Co carbonic anhydrase

Nitrate reductase (assimilatory and respiratory) Nitrite reductase (assimilatory and respiratory) Co Carbonic anhydrase... 

The measurement of the spin-lattice relaxation of solvent water protons in solutions of Co" carbonic anhydrase leads to the establishment of pK values of 6.1 and 8.5. The former value probably refers to the Co—OH2 group, while the latter pK is suggested to be due to enzyme-bound bicarbonate. ... 

H and n.m.r. spectroscopy has been used to investigate the interaction between various carboxylate inhibitors and Co carbonic anhydrase. Analysis of the temperature dependence of the transverse relaxation time Ti has yielded the rate constants for formation and dissociation of the complexes, and values in the region of 2 x 10 1 mol and 10 s respectively, have been obtained (Table 4). Anion association is thus two to three orders of magnitude faster than the corresponding process in hexa-aquocobalt(n). 

Carbonic anhydrase red blood corpuscles carbonic acid CO, and H,0 6-8... 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

Fig. 17. Rapid reversed-phase separation of proteins at a flow-rate of 10 ml/min (Reprinted with permission from [127]. Copyright 1999 Elsevier). Conditions Column, 50x4.6 mm i.d. poly(styrene-co-divinylbenzene) monolith,mobile phase gradient 42% to 90% acetonitrile in water with 0.15% trifluoroacetic acid in 0.35 min, UV detection at 280 nm. Peaks ribonucle-ase (1), cytochrome c (2), bovine serum albumin (3), carbonic anhydrase (4), chicken egg albumin (5)...

Fig. 21. Separation of cytochrome (peak 1), ribonuclease, (peak 2), carbonic anhydrase (peak 3), lysozyme (peak 4), and chymotrypsinogen (peak 5) by hydrophobic interaction chromatography on a molded poly(acrylamide-co-butylmethacrylate-co-N,AT,-methylenebisacry-lamide) monolithic column. (Reprinted with permission from [ 135]. Copyright 1998 Elsevier). Conditions column, 50 x8 mm i.d., 10% butyl methacrylate,mobile phase gradient from 1.5 to 0.1 mol/1 ammonium sulfate in 0.01 mol/l sodium phosphate buffer (pH 7) in 3 min, gradient time 3.3 min, flow rate 3 ml/min...

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. 

Carbonic anhydrase is a metalloprotein with a co-ordinate bonded zinc atom immobilized at three histidine residues (His 94, His 96 and Hisl 19) close to the active site of the enzyme. The catalytic activity of the different isoenzymes varies but cytosolic CA II is notable for its very high turnover number (Kcat) of approximately 1.5 million reactions per second. 

Several other nmr procedures have been used for the determination of fractionation factors. These have advantages in some systems. Instead of determining the effect of the concentration of an exchanging site on the averaged chemical shift, the effect on the averaged relaxation rate of water protons can be used in a very similar way (Silverman, 1981 Kassebaum and Silverman, 1989), For example, addition of the enzyme Co(ii)-carbonic anhydrase to an aqueous solution increases the observed value of XjT because the proton-relaxation rate is the average of that for the bulk solvent (cfl. 0.3 s ) and that for water bound to the cobalt ca. 6x 10 s ). The average is different in an H2O/D2O mixture if the bulk solvent and the Cobound solvent have different deuterium contents, and it has been used to determine a value for the fractionation factor of Co-bound water molecules in the enzyme. 

Redox catalysis Zn, Fe, Cu, Mn, Mo, Co, V Se, Cd, Nl Enzymes (see Table 11.4 for more Information) Reactions with oxygen (Fe, Cu) Oxygen evolution (Mn) Nitrogen fixation (Fe, Mo) Inhibition of llpid peroxidation (Se) Carbonic anhydrase (Cd) Reduction of nucleotides (Co) Reactions with H2 (Nl) Bromoperoxidase activity (V)... 

As an example of tetra-coordinate cobalt(II) systems, the NMRD profile of cobalt(II)-substituted carbonic anhydrase (MW 30,000) at high pH is reported (Fig. 14). The metal ion is coordinated to three histidines and to a hydroxide ion (48). The NMRD profile shows a cos Cg dispersion centered around 10 MHz, which qualitatively sets the correlation time around 10 s. As the reorientational correlation time of the molecule is much longer, this value is a measure of the effective electronic relaxation time. A quantitative... 

The importance of the biochemistry of hydration of CO2 and dehydration of HCOg in an aqueous environment has led to extensive and invigorating research on the enzyme carbonic anhydrase pertaining to its structural details, metal ion cofactor, its coordination environment (12) and kinetic activity Model studies, both theoretical and experimental, have been undertaken using primarily the complexes of Zn(II), Mn(II), and Co(II), the latter one being its closest equivalent (13). 

There are several types of -class CAs i.e., a-CA I-VII, reported in the literature, out of which the human carbonic anhydrase II (HCA II), the most extensively studied carbonic anhydrase, has an exceptionally high CO2 hydration rate and a wide tissue distribution 107). The HCA II comprises a single polypeptide chain with a molecular mass of 29.3 kDa and contains one catalytic zinc ion, coordinated to three histidine residues, His 94, His 96, and His 119. A tetrahedral coordination geometry around the metal center is completed with a water molecule, which forms a hydroxide ion with a pK value of 7.0 108). Quigley and co-workers 109,110) reported that the inhibition of the synthesis of HCO3 from CO2 and OH- reduces aqueous humor formation and lowers intra-ocular pressure, which is a major risk factor for primary open-angle glaucoma. 

Compounds which enhance the catalytic activities of the CAs are known as activators. Activators of carbonic anhydrases are less studied because CA is one of the most efficient enzymes known. Carbonic anhydrase II activation by phosphorylation in the presence of protein kinase and cAMP has been reported (195,196). Also some anions are activators for CA III (197,198) the catalytic effect is due to the proton shuttling capacities of such activators. Histamine, a well known activator, for native and Co(II)-substituted isoenzymes I and II CA is reported by Briganti et al. (199). Amines [Ar-CH(R3)CH(R2)NH(R1) Ar =Aromatic/heterocyclic group R1 =R2 = H, Me R3 = H, OH, COOH] and amino acids are efficient activators for CA I—III (200-207). These amines possess a bulky aromatic/heterocyclic moiety in their molecular structure and act as proton acceptor (204-207). 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

This reaction is rather slow in the absence of the enzyme carbonic anhydrase, which is usually the case with fermentation broths, although this enzyme exists in the red blood cells. Thus, any increase of for CO, desorption from fermentation broths due to simultaneous diffusion of HCO3 seems negligible. 

Most of the CO2 that is physically absorbed by blood becomes H2CO3 by the above-mentioned reaction (A), in the presence of carbonic anhydrase. Thus, [H2CO3] is practically equal to [CO2], which should be proportional to the partial pressure of CO, that is,/)CO2. 

Some other reactions, such as aldehyde hydration (29) and e ter hydrolyses (30—33) are also catalyzed by the enzyme, but much j ess efficiently than the reversible hydration of CO 2. The esterase reaction, in particular, has been very useful in the kinetic analysis of carbonic anhydrase function, however. 

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