Tricarboxylic acid cycle oxaloacetate production

The consequent interpretation, accepted by Krebs in his review of the tricarboxylic acid cycle in 1943, was therefore that citric acid could not be an intermediate on the main path of the cycle, and that the product of the condensation between oxaloacetate and acetyl CoA would have to be isocitrate, which is asymmetric. This view prevailed between 1941 and 1948 when Ogston made the important suggestion that the embarrassment of the asymmetric treatment of citrate could be avoided if the acid was metabolized asymmetrically by the relevant enzymes, citrate synthase and aconitase. If the substrate was in contact with its enzyme at three or more positions a chiral center could be introduced. 

The name stems from the first step in the cycle, which is a condensation of oxaloacetate with acetyl-CoA to form citric acid. However, as this product is a tricarboxylic acid, the cycle has an alternative name, the tricarboxylic acid cycle. 

Aerobic glycolysis first involves a ten-step conversion of glucose to pyruvic acid or pyruvate, called the Embden-Meyerhoff-Pamas pathway, followed by its further conversion to carbon dioxide and water via what is variously called the tricarboxylic acid cycle, or citric acid cycle, or Krebs cycle after its discoverer. The net products discharged from the cycle are carbon dioxide and water, with recycle of a further product called oxaloacetic acid or oxaloacetate. Successive organic acids that contain three carboxyl groups (-COOH), are initially involved in the cycle starting with citric acid or a neutral salt of citric acid (citrate). Hence the designator tricarboxylic. 

Since pyruvic acid carboxylation via the malic enzyme is the main source of oxaloacetate, slow glycolysis may result in deceleration of the Krebs cycle. The slowing down of glycolysis may result from reduced enzyme activity or from insufficient amounts of substrate the former possibility has been eliminated by the experiments of Chaikoff and his group. These investigators injected lactate, pyruvate, and acetate, and measured their conversion to CO2. They observed that CO2 formation was the same in diabetics as in nondiabetics. Insufficiency of Krebs cycle substrate is also unlikely, because CO2 production, which is derived mainly from the tricarboxylic acid cycle, is unimpaired in diabetes. In addition to being used for citric acid formation, acetyl CoA is also a key building block for fatty acids. 

Now, with aspartate (Asp, D) in hand, for example, it should be clear that transamination will allow production of both essential and nonessential amino adds. Alanine (Ala, A), as shown in Scheme 12.2, can be formed when pyruvate (derived, e.g., from phosphoenolpyruvate via 2-phosphoglycerate, as shown inter alia Scheme 11.25) undergoes transamination from aspartate (Asp, D) (produced as in Equation 12.3 from fumarate). Fumarate, it will be remembered (Scheme 11.89), is produced in the tricarboxylic acid cycle. So, as shown in Scheme 12.2, the aminotransferase cofactor, pyridoxal, serves to deaminate aspartate (Asp, D) to produce oxaloacetate. The amino group is then transferred to pyruvate to yield alanine (Ala, A) and the pyridoxal cofactor is regenerated. ... 

Glycolysis proceeds to the pyruvic acid level in the cytoplasm, and then the pyruvic acid or the 2-carbon substances produced from it must come into contact with, or enter into, the mitochondria for the next stage of respiration to take place. The 2-carbon substances are drawn into the Krebs tricarboxylic acid cycle within or on the surface of the mitochondria, become condensed with oxaloacetic acid to form citric acid, and then pass round the cycle giving off (for each molecule of pyruvic acid entering the system) three molecules of carbon dioxide and five pairs of hydrogen atoms. These pairs of hydrogen atoms are swept away from the Krebs cycle and, through the aid of coenzyme I, coenzyme II, and cytochrome c form water and simultaneously reduce the cytochrome which is reoxidized by cytochrome oxidase. It should be stressed that the whole of this process, from pyruvic acid to the production of CO2 and water, can be carried out by mitochondria, which appear to have sufficient of all the enzymes necessary for... 

