Refined structure of bovine carbonic anhydrase III at 2.0 A resolution

Marcus rate theory applied to enzymatic proton transfer

The hydration of CO(2) and the dehydration of HCO(3)(-) catalyzed by the carbonic anhydrases is accompanied by the transfer of protons between solution and the zinc-bound water molecule in the active site. This transfer is facilitated by amino acid residues of the enzyme which act as intramolecular proton shuttles; variants of carbonic anhydrase lacking such shuttle residues are enhanced or rescued in catalysis by intermolecular proton transfer from donors such as imidazole in solution. The resulting rate constants for proton transfer when compared with the values of the pK(a) of the donor and acceptor give Bronsted plots of high curvature. These data are described by Marcus theory which shows an intrinsic barrier for proton transfer from 1 to 2 kcal/mol and work terms or thermodynamic contributions to the free energy of reaction from 4 to10 kcal/mol. The interpretation of these Marcus parameters is discussed in terms of the well-studied pathway of the catalysis and structure of the enzymes.

Kinetic and spectroscopic characterization of the gamma-carbonic anhydrase from the methanoarchaeon Methanosarcina thermophila

The zinc and cobalt forms of the prototypic gamma-carbonic anhydrase from Methanosarcina thermophila were characterized by extended X-ray absorption fine structure (EXAFS) and the kinetics were investigated using steady-state spectrophotometric and (18)O exchange equilibrium assays. EXAFS results indicate that cobalt isomorphously replaces zinc and that the metals coordinate three histidines and two or three water molecules. The efficiency of either Zn-Cam or Co-Cam for CO(2) hydration (k(cat)/K(m)) was severalfold greater than HCO(3-) dehydration at physiological pH values, a result consistent with the proposed physiological function for Cam during growth on acetate. For both Zn- and Co-Cam, the steady-state parameter k(cat) for CO(2) hydration was pH-dependent with a pK(a) of 6.5-6.8, whereas k(cat)/K(m) was dependent on two ionizations with pK(a) values of 6.7-6.9 and 8.2-8.4. The (18)O exchange assay also identified two ionizable groups in the pH profile of k(cat)/K(m) with apparent pK(a) values of 6.0 and 8.1. The steady-state parameter k(cat) (CO(2) hydration) is buffer-dependent in a saturable manner at pH 8. 2, and the kinetic analysis suggested a ping-pong mechanism in which buffer is the second substrate. The calculated rate constant for intermolecular proton transfer is 3 x 10(7) M(-1) s(-1). At saturating buffer concentrations and pH 8.5, k(cat) is 2.6-fold higher in H(2)O than in D(2)O, suggesting that an intramolecular proton transfer step is at least partially rate-determining. At high pH (pH > 8), k(cat)/K(m) is not dependent on buffer and no solvent hydrogen isotope effect was observed, consistent with a zinc hydroxide mechanism. Therefore, at high pH the catalytic mechanism of Cam appears to resemble that of human CAII, despite significant structural differences in the active sites of these two unrelated enzymes.

Human carbonic anhydrase XIV (CA14): cDNA cloning, mRNA expression, and mapping to chromosome 1

A full-length cDNA clone of a human carbonic anhydrase XIV (HGMW-approved gene symbol CA14) was obtained and sequenced. The cDNA sequence was 1757 bp long and was predicted to encode a 337-amino-acid polypeptide with a molecular mass of 37.6 kDa. The deduced amino acid sequence of CA XIV showed an overall similarity of 29-46% to other active CA isozymes. The highest percentage similarity was with a transmembrane CA isoform, CA XII. As observed for CA XII, CA XIV has hydrophobic segments at both termini of the deduced protein for a putative signal sequence and a transmembrane domain. CA XIV showed low activity and was sensitive to acetazolamide, but not to sulfonamide. Northern blot analysis demonstrated an approximately 1.7-kb transcript in the adult human heart, brain, liver, and skeletal muscle. RNA dot-blot analysis for CA XIV mRNA expression showed a strong signal in all parts of the human brain and a weaker signal in the colon, small intestine, urinary bladder, and kidney. RT-PCR analysis showed an intense signal in the liver and spinal cord and a faint signal in the kidney. No CA XIV mRNA was seen in the salivary gland and pancreas. In contrast, CA XII mRNA was expressed in the kidney, salivary gland, and pancreas, but not in the liver or spinal cord. The CA XIV gene was localized to human chromosome 1q21. These findings indicate genetically distinct but closely related isoforms of human transmembrane CAs, CA XII and CA XIV, which have different patterns of tissue-specific expression.

