Proton translocation models

The proton translocation models (2) encounter several difficulties which seriously question their applicability ... 

The structure of the intermediate states in Rh7 and Rh8 has been studied recently by theoretical investigation [42], Regarding the proton translocation model, it should be noted that the excitation photon density was extremely high in the low-temperature picosecond experiments [10,35]. Therefore, the non-Arrhenius dependence of the formation rate of bathorhodopsin on temperature and the deuterium isotope effect may be results which could be detected only under intense excitation conditions. In fact, a deuterium isotope effect was not observed in the process from photorhodopsin to bathorhodopsin under weak excitation conditions [43],... 

Fig. 10. Models proposed for proton translocation (Model A) single proton translocation with carbonium ion formation. (Model B) concerted double proton translocation with retroretinal formation, (c) Tunneling potential energy barriers for formation of prelumirho-dopsin. represents the energy barrier and cIq the width or translocation distance. 

The proton-motive Q-cycle model, put forward by Mitchell (references 80 and 81) and by Trumpower and co-workers, is invoked in the following manner (1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-quinone see Figure 7.27) to the Rieske [2Fe-2S] center at the Qo site, the site nearest the intermembrane space or p side (2) this electron can leave the bci complex via an attached cytochrome c or be transferred to cytochrome Ci (3) the reactive ubisemiquinone reduces the low-potential heme bL located closer to the membrane s intermembrane (p) side (4) reduced heme bL quickly transfers an electron to high-potential heme bn near the membrane s matrix side and (5) ubiquinone or ubisemiquinone oxidizes the reduced bn at the Qi site nearest the matrix or n side. Proton translocation results from the deprotonation of ubiquinol at the Qo site and protonation of ubisemiquinone at the Qi site. Ubiquinol generated at the Qi site is reoxidized at the Qo site (see Figure 7.27). Additional protons are transported across the membrane from the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f). The overall reaction can be written... 

Brandt, U. (1997) Proton-translocation by membrane-bound NADH ubiquinone-oxidoreductase (complex I) through redoxgated ligand conduction. Biochim Biophys. Acta 1318, 79-91. Advanced discussion of models for electron movement through Complex I. 

The chemiosmotic model requires that flow of electrons through the electron-transport chain leads to extrusion of protons from the mitochondrion, thus generating the proton electrochemical-potential gradient. Measurements of the number of H+ ions extruded per O atom reduced by complex IV of the electron-transport chain (the H+/0 ratio) are experimentally important because the ratio can be used to test the validity of mechanistic models of proton translocation (Sec. 14.6). 

MECHANISTIC MODELS OF PROTON TRANSLOCATION Loop Mechanisms... 

In the chemiosmotic model, as first developed by Mitchell in the early 1960 s, proton translocation arises from transfer of electrons from an (H + + e ) carrier (such as FMNH2) to an electron carrier (such as an iron-sulfur protein), with expulsion of protons to the outer compartment of the inner mitochondrial membrane. This process is followed by electron transfer to an (H+ + e ) carrier, with uptake of protons from the matrix. In this model, the electron-transport chain is organized into three such loops, as shown in Fig. 14-5. 

Complex IV catalyzes electron transfer from cytochrome c to O2 this process appears to be coupled to proton translocation, with an H+/e value of 2. Two models have been developed to account for these values (Fig. 14-7). Current understanding is that complex IV is capable of acting as a proton pump. 

Fig. 14-9 A model for the cyclic synthesis of ATP by ATP synthase coupled to conformation changes in the P subunits brought about by proton translocation through the F0 complex. ATP synthesis occurs in the tightly bound" (T) state, but the ATP can only be released from the open (O) state. The energy of the electrochemical gradient is used to switch the T to the O state. The third state, L, can bind ADP.

In the binuclear haem-copper centre of cytochrome oxidases there is no cation radical formed at the active site. Instead the extra positive charge is held by the copper atom as it converts from cuprous (Cu1+) to cupric (Cu2+). In fact there is growing evidence to support the model of Mitchell [56] that it is the protonation steps associated with oxidation state changes in this copper atom (Cub) that provide the link between the electron transfer and proton translocation activities of this enzyme. 

In all these models cis-trans isomerization is assumed to follow the light-initiated intramolecular proton translocation. 

Proton translocation to the Schiff base nitrogen was proposed to occur by concerted double proton transfer (as shown in Fig. 15) leading to a retro retinal structure in the batho intermediate [127, 196], However, this model can be eliminated as it is inconsistent with the formation of batho intermediates from pigment analogs based on 5-desmethylretinal [145] and y-retroretinal [146,147], It also disagrees with the resonance Raman results. 

In a model proposed by Lewis [228] the effect of the excited state of retinal on the conformational state of the protein is considered to be the first step of the excitation mechanism. Charge redistribution in the retinal by excitation with light would have the consequence of vibrationally exciting and perturbing the ground state conformation of the protein, i.e., excited retinal would induce transient charge density assisted bond rearrangements (e.g., proton translocation). Subsequently, retinal would assume such an isomeric and conformational state so as to stabilize maximally the new protein structure established. In this model, 11-m to trans isomerization would not be involved in the primary process, but would serve to provide irreversibility for efficient quantum detection. It was also proposed that either the 9-m-retinal (in isorhodopsin) or the 11-m-retinal (in rhodopsin) could yield the same, common... 

Enzyme-catalyzed reactions, including the electron transport chain and proton translocation, are composed of series of elementary reactions that proceed forward and backward. One of the methods in describing this thermodynamically and mathematically coupled complex chemical reaction-transport system is the nonequilibrium thermodynamic model, which does not require the detailed knowledge of the system. 

The isomerization models discussed above differ from that described by Warshel, where J625 and PBAT are partially isomer-ized chromophores (90°) and their decay to Kfc g and BAT, respectively, is described by a motion on a potential surface involving both protein relaxation (proton translocation) and additional chromophore isomerization. This model implies that in K ig and BAT, proton translocation has taken place in the opsin, but it is not discriminative concerning whether a chromophore isomerization has taken place at this stage. 

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