Energy transduction in ATP synthase

Abstract

Mitochondria, bacteria and chloroplasts use the free energy stored in transmembrane ion gradients to manufacture ATP by the action of ATP synthase. This enzyme consists of two principal domains. The asymmetric membrane-spanning F0 portion contains the proton channel, and the soluble F1 portion contains three catalytic sites which cooperate in the synthetic reactions. The flow of protons through F0 is thought to generate a torque which is transmitted to F1 by an asymmetric shaft, the coiled-coil gamma-subunit. This acts as a rotating 'cam' within F1, sequentially releasing ATPs from the three active sites. The free-energy difference across the inner membrane of mitochondria and bacteria is sufficient to produce three ATPs per twelve protons passing through the motor. It has been suggested that this proton motive force biases the rotor's diffusion so that F0 constitutes a rotary motor turning the gamma shaft. Here we show that biased diffusion, augmented by electrostatic forces, does indeed generate sufficient torque to account for ATP production. Moreover, the motor's reversibility-supplying torque from ATP hydrolysis in F1 converts the motor into an efficient proton pump-can also be explained by our model.

Intragenic and intergenic suppression of the Escherichia coli ATP synthase subunit a mutation of Gly-213 to Asn: functional interactions between residues in the proton transport site

“…The suppression of aG213N by cD61E reinforces this conclusion. Our results provide, for the first time, functional evidence supporting various models [10,12,16,18,28,29] that suggest a direct interaction between the critical transport residues, aArg-210 and cAsp-61. These residues, along with aGln-252, which is an important but not essential component, constitute the ' transport site '.…”

“…Importantly, recent evidence now demonstrates that the c subunits also undergo ATPase-dependent rotation [15], which is consistent with several models of transport [12,[16][17][18]. Nevertheless, because biochemical and structural analyses of the Abbreviations used : ACMA, 9-amino-6-chloro-2-methoxyacridine ; CCCP, carbonyl cyanide m-chlorophenylhydrazone ; FLAG, Asp-Tyr-Lys-Asp-AspAsp-Asp-Lys ; LB, Luria-Bertani.…”

“…Because of the spatial proximity of aGly-213 and aArg-210, we suggest that the aG213N and aG213L mutations cause aArg-210 to strengthen existing interactions or to form new ones, possibly with cAsp-61, that must be broken before the enzyme can proceed through its reaction cycle. This effect might involve the steps of protonation or deprotonation of cAsp-61 or the rotation of cAsp-61 past aArg-210 [16]. A larger activation energy for transport is expected if, for example, the pK of cAsp-61 has shifted, thus affecting the rate-limiting transition state for the transport of each proton.…”

“…A critical question is whether cAsp-61 and aArg-210 interact physically and functionally, as has been suggested in many models for transport [10,12,16,29]. This is a difficult question to answer because all substitutions of aArg-210 are null mutations.…”

The rotary mechanism of the ATP synthase

“…There are likely five transmembrane segments, some of which interact with the ring of c subunits to create the pathway for H + or Na + . In current models of rotational transport [79][80][81][82], H + are guided to the subunit c carboxylic acid near the center of the bilayer via pathways created by the interface between subunit a and the c subunit ring on the cytoplasmic half of the membrane, or within subunit a on the periplasmic half of the membrane [83,84]. In order to couple rotation direction to the direction of proton flow, the pathways from either side of the membrane lead to adjacent c subunits.…”