Tricarboxylic Acid TCA Cycle

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

With oxygen present, pyruvate is oxidized by the tricarboxylic acid cycle (TCA). 

The tricarboxylic acid cycle (TCA cycle, also known as the citric acid cycle or Krebs cycle) is a cyclic metabolic pathway in the mitochondrial matrix (see p. 210). in eight steps, it oxidizes acetyl residues (CH3-CO-) to carbon dioxide (CO2). The reducing equivalents obtained in this process are transferred to NAD"" or ubiquinone, and from there to the respiratory chain (see p. 140). Additional metabolic functions of the cycle are discussed on p. 138. 

Alternate fates of pyruvate Compounds other than lactate to which pyruvate can be converted ALTERNATE FATES OF PYRUVATE (p. 103) Pyruvate can be oxidatively decarboxylated by pyruvate dehydrogenase, producing acetyl CoA—a major fuel for the tricarboxylic acid cycle (TCA cycle) and the building block for fatty acid synthesis. Pyruvate can be carboxylated to oxaloacetate (a TCA cycle intermediate) by pyruvate carboxylase. Pyruvate can be reduced by microorganisms to ethanol by pyruvate decarboxylase. 

Reactions of the TCA cycle Enzyme that oxidatively decarboxylates pyruvate, its coenzymes, activators, and inhibitors REACTIONS OF THE TRICARBOXYLIC ACID CYCLE (p. 107) Pyruvate is oxidatively decarboxylated by pyruvate dehydrogenase complex producing acetyl CoA, which is the major fuel for the tricarboxylic acid cycle (TCA cycle). The irreversible set of reactions catalyzed by this enzyme complex requires five coenzymes thiamine pyrophosphate, lipoic acid, coenzyme A (which contains the vitamin pantothenic acid), FAD, and NAD. The reaction is activated by NAD, coenzyme A, and pyruvate, and inhibited by ATP, acetyl CoA, and NADH. 

The material within the cell membrane is gellike and is termed the cytoplasm or cytosol. It is composed of ions, water, soluble proteins, and enzymes that are involved in generation of energy in the form of ATP by a process termed the tricarboxylic acid cycle (TCA) or Krebs cycle in the absence of oxygen (Figure 1.4). It is also involved in activation of amino acids for protein synthesis (Table 1.5 and Figure 1.4). 

Tricarboxylic acid cycle (TCA cycle - Krebs cycle - citric acid cycle)... 

When yeast cells grow aerobically, they can oxidize the pyruvate entirely to carbon dioxide and water. These reactions take place in structures called mitochondria. The reactions form a cyclic scheme termed the tricarboxylic acid cycle (TCA cycle) or Krebs cycle in which in one cycle, the substrate, pyruvate, is converted entirely to carbon dioxide and water (Fig. 3-10). Both substrate level phosphorylation and electron transport mediated phosphorylation occur during the process. In electron... 

Fig. 3-10 The biochemical pathway of the tricarboxylic acid cycle (TCA cycle). Pyruvate, generated from glycolysis (Fig. 3-8), enters the cycle as acetyl-CoA (acetyl-coenzyme A), and is then degraded through a series of reactions to a 4-carbon compound, oxaloacetate. The energy resulting from the reactions is stored as ATP, which is produced through electron transport and oxidative phosphorylation (see Fig. 3-11).

Fluoroacetyl CoA is incorporated into the tricarboxylic acid cycle (TCA cycle) in an analogous manner to acetyl CoA, combining with oxaloacetate to give fluorocitrate (figure 7,39). However, fluorocitrate inhibits the next enzyme of the TCA cycle, aconitase, and there is a build-up of both fluorocitrate and... 

In class VI are ADP-Glc PPases from anaerobic bacteria Rhodospirillum, capable of growing in either hetero-trophic conditions in the dark or autotrophic conditions in the light under anoxygenic photosynthesis (Table 1). These organisms cannot catabolize glucose but grow very well on pyruvate and tricarboxylic acid cycle (TCA) intermediates. ADP-Glc PPases from class VI are specifically regulated by pyruvate (Table... 

The tricarboxylic acid cycle (tca cycle) provides a logical starting point. Relationships between carbon skeletons in that cycle are shown in Figure 16. The tca... 

Succinic acid, known as amber acid or butanedioic acid, is a four-carbon dicarboxylic acid produced as an intermediate of the tricarboxylic acid cycle (TCA) [1,2]. Succinic acid and its derivative have wide industrial applications such as the feedstock of food and pharmaceutical products, as the intermediate of chemical synthesis of surfactants, detergents, green solvents, and biodegradable plastics, and also as ingredients of animal feeds to stimulate animal and... 

Figure 8.2 Malate Dehydrogenase (MDH). (a) Reversible reaction catalyzed by MDH where NADH is nicotinamide adenine dinucleotide, reduced form (b) ribbon display structure of MDH (porcine heart) (side view) (pdb 4mdh). The homo-dimeric protein consists of two polypeptides chains (yellow and red), with nicotinamide adenine dinudeotide (NAD+) in both independent catalytic sites illustrated in a ball and stick (blue) representation (c) chemical illustration of the tricarboxylic acid cycle (TCA) to demonstrate the importance of MDH catalysis in cycle closure. Enzyme abbreviations are PDH, pyruvate dehyrogensase CS, citrate synthase.

After pyruvate enters the mitochondria, it encounters the second pathway, which is aerobic and cyclic, and is called the Krebs or tricarboxylic acid cycle TCA cycle). The process is illustrated in Fig. 22-3, and the structures of intermediates and enzymes are illustrated in Fig. 22-4. 

The metabolic intermediate (e.g., pyrnvate) nndergoes complete oxidation to CO2, throngh the pathway referred to as tricarboxylic acid cycle (TCA cycle). The following reactions are involved in TCA cycle. The hrst step involves conversion of pyrnvate to acetyl-CoA throngh decarboxylation and prodnction of NADH. The acetyl-CoA (2 carbon) combines with the fonr-carbon componnd oxalacetate, leading to the formation of citric acid (6 carbon). The TCA cycle is also referred to as citric acid cycle. A series of reactions inclnding dehydration, decarboxylation, and oxidation are involved in the conversion of citric acid to carbon dioxide. The electrons released are transferred to enzymes containing the coenzyme NAD+. 

Fig. 7. Proposed pathway for assimilation of ammonia produced by Nj reduction in the bacteroids and synthesis of asparagine by enzymes located in the plant fraction of lupin nodules. The utilization of photosynthate to provide oxaloacetate, NADH and ATP is also indicated. Phosphoenolpyruvate, PEP tricarboxylic acid cycle, TCA. (Based on Scott et al., 1976, and reproduced with permission.)...

Stage in in the oxidation of fnel molecnles (Section 12.6) begins when the two-carbon acetyl units (of acetyl CoA) enter the citric acid cycle. This process is called the citric acid cycle because one of the key intermediates is citric acid. However, it is also called the tricarboxylic acid cycle (TCA) in reference to the three carboxylic acid groups in citric acid, and the Krebs cycle in honor of Sir Hans A. Krebs, who deduced its reaction sequence in 1937 (see h Figure 13.6). 

The principal mechanism by which fatty acids are broken down in plants is P-oxidation (Fig. 2.11) (Kindi, 1987). On germination, seeds with high oil content form glyoxysomes that contain the enzymes of P-oxidation. In this process, much of the energy stored in the lipids is converted to acetyl-CoA and is trapped in the thioester bond. Acetyl-CoA then enters the tricarboxylic acid cycle (TCA). P-Oxidation in plants appears to be identical to that of animals. 

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