A plant-type (beta-class) carbonic anhydrase in the thermophilic methanoarchaeon Methanobacterium thermoautotrophicum

Carbonic anhydrase, a zinc enzyme catalyzing the interconversion of carbon dioxide and bicarbonate, is nearly ubiquitous in the tissues of highly evolved eukaryotes. Here we report on the first known plant-type (beta-class) carbonic anhydrase in the archaea. The Methanobacterium thermoautotrophicum DeltaH cab gene was hyperexpressed in Escherichia coli, and the heterologously produced protein was purified 13-fold to apparent homogeneity. The enzyme, designated Cab, is thermostable at temperatures up to 75 degrees C. No esterase activity was detected with p-phenylacetate as the substrate. The enzyme is an apparent tetramer containing approximately one zinc per subunit, as determined by plasma emission spectroscopy. Cab has a CO(2) hydration activity with a k(cat) of 1.7 x 10(4) s(-1) and K(m) for CO(2) of 2.9 mM at pH 8.5 and 25 degrees C. Western blot analysis indicates that Cab (beta class) is expressed in M. thermoautotrophicum; moreover, a protein cross-reacting to antiserum raised against the gamma carbonic anhydrase from Methanosarcina thermophila was detected. These results show that beta-class carbonic anhydrases extend not only into the Archaea domain but also into the thermophilic prokaryotes.

Proton transfer to residues of basic pK(a) during catalysis by carbonic anhydrase

The maximal velocity in the hydration of CO(2) catalyzed by the carbonic anhydrases in well-buffered solutions is limited by an intramolecular proton transfer from zinc-bound water to acceptor groups of the enzyme and hence to buffer in solution. Stopped-flow spectrophotometry was used to accumulate evidence that this maximal velocity is affected by residues of basic pK(a), near 8 to above 9, in catalysis of the hydration of CO(2) by carbonic anhydrases III, IV, V, and VII. A mutant of carbonic anhydrase II containing the replacement His-64-->Ala, which removes the prominent histidine proton shuttle (with pK(a) near 7), allows better observation of these basic groups. We suggest this feature of catalysis is general for the human and animal carbonic anhydrases and is due to residues of basic pK(a), predominantly lysines and tyrosines more distant from the zinc than His-64, that act as proton acceptors. These groups supplement the well-studied proton transfer from zinc-bound water to His-64 in the most efficient of the carbonic anhydrases, isozymes II, IV, and VII.

Characterization of a molten globule state of bovine carbonic anhydrase III: loss of asymmetrical environment of the aromatic residues has a profound effect on both the near- and far-UV CD spectrum

Bovine muscle carbonic anhydrase (isoenzyme III; BCAIII) exhibited a three-state unfolding process at equilibrium upon denaturation in guanidine hydrochloride (GuHCl). The stable folding intermediate appeared to be of molten globule type. The stability towards GuHCl in terms of mid-point concentrations of denaturation were very similar for BCAIII and human CAII (HCAII). It was further demonstrated that the aromatic amino acid residues contributed significantly to the circular dichroism (CD) spectrum in the far-UV wavelength region during the native-->molten globule state transition. Thus, the ellipiticity change at 218 nm was shown to monitor the loss of tertiary interactions of aromatic side chains at the first unfolding transition as well as the rupture of secondary structure at the second unfolding transition. Similar aromatic contributions to the far-UV CD spectrum, but with varying magnitudes, were also noted for BCAII and HCAII, further emphasizing that interference of aromatic residues should not be neglected at wavelengths that normally are assigned to secondary structural changes.

Evolutionarily conserved, "acatalytic" carbonic anhydrase-related protein XI contains a sequence motif present in the neuropeptide sauvagine: the human CA-RP XI gene (CA11) is embedded between the secretor gene cluster and the DBP gene at 19q13.3

Conserved amino acid motifs are found in numerous expressed genes. Proteins and peptides with functional relationships may be identified using probes designed to hybridize with these motifs. An oligonucleotide probe was prepared to match the sequence of the expected active region of a frog corticotropin-releasing factor-like peptide sauvagine and used to screen a sheep brain cDNA library. A novel 1331-bp cDNA encoding a putative 328-residue protein with a theoretical mass of 36 kDa was identified. The presence of a strong signal sequence indicates that it is a secreted protein. The amino- and carboxy-terminal regions are characterized by several potential phosphorylation sites and binding motifs, suggesting a role in intracellular signal transduction. Although the protein possesses a 7-residue sequence identical to that found in sauvagine, its overall primary structure most closely resembles those of the alpha-carbonic anhydrases (alpha-CAs). Moreover, the detection of the human and mouse orthologues in the EST databases, together with an evolutionary analysis, indicates that the protein represents a new member of the alpha-CA gene family, which we designate carbonic anhydrase-related protein XI (CA-RP XI), encoded by CA11 (human) and Car11 (mouse, rat). The human CA11 gene appears to be located between the secretor type alpha(1,2)-fucosyltransferase gene cluster (FUT1-FUT2-FUT2P) and the D-site binding protein gene (DBP) on chromosome 19q13.3. Despite potentially inactivating changes in the active-site residues, CA-RP XI is evolving very slowly in mammals, a property indicative of an important function, which has also been observed in the two other "acatalytic" CA isoforms, CA-RP VIII and CA-RP X, whose functions are unknown.

