The ATP synthase: Parts and properties of a rotary motor

Publisher Summary This chapter discusses (1) the experimental approaches that have documented subunit rotation in F 1 and tentatively in F 0 F 1 and (2) the studies of the way the structural organization and interactions of subunits within the adenosine triphosphate (ATP) synthase may fit as functional parts of a molecular motor. ATP has a central role in cellular metabolism with cleavage of its high-energy phosphoanhydride bond(s) serving as a common “currency” to drive a variety of energy-requiring processes. These include mechanical transport processes catalyzed by molecular motors, such as muscle contraction by myosin ATPase, the transport of vesicles and organelles along microtubules by dynein and kinesin ATPases, and the vectorial unwinding of double-stranded (ds)DNA by DNA helicases. Most cellular ATP is synthesized from adenosine disphosphate (ADP) and P i by a ubiquitous coupling enzyme, the H + - transporting ATP synthase, in the terminal step of oxidative- and photo-phosphorylation. The ATP synthase is found embedded in the inner membrane of mitochondria, the thylakoid membrane of chloroplasts, and the plasma membrane of eubacteria. It is referred to as “F 0 F 1 ATP synthase,” because, experimentally, it can be separated into two complexes that retain the distinct partial functions of the intact synthase. The F 0 portion is embedded in the membrane and functions in proton transport; in the absence of F 1 , F 0 catalyzes the passive flux of protons across the bilayer. F 1 can be released from F 0 as a water-soluble complex that contains the catalytic sites for ATP synthesis but, uncoupled from a proton current, soluble F 1 can catalyze only the net hydrolysis of ATP. Thus, it is often referred to as “F 1 -ATPase.”