Translocation mechanism, proton

Krab, K. Wikstrom, M. (1987). Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase. Biochim. Biophys. Acta 895,25-39. 

Several details of the proton translocation mechanism have been revealed since the previous edition of this series. Thus, the proton transfer pathways have been elucidated (see above), and near consensus has been reached that all protons that... 

As indicated above, the number of subunits c present in Fq has been reported to be 9-12. From consideration of the model for the proton translocation mechanism, it is believed that either 9 or 12 c subunits should be present in each Fq, and that they are presumably arranged either as three trimers or three tetramers in a ring. In accord with the ratio of 3 that has been generally accepted at least up until recently for the number of protons translocated per ATP synthesized, nine protons have to be translocated during a complete catalytic cycle, since each ofthe three (a P) pairs synthesizes an ATP thus nine c-subunits are needed, one for each translocated. However, based on the recently determined ff/ATP ratio of 4 for chloroplast and cyanobacterial ATP synthasetwelve c subunits organized in three tetramers ofc subunits would be favored. 

Lanyi J K 1993 Proton translocation mechanism and energetics in the light-driven... 

Schiff base is deprotonated in M, it has been concluded that this aspartic acid is the proton acceptor for Schiff base deprotonation. In the BR M/N spectrum the negative band at 1742 cm indicates that another carboxyl group assigned to another aspartic acid deprotonates, transferring the proton to the Schiff base for reprotonation. Thus, infrared difference spectroscopy has identified essential steps in the proton translocating mechanism. 

The electron transfer and proton translocation mechanism of the mammalian and bacterial bc complexes is now relatively well understood. Their catalytic Q-cycle can occur in the monomeric enzyme and is always coupled to a net proton translocation. The key step is electro-genic electron transfer through the haems b from a site of quinol oxidation to one of quinone reduction. Associated (de)protonations are probably non-electrogenic but result in net proton translocation [1]. 

Lanyi, I.K., Proton translocation mechanism and energetics in the hght-driven pump bacterior-hodopsin, Biochim. Biophys. Acta, 1183, 241,1993. 

Why has nature chosen this rather convoluted path for electrons in Complex 111 First of all. Complex 111 takes up two protons on the matrix side of the inner membrane and releases four protons on the cytoplasmic side for each pair of electrons that passes through the Q cycle. The apparent imbalance of two protons in ior four protons out is offset by proton translocations in Complex rV, the cytochrome oxidase complex. The other significant feature of this mechanism is that it offers a convenient way for a two-electron carrier, UQHg, to interact with the bj and bfj hemes, the Rieske protein Fe-S cluster, and cytochrome C, all of which are one-electron carriers. 

Figure 7. Mechanism of the proton-translocating ubiquinol cytochrome c reductase (complex III) Q cycle. There is a potential difference of up to 150 mV across the hydrophobic core of this complex (potential barrier represented by the vertical broken line). Cytochromes hour and b N are heme groups on the same peptide subunits of complex III which can transfer electrons across the hydrophobic core. The movement of two electrons provides the driving force to transfer two protons from the matrix to the cytosol. Diffusion of UQ and UQHj, which are uncharged, in the hydrophobic core, and lipid bilayer of the inner membrane is not influenced by the membrane potential (see Nicholls and Ferguson, 1992).

Figure 9. Proposed cyclic mechanism for ATP synthesis by complex V involving all three catalytic sites of F,. In this scheme only the a and p subunits of F, are shown these are connected by a short stalk to F, in the inner membrane. Proton translocation through Fq driven by the proton motive force (AP) causes sequential conformational changes in each of the p-subunits and ATP synthesis as described in the text hexagons, high-affinity sites semicircles, low affinity sites parallelepipeds, intermediate-affinity sites (with no movement of F,).

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

The mechanism of proton translocation in complexes I and IV is not yet understood. Here, the electron-transfer reactions may cause protein conformational changes that open gates for proton movement first on one side of the membrane and then on the other. 

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