Carboxylase enzymes

Pantothenic acid is present in coenzyme A and acyl carrier protein, which act as carriers for acyl groups in metabolic reactions. Pyridoxine, as pyridoxal phosphate, is the coenzyme for several enzymes of amino acid metabolism, including the aminotransferases, and of glycogen phosphorylase. Biotin is the coenzyme for several carboxylase enzymes. 

Carbonic anhydrase (CA, also called carbonate dehydratase) is an enzyme found in most human tissues. As well as its renal role in regulating pH homeostasis (described below) CA is required in other tissues to generate bicarbonate needed as a co-substrate for carboxylase enzymes, for example pyruvate carboxylase and acetyl-CoA carboxylase, and some synthase enzymes such as carbamoyl phosphate synthases I and II. At least 12 isoenzymes of CA (CA I—XII) have been identified with molecular masses varying between 29 000 and 58 000 some isoenzymes are found free in the cytosol, others are membrane-bound and two are mitochondrial. 

Vitamin K is a component of the carboxylase enzyme that carboxylates the amino acid glutamate in proteins to form y-carboxyglutamate, which binds calcium ions i.e. it catalyses a post-transcriptional modification. Proteins so carboxylated include clotting factors (Factors 11, Vll, IX, and X) and two proteins in bone oesteocalcin (known as matrix-gln-protein) and bone gin protein (BGP). The... 

Biochemically, biotin serves as a prosthetic group and a carboxyl carrier for a number of enzymes (179,180). In humans, biotin is required by four carboxylase enzymes ... 

Structures of biotinyl enzyme and N -carboxybiotin. The reactive portions of the coenzyme and the active intermediate are shown in red. In carboxylase enzymes, biotin is covalently bonded to the proteins by an amide linkage between its carboxyl group and a lysyl- -NH2 group in the polypeptide chain. 

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

H ydroxypropionate / 4- hydroxybutyrate cycle 9 6 NAD(P)H Acetyl-CoA/propionyl-CoA carboxylase HCOJ Acetyl-CoA, succinyl-CoA Acetyl-CoA/propionyl-CoA carboxylase", enzymes reducing malonyl-CoA to propionyl-CoA, methylmalonyl-CoA rnutase, 4-hydroxybutyryl-CoA dehydratase... 

In this chapter attention will be focused on the chemical transformations of C02 to yield carboxylic acids and related molecules (e.g., cyclic esters), while natural carboxylations reactions such as those dealing with RuBisCO (ribulose-1,5 bis(phosphate) carboxylase/oxidase) and its genetic engineering or the carboxylation of phenol via a phenol phosphate intermediate by the phenol-carboxylase enzyme, will be left aside. 

C. Dileo, and I. Tommasi, Enzymic synthesis of 4-hydroxybenzoic acid from phenol and carbon dioxide the first example of a biotechnological application of a carboxylase enzyme, Tetrahedron 1998, 54, 8841-8846. 

Malathion has two carboxyester linkages, which are hydrolyzable by carboxylase enzymes to relatively nontoxic products, as shown in reaction 18.7.1. The enzymes that accomplish this reaction are possessed by mammals, but not by insects, so that mammals can detoxify malathion, whereas insects cannot. The result is that malathion has selective insecticidal activity. For example, although malathion is a very effective insecticide, its LD50 for adult male rats is about 100 times that of parathion, reflecting the much lower mammalian toxicity of malathion than those of some of the more toxic organophosphate insecticides, such as parathion. 

Carboxylase enzymes are inhibited by organophosphates other than malathion. The result of exposure of mammals to malathion plus another organophosphate is potentiation (enhancement of the action of an active substance by an otherwise inactive substance) of the toxicity of malathion. 

Biotin (60), a water-soluble vitamin with widespread application in the growing market for health and nutrition, acts as a co-factor for carboxylase enzymes and its essential fatty acid synthesis. The key step in the chemical synthesis of biotin is the asymmetric reduction of the tetrasubstituted olefins 61 by in situ Rh(I)-4i catalyst (Scheme 12.1 S).79-83-85-86 Substrate-to-catalyst ratios of 2000 with diastereoselectivities of 99% de were achieved with Rh-4i at the multi-ton scale before production was terminated.87... 

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. 

The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The thiol group acts as a carrier of acyl group. It is an important coenzyme involved in fatty acid oxidation, pyruvate oxidation and is also biosynthesis of terpenes. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl carrier protein (BCCP or biocytin) and serve as an intermediate carrier of C02. Acetyl CoA pyruvate and propionyl carboxylayse require the participation of BCCP. The coenzyme form of folic acid is tetrahydro folic acid. It is associated with one carbon metabolism. The oxidised and reduced forms of lipoic acid function as coenzyme in pyruvate and a-ketoglutarate dehydrogenase complexes. The 5-deoxy adenosyl and methyl cobalamins function as coenzyme forms of vitamin B12. Methyl cobalamin is involved in the conversion of homocysteine to methionine. 

STEPS 3-4 Carboxylation and acyl transfer. The third priming reaction again starts with acetyl CoA, which is carboxylated by reaction with HCO3" and ATP to yield malonyl CoA plus ADP. This step involves the coenzyme biotin, which is bonded to the lysine residue of acetyl CoA carboxylase enzyme and acts as a carrier of CO2. The enolate ion of acetyl CoA reacts with carboxylated biotin and transfers the CO2 group in a nucleophilic acyl substitution reaction. 

Our interest in the enzymatic functionalization of phenol derived from our discovery of a Mn- and K-dependent Carboxylase enzyme isolated from some anaerobic bacteria growing on phenol. This enzyme converts, under mesophilic conditions, very specifically phenol into 4-OH-benzoate that is thereafter dehydroxylated to benzoic acid and metabolized (Scheme 1) [5]. 

A gla protein is secreted into the endoplasmic reticulum (ER) of the liver where its N-terminal domain binds to and activates an enzyme that converts glutamic acid residues in the gla domain to y-carboxyglutamic acid. The activated carboxylase enzyme then binds to a reduced form of vitamin K, a diphenol (KH2) to which it adds molecular oxygen. The... 

Biotin is bound to carboxylase enzymes by an amide bond between this carboxyi group and the e-amino group of a lysine... 

Biotin is a vitamin and a coenzyme commonly associated with enzymes performing carboxylation reactions. Biotin is typically linked covalently to carboxylase enzymes through the -amino nitrogen of lysine. Biotin is very tightly bound by avidin, a protein found in egg white. The strong interaction between these molecules is exploited in numerous purification techniques in biotechnology. 

CH..CO.COOH + (carboxylase enzyme) = CH.CHO 4- CO pyruvic acid acetic aldehyde... 

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