Circular dichroism and nucleotide and phosphate-induced conformational changes of mitochondrial adenosinetriphosphatase.

The conformational changes induced by the binding of different effectors on F1-ATPase are investigated by using circular dichroism and are related to enzyme activity. The hydrophilic part of the terminal enzyme of oxidative phosphorylation, F1-ATPase, solubilized from the pig heart mitochondrial membrane contains both regulatory and catalytic sites which can bind nucleotides and phosphate. The circular dichroic spectra of F1-ATPase in the absence or in the presence of ADP, Mg2+, phosphate, and the substrate analogue guanosine 5'-(beta, gamma-imidotriphosphate) [GMP-P-(NH)P] were recorded and analyzed in terms of secondary structure. The most significant result is a sizable increase from 35% to 42% of the alpha-helix content when the enzyme is incubated with all the effectors. Since the kinetic study showed that GMP-P(NH)P is a competitive inhibitor of MgATP with or without preincubation of the enzyme with ADP and phosphate, it was concluded that the catalytic and regulatory sites can be simultaneously occupied by ADP and GMP-P-(NH)P. The increase of alpha-helix content is then interpreted by a conformational change that occurs only after occupation of both types of sites.