Dehydratases carbonate dehydratase

Oinuma K-I, Y Hashimoto, K Konishi, M Goda, T Noguchi, H Higashibata, M Kobayashi (2003) Novel aldox-ime dehydratase involved in carbon-nitrogen triple bond synthesis of Pseudomonas chlororaphis B23. J Biol Chem 278 29600-29608. 

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. 

Part of the metabolic machinery of an osteoclast resembles the red cell and the renal tubule cells because all of these cell types contain the enzyme carbonic anhydrase (carbonate dehydratase) which generates acid, that is protons, and have ion pumps in their plasma membranes. The mechanism of bone resorption requires the action of cathepsin and metalloproteinase-9 working in an acidic environment (Figure 9.8). 

Carbonic anhydrase (carbonate dehydratase, EC 4.2.E1) is a small, monomeric zinc-containing metalloenzyme that catalyzes the reversible hydration of C02 to bicarbonate [101][102], In addition to this activity, carbonic anhydrase also catalyzes the hydrolysis of many aromatic esters [103]. 

Carbonate dehydratase, see Carbonic anhydrase Carbonic anhydrase C (Lindskog et al., 1971)... 

Dual isotope technique The technique uses two heavy isotopes, oxygen ( 0) and deuterium ( H). Water that contains these isotopes is prepared. The subject drinks a glass of this water, as part of a normal meal. Once equilibrated with body water, which occurs quickly, the content of in the water falls due to the production of unlabelled water from the oxidation of fuels. Similarly, the 02 content in the water also falls but the rate is greater than that of since the 02 equilibrates not only with the oxygen atoms in water but also with those in carbon dioxide. An equihbrium between water and carbon dioxide is rapidly estabhshed due to the activity of the enzyme carbonate dehydratase. 

Figure 5.7 An example of counter-transport in the erythrocyte. The transport of CO from peripheral tissues to the lungs for excretion is more complex than simple solution of COj in the plasma and transport in the blood. The CO2 produced by the muscle (or any other tissue) enters the blood and then enters an erythrocyte where it reacts with water to produce hydrogen-carbonate, catalysed by the enzyme carbonate dehydratase ...

The adjustment of the equilibrium between CO2 and HCOs is relatively slow in the uncatalyzed state. It is therefore accelerated in the erythrocytes by carbonate dehydratase (carbonic anhydrase) [1 ])—an enzyme that occurs in high concentrations in the erythrocytes. 

Carbonate dehydratase [Zn " ]— carbonic anhydrase Fumarate hydratase— fumarase ... 

This enzyme [EC 4.2.1.1], also referred to as carbonate dehydratase, is a zinc-dependent enzyme that catalyzes the reaction of carbon dioxide with water to form carbonic acid (or, of bicarbonate and a proton). See also Proton Transfer in Aqueous Solution Manometric Assay Methods Marcus Rate Theory... 

PLP-dependent enzymes catalyze the following types of reactions (1) loss of the ce-hydrogen as a proton, resulting in racemization (example alanine racemase), cyclization (example aminocyclopropane carboxylate synthase), or j8-elimation/replacement (example serine dehydratase) (2) loss of the a-carboxylate as carbon dioxide (example glutamate decarboxylase) (3) removal/replacement of a group by aldol cleavage (example threonine aldolase and (4) action via ketimine intermediates (example selenocysteine lyase). 

Another zinc-utilizing enzyme is carbonate/dehydratase C (Kannan et al., 1972). Here, the zinc is firmly bound by three histidyl side chains and a water molecule or a hydroxyl ion (Fig. 27). The coordination is that of a distorted tetrahedron. Metals such as Cu(II), Co(Il), and Mn(ll) bind at the same site as zinc. Hg(II) also binds near, but not precisely at, this site (Kannan et al., 1972). Horse liver alcohol dehydrogenase (Schneider et al., 1983) contains two zinc sites, one catalytic and one noncatalytic. X-Ray studies showed that the catalytic Zn(II), bound tetrahedrally to two cysteines, one histidine, and water (or hydroxyl), can be replaced by Co(II) and that the tetrahedral geometry is maintained. This is also true with Ni(Il). Insulin also binds zinc (Adams etai, 1969 Bordas etal., 1983) and forms rhombohedral 2Zn insulin crystals. The coordination of the zinc consists of three symmetry-related histidines (from BIO) and three symmetry-related water molecules. These give an octahedral complex... 

