Membrane-bound enzyme complexe

Superoxide is produced by the NADPH oxidoreduc-tase (oxidase), which is a membrane-bound enzyme complex containing a flavoprotein that catalyses the transfer of single electrons from NADPH in the cytosol to extracellular oxygen. NADPH is mainly supplied by the hexose monophosphate shunt. In resting cells, the oxidase complex is inactive and disassembled, but is rapidly reconstituted and activated by chemotactic mechanisms or phagocytosis (Baggiolini and Thelen, 1991). 

Fatty acyl-CoA desaturases are terminal oxidases of a membrane-bound enzyme complex that also includes cytochrome b5 and cytochrome b5 reductase (Bloomfield and Bloch, 1960). They remove substrate hydrogen atoms at a position determined by the specificity of the enzyme. They play essential roles in regulating membrane fluidity and are also involved in insect lipid and pheromone metabolism. They share the presence of three highly conserved histidine-rich sequences (H-boxes) that coordinate the diiron-oxo structure at the active sites (Shanklin and Cahoon, 1998) and four hydrophobic a helices that appear to anchor the protein into the lipid bilayer and situate the H-boxes in their correct position in the active site. 

Heath I.B. 1974. A unified hypothesis for the role of membrane bound enzyme complexes and microtubules in plant cell wall synthesis. J Theor Biol 48 445 149. 

Type III synthetases are responsible for the chain lengthening of preformed fatty acids and are also called elongase enzymes. Most fatty acid elongases are membrane-bound enzyme complexes located on the endoplasmic reticulum. Like the Type I and Type II fatty acid synthetases they use malonyl-CoA as the source of 2C addition units. However, some elongation activity occurs in mitochondria where acetyl-CoA is used for additionally a reversal of -oxidation. 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

The complex activates a G-protein, which then activates a membrane-bound enzyme (e.g. adenyi cyclase, which converts ATP to cyclic AMP Figure 12.6). 

Regulatory enzymes containing multiple polypeptide chains are just beginning to be understood in molecular terms. Considerably more thermodynamic, kinetic, and structural information is required. Several multienzyme complexes are available in a reasonably pure state, but the molecular characterization of their mechanisms is still in a rather primitive state. The situation is even more difficult with membrane-bound enzymes. A few of these enzymes can be obtained as well-defined entities, but in many cases purification of the enzyme system and all its components is quite far off in the future. The small quantity of material usually available is also a great problem with these systems. As might be... 

This cytochrome oxidase ( o is an abbreviation for oxidase) is widely distributed and may completely or partly replace cytochrome oxidase aa3 under conditions where the supply of dioxygen is limited. It is a membrane-bound enzyme which has proved difficult to purify, and whose spectral characteristics are those of b cytochromes. Its identity is usually confirmed by observation of the distinctive spectral features of its complex with carbon monoxide. 

Mucoadhesive polymers exhibiting strong complexing properties are capable of inhibiting intestinal brush border membrane-bound proteases through a far distance inhibitory effect [65]. In vivo, the mucoadhesive polymer is separated from the brush border membrane by a mucus layer [30]. Although there is no direct contact between polymer- and membrane-bound enzymes, it could be shown that inhibition takes place. The exploitation of this far distance effect seems to be a very promising alternative to small molecular mass inhibitors, which are currently used as inhibitors of brush border membrane-bound proteases. 

Several possible mechanisms can be envisaged to explain how Atjt may modulate the activity of the cellulose-synthetase complex. The effects of membrane fluidity on membrane-bound enzymes is well... 

Acyl-CoA dehydrogenase is a flavin-linked, membrane-bound enzyme, associated with the mitochondrial respiratory complexes. When FADH2 is produced, it is oxidized by the respiratory chain (Chap. 14). Hydroxyacyl-CoA dehydrogenase is in the mitochondrial matrix, and the NADH produced by the action of this enzyme on hydroxyacyl-CoA compounds contributes to the pool of NADH in the matrix. It is also oxidized by the respiratory chain (Chap. 14). 

A number of enzymes appear therefore to be localized in a specific micro-environment, which can influence. their biocatalytic activity. Because of the complexity of biological membranes, our understanding of the influence of micro-environmental effects on membrane-bound enzyme is minimal. An important contribution to better understanding the mode of action of membrane-bound enzyme has been the development of the concept of heterogeneous catalysis by enzymes synthetically bound to water-insoluble supports. These immobilized enzymes were viewed as models for the cellular bound enzyme (3,A). 

During cell fractionation, nearly all the cyclopenase activity was found in a fraction containing the cell wall together with the cytoplasmic membrane (83,84). From this fraction, the enzyme could be partially solubilized by detergents (e.g., Triton X-100) to yield a protein-phospholipid complex. By treatment with M-butanol, the solubilized enzyme preparation was split into the lipid fraction and the enzyme protein which retained a considerable part of the total enzyme activity. Compared with that of the membrane-bound enzyme, the substrate affinity of the solubilized protein-lipid complex was decreased, whereas... 

As indicated in Sections 1 and 2, succinate is an electron donor widely utilized for NAD(P) reduction by phototrophic purple bacteria. The membrane-bound enzyme responsible for succinate oxidation has been solubilized and partially characterized in the purple non-sulfur bacteria R. rubrum [73,74] and Rhodopseudo-monas sphaeroides (recently renamed Rhodobacter sphaeroides) [57]. In situ characterization of the iron-sulfur centers likely to be associated with succinate dehydrogenase has been accomplished for Rps. capsulata [59] and C. vinosum [51]. Of particular interest is the presence of a succinate-reducible [51,57,58,73] and fu-marate-oxidizable [51] iron-sulfur cluster with near +50 mV that, like center S-3 [60,61,75,76] of mitochondrial succinic dehydrogenase (Complex II), is paramagnetic in the oxidized state. The enzyme in phototrophic bacteria also appears to have one or two ferredoxin-like (i.e., paramagnetic in the reduced state) iron-sulfur centers that correspond to centers S-1 (succinate-reducible, EJ ranging from... 

This reaction is catalyzed by a complex of three membrane-bound enzymes NADH-cytochrome h reductase, cytochrome b, and a desaturase (Figure 22.29). First, electrons are transferred from NADH to the FAD moiety of NADH-cytochrome b 5 reductase. 

Fig. 3.10. Topography of Complex III. Complex III is a dimer in the two-dimensional crystal form studied by electron microscopy. The shape of the membrane-bound enzyme particle was resolved by image reconstruction of micrographs [230]. The location of various components was predicted by comparing crystals of Complex III with those of a subcomplex lacking the Rieske FeS protein and the core proteins [222,231]. The schematic figure is adapted from Li et al. [222].

An interesting topographic feature of Complex I is that although the flavoprotein and FeS protein subcomplexes are soluble in water, each appears to be buried in the intact membrane-bound enzyme. Thus, the three flavoprotein subunits are inaccessible to surface labelling, and some of the proteins in the ISP fraction are probably transmembranous [289,299]. The corollary of this would be that the enzyme may have a hydrophilic core within the membrane, which is surrounded by hydrophobic protein components (Fig. 3.13B). 

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