ATP synthase and other motor proteins

Proteins, the working machines of the cell, operate as enzymes to catalyze chemical synthesis, as ion pumps to generate electrical voltage, and as motors to generate mechanical force. Myosin, a linear motor, jumps along an actin filament to contract striated muscle, the two-armed kinesin walks hand-over-hand on microtubuli to pull a synaptic vesicle along the nerve axon, and RNA polymerase, helicase, and the ribosome process over nucleotide strands. The flagellar drive, a rotary motor, propels bacteria though the viscous fluid. Whereas the former are powered by the hydrolysis of ATP (or other nucleotide triphosphates), the flagellar motor is powered by an electrochemical potential difference across the cytoplasmic membrane. ATP synthase, the enzyme that produces ATP, links both types of driving forces. It produces ATP at the expense of an electrochemical potential difference or, when operating in the reverse, it generates an electrochemical difference at the expense of ATP hydrolysis, then named F-ATPase. For three decades, it had remained enigmatic how its two functions, the electrochemical and the chemical, are linked to each other. They seemed to be rather well separated in the bipartite construction of the enzyme, with an ion-conducting membrane portion, FO, and a peripheral F1-portion that, by itself, hydrolyzes ATP. Only recently, it has been fully appreciated that ion transport and the synthesis of ATP by the holoenzyme are mechanically coupled. The holoenzyme is made from two rotary motors that are mounted on a central shaft and held together by an eccentric bearing (see Fig. 1A). Depending on the demand for ATP or for ion-motive force, one motor operates in the forward direction and, by rotating the central shaft, drives the other motor backwards to operate as a generator. Of the same size as myosin and by order of magnitude smaller than the …