Succinate dehydrogenase enzymic properties

Moleculab and Enzymic Properties op Representative Preparations op Succinate Dehydrogenase... 

Study of the reaction mechanism of succinate dehydrogenase has. been complicated, partly because of the activation-deactivation properties of the enzyme, and partly because most of the early preparations studied had low iron content and low activities. Kinetic studies with activated, soluble preparations have led Zeylemaker et al. [215) to propose the fol-... 

Hirst, J., Sucheta, A., Ackrell, B. A. C., and Armstrong, F. A., 1996, Electrocatalytic Voltammetry of Succinate Dehydrogenase Direct Quantification of the Catalytic Properties of a Complex Electron-Transport Enzyme, J. Am. Chem. Soc. 118(21) 503195038. 

The formation of succinate from fumarate by fumarate reductase (FR) and anaerobic phosphorylation of ADP to ATP are amongst the important reactions taking place in the mitochondria to provide energy to the helminths. The FR system of helminths differs from succinate dehydrogenase (SDH) of mammalian tissues in several ways. For example (a) FR requires NAD while SDH utilizes flavin nucleotide (FAD) as the coenzyme (b) FR acts only in one direction but SDH is a reversible enzyme (c) FR acts as the terminal electron acceptor under anaerobic conditions while SDH has no such property. Levamisole was shown to inhibit the FR in Ascaris [43]. 

In addition, feedback inhibition has two other characteristics that would be unlikely to arise haphazardly in longer pathways there is often very little structural resemblance between the end product that inhibits and the substrate of the inhibited enzyme and the inhibition is usually cooperative. Let us consider what these two properties mean. When inhibition arises for no obvious biological reason, it is usually for an obvious enough chemical reason the substrate and inhibitor are similar enough in terms of chemical structure that the inhibitor can bind to the same site on the enzyme as the one where the substrate binds however, as it lacks some feature necessary for the chemical reaction it does not react but does nothing. A classical example is provided by the enzyme succinate dehydrogenase, which uses succinate as its substrate but is inhibited by malonate (Figure 10.3). Succinate and malonate have almost the same chemical structures, so either is likely to bind to a site intended for... 

Upon purification, the K -stimulated phosphatase activity is always copurified with the (K )-ATPase activity [63-65]. Mitochondrial markers, such as cytochrome c oxidase, succinate dehydrogenase, monoamino-oxidase, and the ribo-somal marker RNA are largely removed by the purification procedure. The same is true for the anion-sensitive ATPase and 5 nucleotidase activities, but some (Na — K )-ATPase activity is still present in highly purified (K" -I-H )-ATPase preparations. Purification is also characterised by a lowering of the K -insensitive Mg ATPase activity, but even in the purest preparations some Mg -ATPase activity (4% of (K -I- H )-ATPase activity) is still present. This may represent an impurity or an inherent property of the enzyme. 

Succinic dehydrogenase catalyzes the transfer of electrons from succinate to fumarate. Although in the cell the enzyme is tightly bound to mitochondria, it has been prepared in a soluble form from heart muscle and yeast. The enzyme probably contains SH groups and one flavin molecule for every protein molecule. The property of that enzyme is reviewed in more detail in the section on electron transport. 

NADH cytochrome c reductase was isolated from pigeon breast and pig heart muscle. The enzyme was shown to contain four atoms of iron per flavin molecule. NADH cytochrome c reductase, like succinic dehydrogenase, is a ferroflavoprotein. The ratio of iron to flavin is four. The enzyme contains sulfhydryl groups that can be titrated by classical methods, but their oxidation has no effect on the enzymatic activity. In contrast, the removal of the metal leads to a decrease in the ability of the enzyme to reduce cytochrome c. As for succinic dehydrogenase, the structure of the flavin in NADH cytochrome c reductase is not clear. It was demonstrated that it is not flavin mononucleotide, but the identity of the flavin component with flavin adenine dinucleotide is not established in fact, the flavin component differs from the classical FAD by its chromatographic properties and its behavior in enzymic assays. It is not known if it is a structural variation of the flavin nucleotide or if the nucleotide is conjugated to a peptide. 

Metallo-Flavoproteins. As was mentioned in the case of cytochrome reductase, enzymes are known that contain metal cofactors in addition to flavin. These are called metallo-flavoproteins. The presence of metals introduces complexity into the reaction, since the metals involved, iron, molybdenum, copper, and manganese, all exist in at least two valence states and can participate in oxidation-reduction reactions. The enzymes known to be metallo-flavoproteins include xanthine oxidase, aldehyde oxidase, nitrate reductase, succinic dehydrogenase, fatty acyl CoA dehydrogenases, hydrogenase, and cytochrome reductases. Before these are discussed in detail some physical properties of flavin will be presented. 

The succinoxidase system has been described as a particle containing cytochromes b, c, and a in addition to succinic dehydrogenase and any other (unidentified) enzymes needed for oxygen consumption. The oxidation of reduced DPN requires similar factors when particulate preparations are used. The role of cytochrome b in these systems has not been defined to the satisfaction of all. A difficulty in assigning a place to cytochrome b lies in the nature of the kinetic experiments and the assumptions made about the properties of the systems. It is not questioned that this cytochrome is reduced by DPNH or by succinate the question is... 

More than 100 years ago a fluorescent compound was isolated first fi om whey, and later from different biological materials. When it Ijecame clear that the isolated yellow pigments, named lactochrome, ovoflavin, or lactoflavin, had a common structure, the new compound was named riboflavin (vitamin B2) (for historical review see 2). In the years between 1933 and 1935 the structure and the main chemical reactions of riboflavin were studied and the chemical synthesis was performed. Soon afterward, the coenzyme forms, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), were isolated in pure form, and the structures were determined. In the last 50 years many flavoproteins were isolated and their physicochemical properties were studied. Succinate dehydrogenase was the first enzyme found with the prosthetic group (FAD) covalently bound to the protein. About 20 flavoproteins are now known to contain covalently bound coenzyme (mainly via carbon atom 8a) (3). In mammalian tissue, the number of covalently bound flavoproteins appears to be limited. 

Oxidation could inhibit specific enzymes or alter the permeability properties of structural membranes. The activities of several oxidases have been reported to be markedly reduced in liver homogenates of vitamin E-deficient rats. Among them are alcohol dehydrogenase, gulonolactone oxidase, and the oxidation of such Krebs cycle substrates as a-ketoglutarate and succinate. It has been proposed that the alteration in the activity of these enzymes does not result from changes in the molecule per se, but rather from the... 

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