Subunit movement during catalysis by F₁-F₀-ATP synthases

Abstract

The catalytic portion (F₁) of ATP synthases have the subunit composition α₃, β₃, γ, δ, ε. This composition imparts structural asymmetry to the entire complex that results in differences in nucleotide binding affinity among the six binding sites. Evidence that two or more sites participate in catalysis, alternating their properties, led to the notion that the interactions of individual αβ pairs with the small subunits must change as binding site properties alternate. A rotation of the γ subunit within the α₃β₃ hexamer has been proposed as a means of alternating the properties of catalytic sites. Evidence argues that the rotation of the complete γ subunit during ATP hydrolysis is not mandatory for activity. The γ subunit of chloroplast F₁ may be cleaved into three large fragments that remain bound to F₁. This cleavage enhances ATPase activity without loss of evidence of site-site interactions. Complexes of α₃β₃ have been shown to have significant ATPase activity in the absence of γ. Mg²⁺ATP affects the interaction of γ with the different β subunits, and induces other changes in F₁, but whether these changes are induced by catalysis, or are fast enough to be involved in the catalytic turnover of the enzyme has not been established. Likewise, changes in structure and in binding site properties induced in thylakoid membrane bound CF₁ by formation of an electrochemical proton gradient may activate the enzyme rather than be apart of catalysis. Mechanisms other than rotary catalysis should be considered.