Carboxylation carboxylase

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

Rittenberg and Bloch showed in the late 1940s that acetate units are the building blocks of fatty acids. Their work, together with the discovery by Salih Wakil that bicarbonate is required for fatty acid biosynthesis, eventually made clear that this pathway involves synthesis of malonyl-CoA. The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids (Figure 25.2). The reaction is catalyzed by acetyl-CoA carboxylase, which contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase. 

In animals, acetyl-CoA carboxylase (ACC) is a filamentous polymer (4 to 8 X 10 D) composed of 230-kD protomers. Each of these subunits contains the biotin carboxyl carrier moiety, biotin carboxylase, and transcarboxylase activities, as well as allosteric regulatory sites. Animal ACC is thus a multifunctional protein. The polymeric form is active, but the 230-kD protomers are inactive. The activity of ACC is thus dependent upon the position of the equilibrium between these two forms ... 

FIGURE 25.2 (a) The acetyl-CoA carboxylase reaction produces malonyl-CoA for fatty acid synthesis, (b) A mechanism for the acetyl-CoA carboxylase reaction. Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonylphosphate intermediate (Step 1). In a typical biotin-dependent reaction, nncleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2). 

FIGURE 25.3 In the acetyl-CoA carboxylase reaction, the biotin ring, on its flexible tether, acquires carboxyl groups from carbonylphos-phate on the carboxylase subunit and transfers them to acyl-CoA molecules on the transcarboxylase subunits. 

Step 1 of Figure 29.13 Carboxylation Gluconeogenesis begins with the carboxyl-afion of pyruvate to yield oxaloacetate. The reaction is catalyzed by pyruvate carboxylase and requires ATP, bicarbonate ion, and the coenzyme biotin, which acts as a carrier to transport CO2 to the enzyme active site. The mechanism is analogous to that of step 3 in fatty-acid biosynthesis (Figure 29.6), in which acetyl CoA is carboxylated to yield malonyl CoA. 

Biotin is involved in carboxylation and decarboxylation reactions. It is covalently bound to its enzyme. In the carboxylase reaction, C02 is first attached to biotin at the ureido nitrogen, opposite the side chain in an ATP-dependent reaction. The activated C02 is then transferred from carboxybiotin to the substrate. The four enzymes of the intermediary metabolism requiring biotin as a prosthetic group are pyruvate carboxylase (pyruvate oxaloacetate), propionyl-CoA-carboxylase (propionyl-CoA methylmalonyl-CoA), 3-methylcroto-nyl-CoA-carboxylase (metabolism of leucine), and actyl-CoA-carboxylase (acetyl-CoA malonyl-CoA) [1]. 

Bicarbonate as a source of CO2 is required in the initial reaction for the carboxylation of acetyl-CoA to mal-onyl-CoA in the presence of ATP and acetyl-CoA carboxylase. Acetyl-CoA carboxylase has a requirement for the vitamin biotin (Figure 21-1). The enzyme is a multienzyme protein containing a variable number of identical subunits, each containing biotin, biotin carboxylase, biotin carboxyl carrier protein, and transcarboxylase, as well as a regulatory allosteric site. The reaction takes place in two steps (1) carboxylation of biotin involving ATP and (2) transfer of the carboxyl to acetyl-CoA to form malonyl-CoA. 

Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. A holocarboxylase synthetase acts on a lysine residue of the apoenzymes of acetyl-CoA carboxylase, pymvate carboxylase, propi-onyl-CoA carboxylase, or methylcrotonyl-CoA carboxylase to react with free biotin to form the biocytin residue of the holoenzyme. The reactive intermediate is 1-7V-carboxybiocytin, formed from bicarbonate in an ATP-dependent reaction. The carboxyl group is then transferred to the substrate for carboxylation (Figure 21—1). 

Primary structure analysis of phenylphosphate carboxylase of T. aromatica is performed in detail, to clarify the reaction mechanism involving four kinds of subunits. The a, (3, y, 8 subunits have molecular masses of 54, 53, 18, and lOkDa, respectively, which make up the active phenylphosphate carboxylase. The primary structures of a and (3 subunits show homology with 3-octaprenyl-4-hydroxybenzoate decarboxylase, 4-hydroxybenzoate decarboxylase, and vanil-late decarboxylase, whereas y subunit is unique and not characterized. The 18kDa 8 subunit belongs to a hydratase/phosphatase protein family. Taking 4-hydroxybenzoate decarboxylase into consideration, Schiihle and Fuchs postulate that the a(3y core enzyme catalyzes the reversible carboxylation. ... 

Nonphotosynthetic COi fixation via phosphoenolpyruvate carboxylase (PEPC) can contribute a substantial proportion of carbon (>30%) for the biosynthesis of carboxylates in roots of P-deficient plants (Fig. 5) (11,82,101,111-113). Thus, PEPC-mediated COi fixation may be interpreted as an anaplerotic carbon... 

Carboxylation of propionyl-CoA is accomplished by propionyl-CoA carboxylase (biotin, which is the carboxyl group carrier, serves as a coenzyme for this enzyme) the presence of ATP is also required. The methylmalonyl-CoA formed is converted by methylmalonyl-CoA mutase (whose coenzyme, deoxyadenosylcobalamin, is a derivative of vitamin B]2) to succinyl-CoA the latter enters the Krebs cycle. 

Acetyl-CoA carboxylase (ACCase) carboxylates acetyl-CoA into malo-nyl-CoA and therefore represents the first committed step in fatty acid biosynthesis. ACCase is a multimer essential for cell growth whose components are highly conserved among bacteria, making it a promising broad-spectrum target [8]. 

This process (106— 107) required the incubation of an aqueous mixture of AIR (106), potassium carbonate, and enzyme (AIR carboxylase). The equilibrium of the carboxylation reaction was shown to lie to the left (59JBC1799), but in the presence of 0.3 M aqueous potassium bicarbonate solution, yields of C-AIR (107) approaching 50% were obtained. 

Odd-chain fatty acids are an exception. While they are relatively rare in the diet, odd-chain-length fatty acids end up at propionyl-CoA (C3). Propionyl-CoA is carboxylated by propionyl-CoA carboxylase to give methylmalonyl-CoA. Methylmalonyl-CoA is rearranged to succinyl-CoA by the enzyme methylmalonyl-CoA mutase, a vitamin-B12-requiring enzyme. 

Acetyl-CoA carboxylase (ACC) catalyses the ATP-dependent carboxylation of acetyl-CoA to form malonyl-CoA, the rate limiting, first committed step in fatty... 

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

Binding of C02 takes place in aqueous medium by the carboxylation reaction of ribulose-diphosphate (RuDP) with the formation of 3-phospho-glycerine acid (PGA) - table 5. Water molecule and radical C=0 at the distances of molecular interaction have quite similar values of PE-parameters for forming the general structural grouping of dimeric composite type. Total PE-parameter of water molecule and radical C=0 hearly equals PE-parameter of C02 and therefore the molecules of C02 and H20 join RuBP with the formation of two radicals COOH b PGA (table 5). In ferment RuDP- carboxylase, Mg atoms and 0" ions (5.4867 eV and 4.755 eV) play an active role, their PE-parameters similar to PE-parameter of radical COOH. 

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