Carbon dioxide reaction with acetyl coenzyme

Fats and carbohydrates are metabolized down to carbon dioxide via an acetyl unit, CH3C=0, which is attached to a coenzyme, HSCoA, as a thioester called acetyl CoA. Acetyl CoA enters the citric acid cycle and eventually is converted to two molecules of carbon dioxide. The first step in the citric acid cycle is the aldol of acetyl CoA with oxaloacetate (Fig. 8.6). What is so elegant about this aldol is that the acidic and basic groups within the enzyme s active site provide a route that avoids any strongly acidic or basic intermediates. The enzyme accomplishes an aldol reaction at neutral pH, without an acidic protonated carbonyl or basic enolate intermediate via push-pull catalysis (Section 7.4.3). 

Note that this overall reaction requires three coenzymes that we encountered as metabolites of vitamins in chapter 15 NAD+, derived from lucotiiuc acid or nicotinamide FAD, derived from riboflavin and coenzyme A(CoASH), derived from pantothenic acid. In the overall process, acetyl-SCoA is oxidized to two molecules of carbon dioxide with the release of CoASH. Both NAD+ and FAD are reduced to, respectively, NADH and FADH2. Note that one molecule of guanosine triphosphate, GTP, functionally equivalent to ATP, is generated in the process. 

In this reaction, pyruvic acid is oxidized to carbon dioxide with formation of acetyl-SCoA and NAD+ is reduced to NADH. As noted in chapter 15, this reaction requires the participation of thiamine pyrophosphate as coenzyme. Here too the NADH formed is converted back to NAD+ by the electron transport chain. As noted above, the acetyl-SCoA is consumed by the citric acid cycle and CoASH is regenerated. 

Biotin (5) is the coenzyme of the carboxylases. Like pyridoxal phosphate, it has an amide-type bond via the carboxyl group with a lysine residue of the carboxylase. This bond is catalyzed by a specific enzyme. Using ATP, biotin reacts with hydrogen carbonate (HCOa ) to form N-carboxybiotin. From this activated form, carbon dioxide (CO2) is then transferred to other molecules, into which a carboxyl group is introduced in this way. Examples of biotindependent reactions of this type include the formation of oxaloacetic acid from pyruvate (see p. 154) and the synthesis of malonyl-CoA from acetyl-CoA (see p. 162). 

Claisen reactions involving acetyl-CoA are made even more favourable by first converting acetyl-CoA into malonyl-CoA by a carboxylation reaction with CO2 using ATP and the coenzyme biotin (Figure 2.9). ATP and CO2 (as bicarbonate, HC03-) form the mixed anhydride, which car-boxy lates the coenzyme in a biotin-enzyme complex. Fixation of carbon dioxide by biotin-enzyme complexes is not unique to acetyl-CoA, and another important example occurs in the generation of oxaloacetate from pyruvate in the synthesis of glucose from non-carbohydrate sources... 

The important function of biotin is its role as coenzyme for carboxylase, which catalyses carbon dioxide fixation or carboxylation reaction. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl group of biotin to form covalently linked biotinyl carboxyl carrier protein (BCCP or biocytin) (Figure 6.8). This serves as an intermediate carrier of carbon dioxide. The carboxylation of acetyl CoA to malonyl CoA in presence of acetyl CoA carboxylase requires biotin as coenzyme. Propionyl carboxylase and pyruvate carboxylase are also associated with biotin. 

Overall then the Pyruvate DH Complex converts pyruvate into acetyl CoA in a physiologically irreversible reaction with the release of carbon dioxide and the capture of an electron pair as a hydride ion on NADH. Note the cofactors involved for this reaction sequence TPP, FAD, Mg2+, lipoamide. Coenzyme A, and NAD+. 

C-sodium acetate is produced by the reaction of the Grignard reagent, methylmagnesium bromide in diethyl ether, with cyclotron-produced nC-carbon dioxide at -15°C (Oberdorfer et al, 1996). After reaction, the product is allowed to react with O-phthaloyl dichloride to produce nC-acetyl chloride, which is then hydrolyzed to 11C-acetate with saline. The solution is filtered through a 0.22-pm membrane filter. 11C-acetate has been found to be stable at pH between 4.5 and 8.5 for up to 2 h at room temperature. The overall yield is about 10-50%. It is used for the measurement of oxygen consumption (oxidative metabolism) in the heart, since acetyl CoA synthetase converts 11C-acetate to acetyl coenzyme A after myocardial uptake, which is metabolized to 11C-C02 in the tricarboxylic acid cycle. 

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+. 

An entirely different sort of mechanism for the photochemical step in photosynthesis was suggested by Calvin and Barltrop (35). It had been observed that when algae in a steady state of photosynthesis were fed radioactive carbon dioxide, the radioactivity could not be found in those products characteristic of the tricarboxylic acid cycle (Fig. 11, p. 777). If the algae were allowed to undergo photosynthesis for a short time in the presence of radioactive carbon dioxide and then placed in the dark, the radioactive carbon was found to appear in the members of the tricarboxylic acid cycle. These results were interpreted in terms of the reactions known to be necessary for pyruvic acid to enter into the tricarboxylic acid cycle. The pyruvic acid is oxidatively decarboxylated to yield acetyl-coenzyme A and CO2. Acetyl-coenzyme A then enters the tricarboxylic acid cycle by condensing with oxalacetic acid. 

Study of the metabolism, fundamental and vital to living things, has led to a detailed understanding of the processes involved. A complex web of enzyme-catalysed reactions is now apparent, which begins with carbon dioxide and photosynthesis and leads to, and beyond, diverse compounds called primary metabolites, e.g. amino acids, acetyl-coenzyme A, mevalonic acid, sugars, and nucleotides [2, 3]. Critical to the overall energetics involved in metabolism is the coenzyme, adenosine triphosphate (ATP), which serves as a common energy relay and co-operates, like other coenzymes, with particular enzymes in the reactions they catalyse. 

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