The biological motor F1F0-ATP Synthase consists of a part, F1-ATPase, that is outside a bilayer membrane and that has a three-fold symmetry (three alpha and three beta subunits), and a transmembrane part, F0-ATPase. The F1 part permits the “energy rich” ATP (adenosine triphosphate) to be synthesized from ADP (adenosine diphosphate) and Pi, an inorganic phosphate ion. The synthesis is caused by an opening and closing of the successive beta subunits in F1 under the influence of a rotation of an asymmetric spindle of F1 (the gamma subunit). The rotation is driven by a proton gradient across the membrane and occurs via an ingenious offset circular channel in F0 and an acid-base reaction that causes the spindle of the F1 to rotate. The F1-ATPase is the minimal subsystem in which mechanical rotation can induce ATP synthesis. Studies on the reverse process (ATP hydrolysis) revealed that there are several successive kinetic steps, the opening (and later closing) of a cleft in one beta subunit, permitting the ATP to enter into the subunit while ADP is being released from another, followed by hydrolysis of the ATP to form ADP and Pi, and then by their release. We have studied the experimental data, largely single molecule, to obtain and interpret the rotation angle-dependent rate constants of individual steps. The experimental data involve different single molecule experiments (free rotation, controlled rotation, stalling experiments, among others). Important in understanding the overall mechanism are not only the angle-dependent rate constants of individual steps but also the occupation probabilities versus angle profiles of the various ATP, ADP and Pi in the F1 beta-subunits. The kinetic results on the angle-dependent rate constants [which resemble exponential functions] are interpreted in terms of a free energy of activation-standard free energy of reaction formalism for each step, originally designed for electron, atom and group transfer reactions. Here, the transfer can be a breaking and forming of hydrogen bonds accompanying the entrance/exit of the various species and the reaction itself.

Insights into the kinetics and mechanism of a biological motor, F1-ATPAse

ACS Spring Meeting 2021

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