Krebs cycle enzymes

Because the number/ activity of the mitochondria in a given tissue readily varies under several conditions, particularly in the case of an OXPHOS deficiency, it is generally a good idea to quantify the activity of several marker enzymes. Krebs cycle enzymes have often been measured as reference enzymes to estimate variation in the number of mitochondria and it has been widely assumed that the activity of these enzymes are not affected in the case of an OXPHOS deficiency. Citrate synthase (EC... 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

After Gerty s death in 1957, Carl continued his work on the mechanism of enzymes involved in carbohydrate metabolism and remarried in 1960. He continued to pubhsh the results of his research into the early 1980s xmtil shortly before his death in 1984. see also Enzymes Krebs Cycle. 

Although not completely functional, certain enzymes Krebs cycle are, active under anaerobic conditions (Fig. 1.11). These additional pathways are very important to Saccharomyces during fermentation because NADH can be reoxidized and other precursors important for cellular functions synthesized (e.g., a-keto-glutarate involved in NH4 assimilation). 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The citric acid cycle, a nine-step process, also diverts chemical energy to the production of ATP and the reduction of NAD and FAD. In each step of the citric acid cycle (also known as the Krebs cycle) a glucose metabolite is oxidized while one of the carrier molecules, NAD or FAD, is reduced. Enzymes, nature s chemical catalysts, do a remarkable job of coupling the oxidation and reduction reactions so that energy is transferred with great efficiency. 

Carboxylation of propionyl-CoA is accomplished by propionyl-CoA carboxylase (biotin, which is the carboxyl group carrier, serves as a coenzyme for this enzyme) the presence of ATP is also required. The methylmalonyl-CoA formed is converted by methylmalonyl-CoA mutase (whose coenzyme, deoxyadenosylcobalamin, is a derivative of vitamin B]2) to succinyl-CoA the latter enters the Krebs cycle. 

There are also voices critical of the rTCA cycle Davis S. Ross has studied kinetic and thermodynamic data and concludes that the reductive, enzyme-free Krebs cycle (in this case the sequence acetate-pyruvate-oxalacetate-malate) was not suitable as an important, basic reaction in the life evolution process. Data on the Pt-catalysed reduction of carbonyl groups by phosphinate show that the rate of the reaction from pyruvate to malate is much too low to be of importance for the rTCA cycle. In addition, the energy barrier for the formation of pyruvate from acetate is much too high (Ross, 2007). 

Defects of the Krebs cycle. Fumarase deficiency was reported in children with mitochondrial encephalomyop-athy. Usually, there is developmental delay since early infancy, microcephaly, hypotonia and cerebral atrophy, with death in infancy or early childhood. The laboratory hallmark of the disease is the excretion of large amounts of fumaric acid and, to a lesser extent, succinic acid in the urine. The enzyme defect has been found in muscle, liver and cultured skin fibroblasts [16]. 

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. 

Similarly with regard to the fluoroacetate story, other factors in addition to the jamming of the Krebs cycle may be looked for. One profitable line may well be the examination of the mobility, by chemical and enzyme methods, of the firmly... 

The Enzyme Aconitase. The enzyme aconitase catalyzes the elimination or addition of water in the second step of the citric acid (Krebs) cycle, catalyzing the interconversion of citrate and isocitrate via cix-aconitate. See reference 8, pages 190-196, Figure 7.49, and equation 7.13. 

Figure 6.4 Glycolysis divided into five stages. It is not clear whether the reaction in which pyruvate is converted to acetyl-CoA, catalysed by pyruvate dehydrogenase, should be classified as a glycolytic enzyme or an enzyme of the Krebs cycle. Since the enzyme presents acetyl-CoA to the cycle, it is considered in this book as an enzyme of glycolysis (i.e. aerobic glycolysis).

Figure 7.13 Physiological pathway for fatty acid oxidation. The pathway starts with the hormone-sensitive lipase in adipose tissue (the flux-generating step) and ends with the formation of acetyl-CoA in the various tissues. Acetyl-CoA is the substrate for the flux-generating enzyme, citrate synthase, for the Krebs cycle (Chapter 9). Heart, kidney and skeletal muscle are the major tissues for fatty acid oxidation but other tissues also oxidise them.

Figure 9.2 Summary of reactions of the Krebs cycle. The names of the enzymes are dtrate synthase, aconitase, isodtrate dehydrogenase (there are two enzymes, one ubTizes NAD as the cofactor, the other NADPT it is assumed that the NAD -specific enzyme is that involved in the cycle), oxoglutarate dehydrogenase, sucdnyl CoA synthetase, succinate dehydrogenase, fumarate hydratase, malate dehydrogenase.

Table 9.2 Substrate concentrations and K values for some Krebs cycle enzymes...

Figure 9.4 Reactions of glutaminolysis the pathway for glutamine oxidation. Reaction 1 is catalysed by glutaminase, reaction 2 by glutamate aminotransferase, and reaction 8 by aspartate aminotransferase all other enzymes are those of the Krebs cycle (3-7). (See also Chapter 8).

Figure 9.23 Properties of the three enzymes that control the flux through the Krebs cycle. During physical activity. The CoASH/ succinyl CoA concentration ratio increases whereas that of ATP/ADP ratio decrease. These changes increase the flux through the cycle.

Ornithine prodnction by the enzyme arginase is essential for maintenance of flux through the cycle, so that an increase in the concentration of ornithine shonld increase the flnx. This mechanism is analogons to regn-lation of the Krebs cycle by changes in the concentration of oxaloacetate (Chapter 9, Fignre 9.23). 

Fructose is converted to fructose 6-phosphate by hexo-kinase, which phosphorylates both glucose and fructose. It then is converted to pyruvate via glycolysis, which is converted to acetyl-coenzyme-A in the mitochondria for oxidation by the Krebs cycle. The enzyme that is specific for fructose metabolism, fructokinase, has not been found in... 

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