition. Additionally, although PEP has similar affinities for the free enzyme and the M(1)-PYK small amino acid complexes (i.e., the small amino acids have minimal allosteric effects), PEP binding elicits different changes in the SAXS signature of the free enzyme versus the M(1)-PYK small amino acid complexes.