Citric acid cycle enzyme complexes

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). 

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 succinate dehydrogenase complex (complex II) consists primarily of the citric acid cycle enzyme succinate dehydrogenase and two iron-sulfur proteins. Complex II mediates the transfer of electrons from succinate to UQ. The... 

Complex II (succinate dehydrogenase) - Complex II is not in the path traveled by electrons from Complex I (Figure 15.3). Instead, it is a point of entry of electrons from FADH2 produced by the enzyme succinate dehydrogenase in the citric acid cycle. Both complexes I and II donate their electrons to the same acceptor, coenzyme Q. Complex II, like complex I, contains iron-sulfur proteins, which participate in electron transfer. It is also called succinate-coenzyme Q reductase because its electrons reduce coenzyme Q. 

The conversion occurs through a multistep sequence of reactions catalyzed by a complex of enzymes and cofactors called the pyruvate dehydrogenase complex. The process occurs in three stages, each catalyzed by one of the enzymes in the complex, as outlined in Figure 29.11 on page 1152. Acetyl CoA, the ultimate product, then acts as fuel for the final stage of catabolism, the citric acid cycle. All the steps have laboratory analogies. 

When induced in macrophages, iNOS produces large amounts of NO which represents a major cytotoxic principle of those cells. Due to its affinity to protein-bound iron, NO can inhibit a number of key enzymes that contain iron in their catalytic centers. These include ribonucleotide reductase (rate-limiting in DNA replication), iron-sulfur cluster-dependent enzymes (complex I and II) involved in mitochondrial electron transport and cis-aconitase in the citric acid cycle. In addition, higher concentrations of NO,... 

Fig. 5.7. In green sulfur bacteria and in some archaebacteria, a reverse citric acid cycle is used for the assimilation of C02. It must be assumed that this was the original function of the citric acid cycle that only secondarily took over the role as a dissimulatory and oxidative process for the degradation of organic matter. A major enzyme here is 2-oxoglutarate ferredoxin for C02 fixation. Note that it, like several other enzymes in the cycle, uses Fe/S proteins. One is the initial so-called complex I which has eight different Fe/S centres of different kinds but no haem (see also other early electron-transfer chains, e.g. in hydrogenases).

In the EPR of mammalian cells, we do not see much in addition to the signals from the respiratory complexes. The enzyme aconitase from the citric-acid cycle can be detected, and also the protein cytoplasmic aconitase, later identified as the mRNA translation regulatory factor iron regulatory protein IRP-1, which actually started its career in biochemistry as an EPR signal that could not be assigned to the respiratory chain (Kennedy et al. 1992). 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

The PDH complex of mammals is strongly inhibited by ATP and by acetyl-CoA and NADH, the products of the reaction catalyzed by the complex (Fig. 16-18). The allosteric inhibition of pyruvate oxidation is greatly enhanced when long-chain fatty acids are available. AMP, CoA, and NAD+, all of which accumulate when too little acetate flows into the citric acid cycle, allosterically activate the PDH complex. Thus, this enzyme activity is turned off when ample fuel is available in the form... 

The mitochondrial matrix, enclosed by the inner membrane, contains the pyruvate dehydrogenase complex and the enzymes of the citric acid cycle, the fatty... 

This reaction looks simple but actually occurs in four discrete steps that involve a complex of enzymes having a molecular weight of about 4,500,000. We shall pass over this interesting and rather well-studied reaction as we describe the citric acid cycle. A simplified representation of the citric acid cycle is shown in Figure 20-10, and it will help to refer to this diagram as each of the steps in it are discussed in more detail. 

The classic example of competitive inhibition is inhibition of succinate dehydrogenase, an enzyme, by the compound malonate. Hans Krebs first elucidated the details of the citric acid cycle by adding malonate to minced pigeon muscle tissue and observing which intermediates accumulated after incubation of the mixture with various substrates. The structure of malonate is very similar to that of succinate (see Figure 1). The enzyme will bind malonate but cannot act further on it. That is, the enzyme and inhibitor form a nonproductive complex. We call this competitive inhibition, as succinate and malonate appear to compete for the same site on the enzyme. With competitive inhibition, the percent of inhibition is a function of the ratio between inhibitor and substrate, not the absolute concentration of inhibitor. 

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