Intracellular beta-carbonic anhydrase of the unicellular green alga Coccomyxa. Cloning of the cdna and characterization of the functional enzyme overexpressed in Escherichia coli

Carbonic anhydrase (CA) (EC 4.2.1.1) enzymes catalyze the reversible hydration of CO2, a reaction that is important in many physiological processes. We have cloned and sequenced a full-length cDNA encoding an intracellular beta-CA from the unicellular green alga Coccomyxa. Nucleotide sequence data show that the isolated cDNA contains an open reading frame encoding a polypeptide of 227 amino acids. The predicted polypeptide is similar to beta-type CAs from Escherichia coli and higher plants, with an identity of 26% to 30%. The Coccomyxa cDNA was overexpressed in E. coli, and the enzyme was purified and biochemically characterized. The mature protein is a homotetramer with an estimated molecular mass of 100 kD. The CO2-hydration activity of the Coccomyxa enzyme is comparable with that of the pea homolog. However, the activity of Coccomyxa CA is largely insensitive to oxidative conditions, in contrast to similar enzymes from most higher plants. Fractionation studies further showed that Coccomyxa CA is extrachloroplastic.

Properties of intramolecular proton transfer in carbonic anhydrase III

We investigated the efficiency of glutamic acid 64 and aspartic acid 64 as proton donors to the zinc-bound hydroxide in a series of site-specific mutants of human carbonic anhydrase III (HCA III). Rate constants for this intramolecular proton transfer, a step in the catalyzed dehydration of bicarbonate, were determined from the proton-transfer-dependent rates of release of H2 18O from the enzyme measured by mass spectrometry. The free energy plots representing these rate constants could be fit by the Marcus rate theory, resulting in an intrinsic barrier for the proton transfer of deltaG0++ = 2.2 +/- 0.5 kcal/mol, and a work function or thermodynamic contribution to the free energy of reaction wr = 10.8 +/- 0.1 kcal/mol. These values are very similar in magnitude to the Marcus parameters describing intramolecular proton transfer from His64 and His67 to the zinc-bound hydroxide in mutants of HCA III. That result and the equivalent efficiency of Glu64 and Asp64 as proton donors in the catalysis by CA III demonstrate a lack of specificity in proton transfer from these sites, which is indirect evidence of a number of proton conduction pathways through different structures of intervening water chains. The dominance of the thermodynamic contribution or work function for all of these proton transfers is consistent with the view that formation and breaking of hydrogen bonds in such water chains is a limiting factor for proton translocation.

Structures of murine carbonic anhydrase IV and human carbonic anhydrase II complexed with brinzolamide: molecular basis of isozyme-drug discrimination

Carbonic anhydrase IV (CAIV) is a membrane-associated enzyme anchored to plasma membrane surfaces by a phosphatidylinositol glycan linkage. We have determined the 2.8-angstroms resolution crystal structure of a truncated, soluble form of recombinant murine CAIV. We have also determined the structure of its complex with a drug used for glaucoma therapy, the sulfonamide inhibitor brinzolamide (Azopt). The overall structure of murine CAIV is generally similar to that of human CAIV; however, some local structural differences are found in the active site resulting from amino acid sequence differences in the "130's segment" and the residue-63 loop (these may affect the nearby catalytic proton shuttle, His-64). Similar to human CAIV, the C-terminus of murine CAIV is surrounded by a substantial electropositive surface potential that may stabilize the interaction with the phospholipid membrane. Binding interactions observed for brinzolamide rationalize the generally weaker affinity of inhibitors used in glaucoma therapy toward CAIV compared with CAII.

Identification of the structural similarity in the functionally related amidohydrolases acting on the cyclic amide ring

The functionally related amidohydrolases, including D-hydantoinases, dihydropyrimidinases, allantoinases and dihydro-orotases, share a similar catalytic function of acting on the cyclic amide ring. We aligned 16 amidohydrolases by taking account of the conservative substitution and found a number of highly conserved regions and invariant amino acid residues. Analyses of the secondary structure and hydropathy profile of the enzymes revealed a significant degree of similarity in the conserved regions. Among the regions, the long stretched region I is of particular interest, because it is mainly composed of invariant amino acid residues, showing a similarity of 69% for the enzymes. A search of the protein data bank using the sequence of the conserved region I identified a number of proteins possessing a similar catalytic property, providing a clue that this region might be linked with the catalytic function. As a particular sequence, one aspartic acid and four histidine residues are found to be rigidly conserved in the functionally related amidohydrolases. In order to investigate the significance of the conserved residues, site-directed mutagenesis was carried out typically for the D-hydantoinase gene cloned from Bacillus stearothermophilus SD1. These residues were found to be essential for metal binding as well as catalysis, strongly implying that these invariant residues play a critical role in other enzymes as well as in D-hydantoinase. On the basis of the similar catalytic function and existence of the rigidly conserved sequence, we propose a close evolutionary relationship among the functionally related amido hydrolases, including D-hydantoinase, dihydropyrimidinase, allantoinase and dihydroorotase.