The transformation of pyruvate to carbon dioxide is achieved by the several steps in a cyclical series of reactions known as the tricarboxylic acid (TCA) cycle. The name of the cycle comes from the first step where acetyl-CoA is condensed with oxaloacetic acid to form citric acid, a tricarboxylic acid. Once citrate is formed the material is converted back to oxaloacetate through a series of 10 reactions, as illustrated in Fig. 5.22, with the net production of 2 molecules of carbon dioxide and reducing equivalents in the form of 4 molecules of NADH + H and 1 molecule of FADH2, together with 1 mole of ATP. The overall stoichiometry of the TCA cycle from pyruvate is ... 

The metabolic flux distributions around the intermediate pyruvate for different strains and environmental conditions are summarised in Fig. 12. This part of the metabolism has been shown to be an important node for the interconversion between glycolytic C3 metabolites and C4 metabolites of the tricarboxylic acid (TCA) cycle. The different anaplerotic reactions are of special importance for the production of recombinant proteins as they provide precursors, such as oxaloace-tate, for amino acid biosynthesis. Due to that, the flux distribution is noticeably affected by both the cultivation conditions and the carbon source used which indicates flexible adaptation to the environmental situation. The flux from pyruvate to oxaloacetate through the reaction catalysed by pyruvate carboxylase was found to be the main anaplerotic pathway in B. megaterium. 

In tissues other than the RBC, pyruvate has alternative metabolic fates that, depending on the tissue, include gluconeogenesis, conversion to acetyl-CoA by pyruvate dehydrogenase for further metabolism to CO in the tricarboxylic acid (TCA) cycle, transamination to alanine or carboxylation to oxaloacetate by pyruvate carboxylase (Table 23-1). In the RBC, however, the restricted enzymatic endowment precludes all but the conversion to lactate. The pyruvate and lactate produced are end products of RBC glycolysis that are transported out of the RBC to the liver where they can undergo the alternative metabolic conversions described above. 

The sequence of events known as the Krebs cycle is indeed a cycle ox-aloacetate is both the first reactant and the final product of the metabolic pathway (creating a loop). Because the Krebs cycle is responsible for the ultimate oxidation of metabolic intermediates produced during the metabolism of fats, proteins, and carbohydrates, it is the central mechanism for metabolism in the cell. In the first reaction of the cycle, acetyl CoA condenses with oxaloacetate to form citric acid. Acetyl CoA utilized in this way by the cycle has been produced either via the oxidation of fatty acids, the breakdown of certain amino acids, or the oxidative decarboxylation of pyruvate (a product of glycolysis). The citric acid produced by the condensation of acetyl CoA and oxaloacetate is a tricarboxylic acid containing three car-boxylate groups. (Hence, the Krebs cycle is also referred to as the citric acid cycle or tricarboxyfic acid cycle.)... 

The two-carbon acetyl group of acetyl coenzyme A enters the cycle by an enzyme-catalyzed aldol reaction (Section 15.2) between the alpha carbon of acetyl-CoA and the ketone group of oxaloacetate. The product of this reaction is citrate, the tricarboxylic acid from which... 

Acetyl CoA next enters the citric acid cycle, also called the tricarboxylic acid (TCA) cycle, or Krebs cycle. The overall result of the cycle is the conversion of an acetyl group into two molecules of CO2 plus reduced coenzymes by an eight-step sequence. The cycle is a closed loop of reactions in which the product of the final step (oxaloacetate) is a reactant in the first step. The intermediates are constantly regenerated and flow continuously through the cycle, which operates as long as the oxidizing coenzymes NAD" " and FAD are available. 

Malic acid is mainly produced as an acidulant and taste enhancer in the beverage and food industries. The most preferred metabolic pathway for malate production starts from glucose and proceeds with the carboxylation of pyruvate, followed by the reduction of oxaloacetate to malate. These pathways have been identified in bacterial, yeast, and fungal species (Werpy et al., 2004). In the microalgae reduction of oxaloacetate to malate by NADP malate dehydrogenase (Ouyang et al., 2013 Kuo et al., 2013), the condensation of oxaloacetate and acetyl-coenzyme A (acetyl-CoA) to citric acid is followed by oxidation steps of the tricarboxylic acid (TC A) cycle or glyoxyl-ate shunt to malate (Steinhauser et al., 2012 Pearce et al., 1969 Woodward and Merrett, 1975). 

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. 

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