Citric acid cycle enzymes

The citric acid cycle is the final common pathway for the aerobic oxidation of carbohydrate, lipid, and protein because glucose, fatty acids, and most amino acids are metabolized to acetyl-CoA or intermediates of the cycle. It also has a central role in gluconeogenesis, lipogenesis, and interconversion of amino acids. Many of these processes occur in most tissues, but the hver is the only tissue in which all occur to a significant extent. The repercussions are therefore profound when, for example, large numbers of hepatic cells are damaged as in acute hepatitis or replaced by connective tissue (as in cirrhosis). Very few, if any, genetic abnormalities of citric acid cycle enzymes have been reported such ab-normahties would be incompatible with life or normal development. 

NO also has cytotoxic effects when synthesized in large quantities, eg, by activated macrophages. For example, NO inhibits metalloproteins involved in cellular respiration, such as the citric acid cycle enzyme aconitase and the electron transport chain protein cytochrome oxidase. Inhibition of the heme-containing cytochrome P450 enzymes by NO is a major pathogenic mechanism in inflammatory liver disease. 

FIGURE 16-22 Relationship between the glyoxylate and citric acid cycles. The reactions of the glyoxylate cycle (in glyoxysomes) proceed simultaneously with, and mesh with, those of the citric acid cycle (in mitochondria), as intermediates pass between these compartments. The conversion of succinate to oxaloacetate is catalyzed by citric acid cycle enzymes. The oxidation of fatty acids to acetyl-CoA is described in Chapter 17 the synthesis of hexoses from oxaloacetate is described in Chapter 20. 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

Fig. 3. Krebs citric acid cycle. Enzymes involved (1) Condensing enzyme (2) aconitase (3) isocitric acid (4) a-ketoglucaric acid dehydrogenase (4) a succinic acid thiokinasc (5) succinic acid dehydrogenase (6) fumarasc (7) malaic acid dehydrogenase. Abbreviations CA = citric acid ACOM = eij-aconitic acid KG = a-ketoglutaric acid SIC = succinic acid FA = fumaric acid MA = malic acid OA = oxalaceiic acid...

Answer The flavin nucleotides, FMN and FAD, would not be synthesized. Because FAD is required by the citric acid cycle enzyme succinate dehydrogenase, flavin deficiency would strongly inhibit the cycle. 

Answer Anaplerotic reactions replenish intermediates in the citric acid cycle. Net synthesis of a-ketoglutarate from pyruvate occurs by the sequential actions of (1) pyruvate carboxylase (which makes extra molecules of oxaloacetate), (2) pyruvate dehydrogenase, and the citric acid cycle enzymes (3) citrate synthase, (4) aconitase, and (5) isocitrate dehydrogenase ... 

In higher plants and animals, electron transport occurs within the mitochondria. Biochemists have devised methods to isolate intact mitochondria containing all of the functional citric-acid-cycle enzymes and electron-transport components. Such in-... 

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

Boquist, L., Boquist, S., Ericsson, I. (1988). Structural beta-cell changes and transient hyperglycemia in mice treated with compounds inducing inhibited citric acid cycle enzyme activity. Diabetes 37 89-98. 

S.J. Barnes and P.D. Weitzman. 1986. Organization of citric acid cycle enzymes into a multienzyme dusX t FEBS Lett. 201 267-270. (PubMed)... 

P.D.J. Weitzman. 1981. Unity and diversity in some bacterial citric acid cycle enzymes y4c(v. Microbiol. Physiol. 22 185-244. 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

The purification of the IRE-BP and the cloning of its cDNA were sources of truly remarkable insight into evolution. The IRE-BP was found to be approximately 30% identical in amino acid sequence with the citric acid cycle enzyme aconitase from mitochondria. Further analysis revealed that the IRE-BP is, in fact, an active aconitase enzyme it is a cytosolic aconitase that had been known for a long time, but its function was not well understood (Figure... 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

Within the sulphur-dependent thermophilic branch of the archaebacteria, the situation is less clear. Sulfolobus species are facultatively autotrophic [13] and many of the citric acid cycle enzymes have been reported (reviewed in ref. [1]). Thus, when growing heterotrophically in the presence of oxygen, an oxidative cycle may be operative, although respirometric analyses suggest that its use may be limited [13]. 

Metabolic acidosis is the outcome of several factors including accumulation of lactic and pyruvic acids due to toxic interference with citric acid cycle enzymes, and stimulation of lipid metabolism causing increased production of ketone bodies. Late toxic respiratory depression may also cause CO retention. 

Interestingly, cytosolic aconitase was recently shown to function also as an iron sensor. Earlier the cytosolic form of aconitase seemed to be an enzyme- unemployed since the majority of other citric acid cycle enzymes are absent from cytosol. It was found, however, that this enzyme plays a crucial role in regulating both the iron delivery to the cell and iron storage [5]. 

Braun-Falco and Petzoldt (B30), Rassner (Rl), and Halprin and Ohkawara (H2) presented extensive surveys of the glycolytic, pentose shunt, and citric acid cycle enzymes in normal skin, unaffected skin of psoriatic patients, and in the lesions themselves. Figure 10 is reprinted from this work (H2) and shows the reorganization of the cellular metabolic activity made necessary by the high synthetic and mitotic rate of the tissue. 

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