Membrane-bound enzymes coupling with transport

The H" "-translocating ATPase ( ATP-synthase ) from chloroplasts is a membrane-bound enzyme which can couple a transmembrane proton transport with ATP synthesis/hydrolysis. 

Table 8 relates the specific principles of metabolic control to the hierarchical organization of metabolic control processes. The principles of metabolic control ascend through echelons of organizational complexity within hydrophobic and hydrophilic systems. As cumulative acquisitions of functions occur in the hydrophobic and hydrophilic systems, various metabolic functions emerge, with their attendent control mechanisms. In the hydrophobic system, there appear membrane systems, membrane transport, hormonal membrane receptors coupled to signal-molecule generating systems, membrane-bound enzymes, elec-... 

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

Generally, the assimilatory nitrate and nitrite reductases are soluble enzymes that utilize reduced pyridine nucleotides or reduced ferrodoxin. In contrast, the dissimilatory nitrate reductases are membrane-bound terminal electron acceptors that are tightly linked to cytochrome by pigments. Such complexes allow one or more sites of energy conservation (ATP generation) coupled with electron transport. 

DNA. Steroid receptors have been reported in mitochondria (17) and cell membranes, although it is not yet clear whether all of these receptors are the same as the intracellular steroid receptors (16, 18-22 vs. 23). Some of the membrane-bound receptors for steroids are G protein-coupled receptors (24—27). A recent report suggests that membrane-bound steroid receptors can interact with, and augment the transcriptional activity of, the intracellular receptors (24). Finally, steroids can bind to nonreceptor molecules such as enzymes and transport proteins (see above), which may have yet undiscovered consequences. 

CH3-S-C0M is reduced to methane via the heterodisulfide of H-S-CoM and H-S-HTP. The reduction of the heterodisulfide has been shown to be coupled with ATP synthesis according to a chemiosmotic mechanism (see above). The electrons required for the reduction are derived from the oxidation of enzyme-bound CO ([CO]) which is oxidized to CO2 via CO-DH. It is assumed that electron transport from [CO] to the heterodisulfide is coupled with the generation of an electrochemical proton potential which then drives ATP synthesis. Possible eleetron transport components, a cytochrome b and a membrane-bound hydrogenase, have been identified [232]. Probably two H" -translocating sites are present in electron transport from CO to the heterodisulfide the oxidation of CO to CO2 and H2, and the reduction of the heterodisulfide (or methyl-CoM) by H2. Both H2 and... 

Regio- and stereospecific monohydroxylations of elaborated molecules have been achieved with monooxygenases that are in most cases membrane-bound cytochrome P450 enzymes. Because these are coupled with a complex redox system for electron transport from NADPH, the reactions are preferably carried out with whole cells [52], The same is true for enantioselective enzymatic epoxida-tions. An impressive example documented in eq. (6) is the preparation of 18 in 78 % yield and high ee [53]. 

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