Carboxylases and Carboxyltransferases

Biotin (D 10) is the prosthetic group of carboxylating and transcarboxylating enzymes. It is reversibly transformed to N -carboxybiotin  

Carboxylases bind free carbon dioxide in an ATP-dependent reaction, form N -carboxybiotin, and transfer the carboxy group of this compound to the acceptor molecule  

CO2 -j- ATP -j- Biotin-enzyme — HOOC—Biotin-enzyme -j ADP -f- Pi HOOC—Biotin-enzyme RH — R—COOH -f Biotin-enzyme 

Carboxyl transferases catalyze the transfer of a carboxy group from a donor via N -carboxybiotin to an acceptor  

Rj—COOH -)- Biotin-enzyme — Rj H HOOC—Biotin-enzyme 

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Reactions catalyzed by acetyl-CoA carboxylase. In E. coli, BCCP and the two enzymatic activities (biotin carboxylase and carboxyltransferase) can be separated from each other. In contrast, in the liver all three components exist on a single multifunctional polypeptide. 

Acetyl-CoA carboxylase of E. coli is a multienzyme complex that consists of three protein components that can be isolated individually Biotin carboxyl carrier protein (BCCP), biotin carboxylase, and carboxyltransferase (fig. 

In contrast, the carboxylase from Escherichia coli readily dissociates into its constituent proteins, BCCP, biotin carboxylase and carboxyltransferase. BCCP is a dimer of apparently identical subunits of mol.wt. 22 500. Biotin carboxylase is a dimer of identical units of mol.wt. 51 000. The carboxyltransferase is an a2)82-tetramer with 30000 and 35000 subunits (cf. Volpe and Vagelos, 1976). Neither citrate nor phosphorylation play any role in regulating the bacterial enzyme. Instead guanosine nucleotides, guanosine 3 -diphosphate 5-di-(and tri-) phosphate are used. These inhibit the carboxyltransferase. 

Animal and fungal ACCs are comprised of large multifunctional polypeptides containing the biotin carboxylase, biotinyl carboxyl carrier protein, and carboxyltransferase... 

Fig. 2. Acetyl-CoA carboxylase. (A) Eukaryotic ACCs contain -2300 residues organized into three functional domains — biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), and carboxyltransferase (CT). The role of the region between the biotin carboxyl carrier and carboxyltransferase domains is unknown. The biotin carboxyl carrier protein contains a typical conserved biotin attachment-site motif, VMKMV. The sites of phosphorylation are indicated by asterisks. (B) Electron micrograph of polymerized rat acetyl-CoA carboxylase (F. Ahmad, 1978). (C) Crystal structure of the biotin carboxylase domain of the yeast enzyme. In the presence of soraphen A, the biotin carboxyl carrier protein domain forms an inactive monomer the likely position of the modeled ATP-binding site is shown (adapted from Ref. [2]). (D) Crystal structure of the dimeric carboxyltransferase domain of the yeast enzyme. Although acetyl-CoA was included in the crystallization, density was observed only for CoA at one site and adenine at the other (adapted from Ref. [2]). (E) NMR structure of the biotin carboxyl carrier apoprotein domain of the human ACC2 The lysine attachment site for biotin is shown (RIKEN Structural Genomics/Proteomics Initiative, 2006). (See color plate section, plate no. 3.)...

In prokaryotes and in plastids of some plants, the ACC is a multisubunit enzyme, whereas in eukaryotes the cytosolic isozyme and, in some instances also the plastid isozyme, are multidomain proteins. The latter contain three major functional domains, which account for the biotin carboxylase (BT), biotin carboxyl-carrier (BCC) and carboxyltransferase (CT) activities and, which are organized in one large polypeptide. 

Figure 5. Acetyl-coenzyme-A carboxylase (ACC) has critical roles in fatty acid metabolism. The ACC-catalyzed biotin carboxylase (BC) and carboxyltransferase (CT) reactions [103].

