Reaction of membrane-bound F1-adenosine triphosphatase of Escherichia coli with chemical ligands and the asymmetry of beta subunits

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

Unisite hydrolysis of [gamma 32 P]ATP by soluble mitochondrial F1-ATPase and its release by excess ADP and ATP. Effect of trifluoperazine

Some of the characteristics of unisite hydrolysis of [gamma 32P]ATP as well as the changes that occur on the transition to multisite catalysis were further studied. It was found that a fraction of [gamma 32P]ATP bound at the catalytic sites of F1 under unisite conditions undergoes both hydrolysis and release induced by medium nucleotides upon addition of millimolar concentrations of ADP or ATP. The fraction of [gamma 32P]ATP that undergoes release is similar to the fraction that undergoes hydrolytic cleavage, indicating that the rates of the release and hydrolytic reactions of bound [gamma 32P]ATP are in the same range. As part of studies on the mechanisms through which trifluoperazine inhibits ATP hydrolysis, its effect on unisite hydrolysis of [gamma 32P]ATP was also studied. Trifluoperazine diminishes the rate of unisite hydrolysis by 30-40%. The inhibition is accompanied by a nearly tenfold increase in the ratio of [gamma 32P]ATP/32Pi bound at the catalytic site and a 50% diminution in the rate of 32Pi release from the enzyme into the media. Trifluoperazine also induces heterogeneity of the three catalytic sites of F1 in the sense that in a fraction of F1 molecules, the high-affinity catalytic site has a turnover rate lower than the other two. Trifluoperazine does not modify the release of previously bound [gamma 32P]ATP induced by medium nucleotides. The latter indicates that hindrances in the release of Pi do not necesarily accompany alterations in the release of ATP even though both species lie in the same site.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.