Mechanistic Models of Proton Translocation

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. 

S = oxidized substrate FeS = iron-sulfur protein a, o3, b. c, c = cytochromes  

What experimentally testable predictions does the Mitchell loop mechanism make  

This scheme makes two experimentally testable predictions  

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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). 

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