Work from Sturtevant s laboratory detailed the kinetics and thermodynamics of zinc binding to apocarbonic anhydrase (carbonate dehydratase) selected data are recorded in Table II (Henkens and Sturtevant, 1968 Henkens etal., 1969). The thermodynamic entropy term A5 at pH 7.0 is 88 e.u. (1 e.u. = 1 cal/mol-K), and this is essentially matched by the binding of zinc to the hexadentate ligand cyclohexylenediamine tetraacetate where AS = 82 e.u. At pH 7.0 the enthalpy of zinc-protein association is 9.8 kcal/mol, but this unfavorable term is overwhelmed by the favorable entropic contribution to the free energy (AG = AH - T AS), where —TAS = -26.2 kcal/mol at 298 K (25°C). Hence, the kinetics and thermodynamics of protein-zinc interaction in this example are dominated by very favorable entropy effects. 

Four examples of catalytic or regulatory zinc proteins are reviewed here, and the discussion of metalloprotein function is set within the context of the metal ion and its coordination polyhedron. In the zinc enzymes carbonic anhydrase (carbonate dehydratase) II and carboxypeptidase A, the coordination polyhedron of the metal ion changes as the... 

Lyases Decarboxylases Dehydratases Hydratases Add the elements of water, ammonia, or carbon dioxide across a double bond (or the reverse reaction)... 

Serine can be converted to glycine and N5,N10-methylenetetra-hydrofolate (Figure 20.6A). Serine can also be converted to pyru vate by serine dehydratase (Figure 20.6B). [Note The role of tetrahydrofolate in the transfer of one-carbon units is presented on p. 265.]... 

More recently the biotransformation of limonene by another Pseudomonad strain, P. gladioli was reported [76,77]. P. gladioli was isolated by an enrichment culture technique from pine bark and sap using a mineral salts broth with limonene as the sole source of carbon. Fermentations were performed during 4-10 days in shake flasks at 25°C using a pH 6.5 mineral salts medium and 1.0% (+)-limonene. Major conversion products were identified as (+)-a-terpineol and (+)-perillic acid. This was the first time that the microbial conversion of limonene to (+)-a-terpineol was reported, see pathway 4. The conversion of limonene to a-terpineol was achieved with an enzyme, a-terpineol dehydratase (a TD), by the same group [78]. The enzyme, purified more than tenfold after cell-disruption of Pseudomonas gladioli, stereospecifically converted (4 )-(+)-limonene to (4/ )-(+)-a-terpineol or (4S)-(+)-limonene to (4S)-(+)-a-terpineol. a-Terpineol is widely distributed in nature and is one of the most commonly used perfume chemicals [27]. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

In plants, algae and many bacteria there is an alternative route for ALA synthesis that involves the conversion of the intact five-carbon skeleton of glutamate in a series of three steps to yield ALA. In all organisms, two molecules of ALA then condense to form porphobilinogen in a reaction catalyzed by ALA dehydratase (also called porphobilinogen synthase) (Fig. 2a). Inhibition of this enzyme by lead is one of the major manifestations of acute lead poisoning. 

Eu3+, Tb3+, Sm3+, Dy3+ complexes have different emission wavelengths with sharp peak profiles, which are suitable for multi-component immunoassay. Several Eu-Sm two-color time-resolved immunoassays have been reported. Since the sensitivity of Sm3+ chelates is not high compared to Eu3+ and Tb3+, Eu-Sm two-color assays are used for the simultaneous determination of a low concentration component (Eu) and a relatively high concentration component (Sm) in serum the assayed combinations are lutropin and follitropin, myoglobin and carbonic dehydratase, AFP and free (3-subunit of human chorionic gonadotrophin (Hemmila et al., 1987). Use of Eu-Tb is reported by Eriksson et al. (2000) for the simultaneous determination of human serum free and total PSA. 

Pyridoxal Phosphate Reaction Mechanisms Threonine can be broken down by the enzyme threonine dehydratase, which catalyzes the conversion of threonine to a-ketobutyrate and ammonia. The enzyme uses PLP as a cofactor. Suggest a mechanism for this reaction, based on the mechanisms in Figure 18-6. Note that this reaction includes an elimination at the j8 carbon of threonine. 

Carbonic anhydrase (CAR, carbonate dehydratase) M Box stain Phosphate buffer 2 50 ml 1 locus... 

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