Citric acid cycle individual reactions

Figure 29.12 MECHANISM The citric acid cycle is an eight-step series of reactions that results in the conversion of an acetyl group into two molecules of C02 plus reduced coenzymes. Individual steps are explained in the text.

A total of eight enzyme-catalyzed reactions are involved in the citric acid cycle. We are not going to be concerned with the details or individual reactions. One complete turn of the cycle carries out the following overall reaction ... 

Although the individual reactions of the citric acid cycle were initially worked out in vitro, using minced muscle tissue, the pathway and its regulation have also been studied extensively in vivo. By using radioactively la-... 

As a result of metabolic reactions an isotope may appear at more than one position in a product, yielding two or more isotope isomers or isotopomers. These are seen individually by NMR spectroscopy and the concentration and isotope labeling patterns of the labeled compounds can be followed over a period of time. The use of this isotopomer analysis in studies of the citric acid cycle is illustrated in Box 17-C and its use in studies of glucose metabolism is considered in Chapter 17, Section L. 

A second competitive pathway for the disposal of PA requires the initial conversion of PA into tyrosine. This reaction is catalyzed by the enzyme PAH (phenylalanine-4-monooxygenase EC 1.14.16.1). The resulting tyrosine molecule can then be catabolized into fumarate and ace-toacetate. Both products are nontoxic and can be further catabolized in the citric acid cycle. In Mrs. Urick and the majority of individuals suffering from HPA and PKU, there is a defect in the PAH enzyme system (NIH Consensus State-... 

The overall consumption of one molecule of acetyl-CoA in the citric acid cycle is an exergonic process AG° = —60 kJ mol-1. All but two of the individual reactions are exergonic. Step 2 (citrate— isocitrate) and step 8 (malate —>oxaloacetate) are endergonic (Fig. 12-3). 

The acetyl CoA formed can now enter the citric-acid cycle (also known as the tricarboxylicacid cycle and the Krebs cycle, after the individual who discovered this pathway). The first step is a condensation between oxaloacetate and acetyl CoA to form citrate, catalyzed by citrate synthase. This reaction is also physiologically unidirectional. The coenzyme A released from acetyl CoA can then enter the general coenzyme A pool and participate once more in pyruvate dehydrogenase and other related reactions. 

Step 6. Formation of Fumarate—FAD-Linked Oxidation Succinate is oxidized to fumarate, a reaction that is catalyzed by the enzyme succinate dehydrogenase. This enzyme is an integral protein of the inner mitochondrial membrane. We shall have much more to say about the enzymes bound to the inner mitochondrial membrane in Ghapter 20. The other individual enzymes of the citric acid cycle are in the mitochondrial matrix. The electron acceptor, which is FAD rather than NADA is covalently bonded to the enzyme succinate dehydrogenase is also called a flavoprotein because of the presence of FAD with its flavin moiety. In the succinate dehydrogenase reaction, FAD is reduced to FADHo and succinate is oxidized to fumarate. 

Acetyl CoA feeds into a remarkable biochemical sequence, variously known as the tricarboxylic acid (TCA) cycle, the citric acid cycle or, after its discoverer, the Krebs cycle (Fig. 14.1). It is called a cycle because it does indeed end up where it started, at a compound called oxaloacetate. Oxaloacetate combines with acetyl CoA, releasing the CoA and forming citric acid (so named because plentiful in citrus fruit juices). A sequence of seven further reactions takes citrate back to oxaloacetate. What has happened is that the two carbons in the acetyl group of acetyl CoA have been taken up (into citrate) and two carbons have been separately released as CO2 in the reactions of the cycle. If we tracked individual carbon atoms like ringed birds (as we can with radioactive isotopes - see Chemistry X), we would find that the cycle does not release precisely the same two carbon atoms that entered, but that need not concern us. In net chemical terms, it does not matter where the atoms came from. At the end of the cycle, the amounts of aU the TCA cycle intermediates are unchanged, and in effect an acetyl group has been oxidised to 2CO2 (see Appendix 8). 

Figure 5. Schematic diagrams of the sequential labeling of the individual carbon atoms of different intermediates of the citric acid cycle. A shows reactions of acetyl-CoA C-2 (AC2) and B shows reactions of acetyl-CoA C-1 (Acj) with i -C-labeled and unlabeled isoptomers of oxalacetate. The abbreviations used to denote the different intermediates are O, C, K, S, and M, for oxalacetate, citrate, a-ketoglutarate, succinate and malate, respectively.

This intricate web of vital biochemical reactions is referred to as primary metabolism. It is often displayed usefully in chart form [4], and to the eye appears very much like an advanced model railway layout, not least because of the way primary metabolism proceeds in cycles (e.g. the citric acid cycle). The organic compounds of primary metabolism are the stations on the main lines of this railway, the compounds of secondary metabolism the termini of branch lines. Secondary metabolites are distinguished more precisely from primary metabolites by the following criteria they have a restricted distribution being found mostly in plants and micro-organisms, and are often characteristic of individual genera, species, or strains they are... 

The 0-protection of A -protected amino acids 55 was achieved by decarboxylative esterification in solvent-free conditions by Colacino et al. (Scheme 4.13). It was found that the use of planetary ball mill was more effective than vibratory mill [8], Commonly used 0-activation reagents (diaUcyl dicarbonate (B0C2O), carbonate (A, A -disuccinimidyl carbonate, DSC), and alkyl chloroformates (ROCOCl, R=Bn, Et, allyl)) were employed in combination with DMAP as base. Reaction parameters had to be optimized for each individual reagent to achieve acceptable yields (selection of results. Table 4.3). Due to high reactivity of benzyloxy chloroformate (Z-Cl), the two-step cycled milling was executed by addition of Z-Cl in 2equiv. portions, so as to consume the chloroformate and reduce the formation of the undesired byproducts. Acidic workup with 10% aqueous citric acid of ether extracts eliminates DMAP and affords the A-protected amino ester derivatives 56 in good yields. 

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