Cytochrome proton pumps

Cytochrome P450 2C19, also termed S-mephenytoin hydroxylase, is a mixed-function oxidase localized in the endoplasmic reticulum which is responsible for the biotransformation of S-mephenytoin, some barbiturates, almost all proton pump inhibitors such as omeprazole, diazepam and others. 

Proton Pump Inhibitors and Acid Pump Antagonists retinoid X receptor (RXR) and is also activated by various lipophilic compounds produced by the body such as bile acids and steroids. PXR heterodimerized with RXR stimulates the transcription of cytochrome P450 3A monooxygenases (CYP3A) and other genes involved in the detoxification and elimination of the... 

Michel H. 1999. Cytochrome c oxidase Catalytic cycle and mechanisms of proton pumping— A discussion. Biochemistry 38 15129. 

BA, bioavailability CL, total body clearance CrCI, creatinine clearance CYP450, cytochrome P-450 ER, extended-release IR, immediate-release LA, long-acting PPI, proton pump inhibitor Sign., significantly SR, sustained-release TD, transdermal. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Belevich I, Verkhovsky MI, Wikstrom M (2006) Proton-coupled electron transfer drives the proton pump of cytochrome c oxidase. Nature 440 829-832. 

Complex II (which is not shown in the figure) contains succinate dehydrogenase, the FAD-dependent Krebs cycle enzyme and, like Complex I, transfers its electrons through iron-sulfur centres and a 6-type cytochrome (more of these haem iron proteins will be discussed in Chapter 13) to CoQ. However, here AEI is only 0.085 V, corresponding to AG° of —16.4 kJ/mol, which is not sufficient to allow proton pumping. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

In all protein-protein complexes studied to date in which cytochrome c has been a partner, it has been shown that the ET rates depend strongly on the reaction driving force. It follows that variations in the reorganization energy could control ET rates in these cases [12]. In redox enzymes with two or more active centers, ET between two centers could be turned on by lowering X at roughly constant — AG [1]. Indeed, a proposal has been advanced that this type of mechanism would be an efficient way to gate the electron flow in a redox-linked proton pump such as cytochrome oxidase [75]. 

Cytochrome c oxidase (CcO) has been characterized as a proton pump (see, for instance, M. Saratse s 1999 Science magazine article ), although CcO... 

Ishizaki, T., and Horai, Y. (1999) Review article cytochrome P450 and the metabohsm of proton pump inhibitors—emphasis on rabeprazole. Aliment. Pharmacol. Ther. 13, 27-36. 

The oxidation/reduction of redox cofactors in biological systems is often coupled to proton binding/release either at the cofactor itself or at local amino acid residues, which provides the basic mechanochem-ical part of a proton pump such as that foimd in cytochrome c oxidase (95). Despite a thermodynamic cycle that provides that coupling of protonation of amino acids to the reduction process will result in a 60 mV/pH decrease unit in the reduction potential per proton boimd between the pAa values in the Fe(III) and Fe(II) states, the essential pumping of protons in the respiratory complexes has yet to be localized within their three-dimensional structures. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Complex IV Cytochrome c to 02 In the final step of the respiratory chain, Complex IV, also called cytochrome oxidase, carries electrons from cytochrome c to molecular oxygen, reducing it to H20. Complex IV is a large enzyme (13 subunits Mr 204,000) of the inner mitochondrial membrane. Bacteria contain a form that is much simpler, with only three or four subunits, but still capable of catalyzing both electron transfer and proton pumping. Comparison of the mitochondrial and bacterial complexes suggests that three subunits are critical to the function (Fig. 19-13). 

---