De novo synthesis of fatty acids requires the combined action of acetyl-CoA carboxylase and fatty acid synthetase. The acetyl-CoA carboxylases contain biotin carboxyl carrier protein (BCCP), biotin carboxylase ahd carboxyltransferase. The reaction proceeds in two steps. Firstly, the biotin moiety of BCCP is carboxylated. Secondly, the carboxyl group is transferred to the acceptor acetyl-CoA. This latter reaction proceeds in a concerted fashion (Mildvan et aL, 1966). The mechanism of the carboxylation/decarboxylation reaction has been recently probed using the model compound N-1 -methoxy carbonylbiotin methyl... 

Elborough, K.M., Winz, R., Deka, R.K., Markham, J.E., White, A.J., Rawsthome, S., Slabas, A.R. (1996) Biotin Carboxyl Carrier Protein and Carboxyltransferase of the multisubunit form of Acetyl-CoA carboxylase from Brassica-napus - cloning and analysis of expression during oilseed rape embryogenesis, BiochemicalJournal 315(1), 103-112... 

Elborough, K.M. et al., (1995) Biotin carboxyl carrier protein and carboxyltransferase subunits of the multi subunit form of acetyl-CoA carboxylase from Brassica napus Qoning and analysis of expression during oilseed rape embreyogenesis. Biochemical J. 315 103-112. 

The much studied E. coli enzyme is composed of a 156-residue biotin carboxyl carrier protein,38 a 449-residue biotin carboxylase, whose three-dimensional structure in known 39/393 and a carboxyltransferase subunit consisting of 304 (a)- and 319 (P)- residue chains. These all associate as a dimer of the three subunits (eight peptide chains).40-42... 

The biotin carboxyl carrier subunit of E. coli acetyl-CoA carboxylase contains the covalently bound biotin.55a/b The larger biotin carboxylase subunit catalyzes the ATP-dependent attachment of C02 to the biotin and the carboxyltransferase subunit catalyzes the final transcarboxylation step (Eq. 14-5, step b) by which acetyl-CoA is converted into malonyl-CoA. 

This reaction, which proceeds in two half-reactions, a biotin carboxylase (BC) reaction and a carboxyltransferase (CT) reaction is the first committed step in fatty acid biosynthesis and is the rate limiting reaction for the pathway [103]. 

Acetyl-CoA carboxylase has been purified from several plant tissues. In wheat germ, the BCCP and biotin carboxylase are associated with one fraction while the carboxyltransferase can be isolated independently (Heinstein and Stumpf, 1969). The carboxylase from chloroplasts has prokaryotic ... 

The E. coil carboxyltransferase component is a complex of two non identical subunits. a 35 IcDa a-subunit encoded by accA and a 33 kDa p-subunit encoded by occD. The deduced amino acid sequences of a- and -subunits show marked sequence simtlart-ties to the COOH-terminal and the NH2 terminaI moieties, respectively, of the tat pro-pionyl CoA carboxylase, a biotin-dependent carboxylase that catalyzes a similar carboxyltransferase reaction (71). 

Acetyl-CoA carboxylase (ACCase) catalyzes the synthesis of malonyl-CoA, the first intermediate in fatty acid synthesis. There are two forms of ACCase a prokaryote form consisting of three protein components, biotin carboxylase, carboxyltransferase, and biotin carboxylase carrier protein, and a eukaryote form consisting of three functional domains on a single polypeptide. About 20 years ago, Kannangara and Stumpf reported the existence of the prokaryote form in spinach chloroplasts [1], the major site of fatty acid synthesis, but this finding has been dismissed because the prokaryote form has not yet been purified and the purified ACCases from various plants are all eukaryote form consisting of a subunit size of about 200 kDa, like that of the mammal enzyme. 

A number of biotin-containing enzymes have now been demonstrated in plants. If cyclohexanediones and aryloxyphenoxypropionates interfere with the carboxyltransferase reaction of acetyl-CoA carboxylase, one would... 

In summary, the cyclohexanediones and aryloxyphenoxypropionates are herbicidal because they inhibit the carboxyltransferase partial reaction of acetyl-CoA carboxylase in sensitive plants. Their selectivity appears to be based on structural differences in the target enzyme, and this raises fundamental questions about protein sequence and conformation as well as offering some attractive possibilities for gene manipulation. ... 

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