Proton transfer electron flow path

Chapter 3 covered the proton transfer electron flow path and reviewed the factors that contribute to acidity. Chapter 4 introduced all the rest of the major electron flow paths along with the four reaction archetypes, substitution, elimination, addition, and rearrangement. This chapter gathers together all the major electron flow paths, introduces a few minor paths, and reviews common path combinations. Section 7.4, Variations on a Theme, shows how the 18 electron flow paths might be reasonably extended and modified. 

Before we explore the problem space for a simple proton transfer reaction, we need to understand the basics of bonding and define a consistent nomenclature. In order to use the electron flow paths, you first need to be able to keep track of atoms and electrons— write Lewis structures correctly and easily. 

Biochemical Note An enzyme s active site can have weak acids and bases close to our reactant. It is often possible to have proton transfer occurring in the same step as another electron flow path. These are discussed separately in Section 7.4.3. 

Like the homologous Complex III in mitochondria, the cytochrome bci complex of purple bacteria carries electrons from a quinol donor (QH2) to an electron acceptor, using the energy of electron transfer to pump protons across the membrane, producing a proton-motive force. The path of electron flow through this complex is believed to be very similar to that through mitochondrial Complex III, involving a Q cycle (Fig. 19-12) in which protons are consumed on one side of the membrane and released on the other. The ultimate... 

Chemiosmotic theory readily explains the dependence of electron transfer on ATP synthesis in mitochondria. When the flow of protons into the matrix through the proton channel of ATP synthase is blocked (with oligomycin, for example), no path exists for the return of protons to the matrix, and the continued extrusion of protons driven by the activity of the respiratory chain generates a large proton gradient. The proton-motive force builds up until the cost (free energy) of pumping... 

The green and blue allows represent the flow of electrons. The blue arrows represent the path of UQ in its various oxidation states (the Q cycle) and of protons. UQH, is oxidized to UQ in two steps at an enzyme site adjacent to the intermembrane space. The first electron is transferred to the Fe-S protein. The second electron is transferred to cyt b. (Two molecules of UQH2 undergo those reactions.) One of the two molecules of UQ produced diffuses to the site on the matrix side where it is reduced to form UQH,. (The transfer of electrons from the two b cytochromes is inhibited by antimycin.) Once formed, the UQH, diffuses back to the oxidation site, where it joins the pool of UQH, coming from complexes I and II. The electrons transferred from UQH, to the Fe-S center then reduce cyt c. Four protons are released on the cytoplasmic side of the inner membrane. 

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