SUBUNIT MOVEMENT DURING CATALYSIS BY F-1-F-0-ATP SYNTHASES

The catalytic portion (F-1) of ATP synthases have the subunit composit ion alpha(3), beta(3), gamma, delta, epsilon. This composition imparts structural asymmetry to the entire complex that results in difference s in nucleotide binding affinity among the six binding sites, Evidence that two or more sites participate in catalysis, alternating their pr operties, led to the notion that the interactions of individual alpha beta pairs with the small subunits must change as binding site propert ies alternate. A rotation of the gamma subunit within the alpha(3) bet a(3) hexamer has been proposed as a means of alternating the propertie s of catalytic sites. Evidence argues that the rotation of the complet e gamma subunit during ATP hydrolysis is not mandatory for activity. T he gamma subunit of chloroplast F-1 may be cleaved into three large fr agments that remain bound to F-1. This cleavage enhances ATPase activi ty without loss of evidence of site-site interactions. Complexes of al pha(3) beta(3) have been shown to have significant ATPase activity in the absence of gamma. Mg(2+)ATP affects the interaction of gamma with the different beta subunits, and induces other changes in F-1, but whe ther these changes are induced by catalysis, or are fast enough to be involved in the catalytic turnover of the enzyme has not been establis hed. Likewise, changes in structure and in binding site properties ind uced in thylakoid membrane bound CF1 by formation of an electrochemica l proton gradient may activate the enzyme rather than be apart of cata lysis, Mechanisms other than rotary catalysis should be considered.