Tricarboxylate cycle reactions

Lactate and alanine enter as pyruvate following the activities of lactate dehydrogenase (Figure 11.4) and alanine aminotransferase (Section 16.3). The first of the bypass reactions, the objective of which is to overcome the unfavourable energetics of a reversal of the pyruvate kinase reaction, seems a tortuous route (Figure 11.11). The reaction sequence relies on two important enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Since pyruvate carboxylase is located exclusively in the mitochondrion, pyruvate must cross the inner mitochondrial membrane (Section 12.2). Oxaloacetate produced by pyruvate carboxylase cannot traverse the inner membrane and is reduced by malate dehydrogenase into l-malate. This step is the reversal of the tricarboxylate cycle reaction (Section 12.4). Malate may, of... 

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic acid... 

Referring to reactions, pathways, or processes that replenish or add to intermediates of a metabolic cycle (usually the tricarboxylic cycle). The process itself is referred to as anaplerosis. 

Fig. 2.18 Simplified scheme of the cycle/aUocation of magnesium in a green plant. Mg is involved - inter aha - in making peptide bonds, in tricarboxylate cycle, hydrolysis of molecules and binding of CO (ribulose-bisphosphatecarboxylase/oxidase or PEP carboxylase in plants which also employs Mg or Mtf+ in some plants (Kai et al. 2003)). Reaction steps in which Mg takes part as biocatalyst are marked by broken lines/arrows. Citrate and other intermediates of the tricarboxylate cycle, particularly malate, are employed by higher plants for extraction of essential metals, including Mg, Fe and Mn (thus the closed loop) from soil via and by means of the roots. This closed loop depicts a manner of autocatalysis. Amino acids which are required for protein biosynthesis are produced by reductive amination from tricarboxylate cycle intermediates and other 2-oxoadds which likewise eventually...

COs to form oxalacetate which under anaerobic conditions is reduced to malate. The malate in turn may be converted to fumarate and succinate (Fig, 5). The last step in this series of reactions is blocked by malonate. The second pathway involves the aerobic condensation of pyruvate and oxalacetate followed by oxidation of the condensation product to form -ketoglutarate and succinate. Wood has proposed that the first condensation product of the aerobic tricarboxylic cycle is cfs-aconitic acid which is then converted to succinate by way of isocitric, oxalosuccinic, and a-ketoglutaric acids. The a-ketoglutarate is decarboxylated and oxidized to succinic acid. Isotopic a-ketoglutarate containing isotopic carbon only in the carboxyl group located a to the carbonyl would be expected to yield non-isotopic succinate after decarboxylation. This accounts for the absence of isotopic carbon in succinate isolated from malonate-poisoned liver after incubation with pyruvate and isotopic bicarbonate. 

The reaction that uses most of the acetyl-CoA formed by pyruvic acid is the condensation of acetic acid and the keto form of oxaloacetic acid to yield citric acid by forming a carbon-to-carbon bond between the methyl carbon of acetyl-CoA and the carbonyl carbon of oxaloacetate. This reaction is interesting in more than one respect. In addition to the fact that the reaction introduces the oxidation product of fatty acids, carbohydrates, and proteins into the tricarboxylic cycle by converting the 4-carbon chain of oxaloacetic acid into a 6-carbon chain (citric acid), the condensing enzyme presents a fascinating stereospecificity. 

It is generally accepted that only NADPH would be able to supply the hydrogen necessary for lipogenesis in the condensation system (Lynen, 1961 Ball, 1966). Lowenstein, therefore, has suggested another pathway for the formation of NADPH in the cytoplasm (Lowenstein, 1961b). It is dependent on the capacity of the cytoplasm to accomplish the first reactions of the tricarboxylic cycle, i.e., the reactions that yield a-ketoglutaric acid. A cytoplasmic NADP-isocitrate dehydrogenase, in contrast to the mitochondrial enzyme, the coen-... 

Aerobic utilization of pyruvate in eukaryotes necessitates its entry into the mitochondrial matrix where the reactions of the tricarboxylate cycle are considered to occur although some of the enzymes are bound or may bind to the inner membrane. 

Oxaloacetate may also be produced by amino transfer reactions involving aspartate or indirectly from pyruvate through the concerted action of two malate dehydrogenase enzymes, EC 1.1.1.40 and EC 1.1.1.37. However, the amino transfer reaction is not anaplerotic since it does not accomplish net synthesis of a tricarboxylate cycle intermediate as it employs 2-oxoglutarate. Some glucogenic amino acids (Table 16.4) may contribute to anaplerosis (Section 16.3). 

Anaplerotic reactions may also be employed in anabolic functions. Through their reversal, tricarboxylate cycle intermediates may serve as precursors of glucose (Section 11.6). This function is demonstrated in certain species of plants and microorganisms which utilize the glyoxylate cycle (Section 12.8) in the synthesis of carbohydrate from acetyl-CoA produced by the p-oxidation of fatty acids. 

The main purpose of the glyoxylate cycle (Figure 12.7) which is located in glyoxysomes of plants is the synthesis of succinate from which carbohydrate may be produced. The reaction sequence utilizes organelle-specific isoenzymes of three enzymes of the tricarboxylate cycle citrate synthase, aconitate hydratase and malate dehydrogenase. These enzymes together with two enzymes... 

The utilization of acetyl-CoA by the tricarboxylate cycle is dependent upon the availability of an appropriate intramitochondrial concentration of oxaloacetate which is maintained by anaplerotic reactions (Section 12.6). The intracellular concentration of oxaloacetate therefore depends upon the levels of certain glycolytic intermediates. If carbohydrate metabolism is depressed and fatty acid degradation predominant such as during starvation, fasting or diabetes mellitus, acetyl-CoA cannot enter the tricarboxylate cycle and is utilized by a reaction sequence leading to ketone body formation. There are three so-called ketone bodies ... 

In bacteria and plant cells, the 20 amino acids which occur in their proteins may be synthesized from glutamate or glutamine and intermediates of glycolysis, pentose phosphate pathway and tricarboxylate cycle (Table 16.1). In some cases, the synthesis involves a single reaction whilst other pathways may involve as many as 12 reactions. Only the synthesis of tryptophan from phosphoenol-pyruvate and erythrose 4-phosphate does not involve at least one amino transfer reaction... 

---