This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We propose to use small-angle x-ray scattering (SAXS) to study conformational changes of an allosteric protein, rabbit muscle pyruvate kinase (M1-PYK) that are important to the allosteric mechanism. Despite the importance of allosteric regulation in allowing biological systems to adapt to changing environments, the molecular mechanisms by which proteins are allosterically regulated are poorly understood. Our results with allosteric effectors of M1-PYK demonstrate that discrete substructures within the inhibitor, Phe, have functionally specific roles, that is, substructures required for binding differ from those that elicit an allosteric response (Williams et al. (2006) Biochemistry 45, 5421-5429). The resulting hypothesis is that the protein properties that are altered upon effector binding must also be divided into the same functional subsets. Using this information, Ala can be considered as a non-allosteric effector analog, one which binds competitively with Phe but does not elicit an allosteric response. Initial SAXS data indicate that the global conformational changes observed when Phe binds are also elicited when Ala binds. As such, these global changes may be confined to binding functions. Scattering profiles with improved statistical data are needed to characterize small differences between the Phe and the Ala elicited changes. These small differences are likely consequential to allosteric functions. Experiments have been designed to characterize SAXS profiles of M1-PYK in various enzyme complexes with substrate, Phe, Ala, and/or other effector analogs.