The chloroplast F1Fo ATP synthase is a rotary motor that converts the light-driven proton-motive force into the chemical energy of ATP. The number of c-subunits in its rotor fixes the number of protons translocated per ATP formed, a fundamental parameter of bioenergetic systems. The reference spinach enzyme possesses fourteen c-subunits and a H+/ATP ratio of 4.67. Green algae additionally operate a carbon-concentrating mechanism that sustains CO2 fixation in water at a substantial cost in ATP, yet the structure of the algal motor, and whether its bioenergetic parameters differ from those of vascular plants, remains unresolved. Here, a 2.2 [A] structure of the ATP synthase of Chlamydomonas reinhardtii reveals that the enzyme carries a thirteen-membered c-ring, the first departure from c14 in a chloroplast, and with a lower predicted H+/ATP ratio of 4.33. Ordered waters trace a Grotthuss proton relay through the membrane, where an insulating triad separates the proton loading and unloading sites and couples flux to rotation. A single substitution in the redox switching {gamma}-subunit abolishes the contact with the catalytic {beta}-subunit that idles the enzyme in darkness in vascular plants. These unique features of the algal ATP synthase lower the H+/ATP cost of carbon fixation in the light and facilitate acetate metabolism in the dark.
Lorencik, K., Pintscher, S., Richardson, K., Takahashi, H., Proctor, M., Rawski, M., Hunter, C. N., Hitchcock, A., Blaza, J. N., Johnson, M. P.
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