Isolation and expression of murine carbonic anhydrase IV

A 1193-bp cDNA containing the complete murine carbonic anhydrase IV coding sequence was isolated from a Balb/c kidney cDNA library. The entire coding sequence plus shorter segments was used in an Escherichia coli T7 expression vector system to produce four forms of murine CA IV, including (1) a protein representing the full-length coding sequence, (2) an amino-truncated protein lacking the 18 N-terminal amino acid plasma membrane targeting sequence, (3) a protein which lacked the plasma membrane targeting sequence and 26 C-terminal amino acids, and (4) a protein which lacked both 36 N-terminal residues (the plasma membrane targeting sequence plus 18 additional amino acids which included the first two cysteines) and 26 C-terminal residues. All four proteins were expressed as catalytically inactive inclusion bodies. After rapid dilution of washed, guanidine hydrochloride-denatured inclusion bodies into a glutathione-, l-arginine-containing renaturation buffer, an active carbonic anhydrase IV at yields of 3-4 mg/liter was easily purified from cultures expressing the form lacking the N-terminal targeting sequence and 26 C-terminal residues. The longest and shortest forms of carbonic anhydrase IV failed to refold into active enzyme under these conditions. The activity of purified recombinant carbonic anhydrase IV was highly resistant to sodium dodecyl sulfate, as is the native enzyme. This resistance presumably results from intramolecular disulfide bonds maintaining a functional active site configuration even in the presence of denaturing agents.

A different structural feature for carbonic anhydrases in human erythrocytes

This study presents a different structural feature for carbonic anhydrase in human erythrocytes. Carbonic anhydrase isozymes (CA-I and CA-II) were purified from an erythrocyte pool of 20 healthy subjects. For purification, Sepharose-4B-L-tyrosine-sulfanilamide affinity column was used. Resnets from 3-10% discontinuous SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed a single band for CA-I and two distinct bands for CA-II. The molecular weights of the two bands were similar. One peak for CA-I and two peaks for CA-II were obtained in gel filtration. The enzymatic activities of the bands in question were also of different value. Native electrophoresis showed two bands for CA-I, and it showed three bands for CA-II. It can be concluded that CA-I is a polymer composed of a single promoter and CA-II has three different polymers composed of two distinct promoters, suggesting a new structural feature of human erythrocyte carbonic anhydrase isozymes.

Carbonic anhydrase from Camelia sinensis (tea) leaves

Carbonic anhydrase (CA) (carbonate hydrolyase; E,C,4.2.1.1) from leaves of mature Camelia sinensis was purified and characterized. The purification level was 53 fold. The optimum temperature for maximal enzyme activity is 50 degrees C. The optimum pH was 6.8 and this pH varied between 6.5 and 7.5. Each enzyme molecule is a hexamer with an M(r) of 169,000 with subunits of M(r) = 28,000.

Structure and mechanism of carbonic anhydrase

Carbonic anhydrase (CA; carbonate hydro-lyase, EC 4.2.1.1) is a zinc-containing enzyme that catalyzes the reversible hydration of carbon dioxide: CO2+ H2O<-->HCO3(-)+H+. The enzyme is the target for drugs, such as acetazolamide, methazolamide, and dichlorphenamide, for the treatment of glaucoma. There are three evolutionarily unrelated CA families, designated alpha, beta, and gamma. All known CAs from the animal kingdom are of the alpha type. There are seven mammalian CA isozymes with different tissue distributions and intracellular locations, CA I-VII. Crystal structures of human CA I and II, bovine CA III, and murine CA V have been determined. All of them have the same tertiary fold, with a central 10-stranded beta-sheet as the dominating secondary structure element. The zinc ion is located in a cone-shaped cavity and coordinated to three histidyl residues and a solvent molecule. Inhibitors bind at or near the metal center guided by a hydrogen-bonded system comprising Glu-106 and Thr-199. The catalytic mechanism of CA II has been studied in particular detail. It involves an attack of zinc-bound OH- on a CO2 molecule loosely bound in a hydrophobic pocket. The resulting zinc-coordinated HCO3- ion is displaced from the metal ion by H2O. The rate-limiting step is an intramolecular proton transfer from the zinc-bound water molecule to His-64, which serves as a proton shuttle between the metal center and buffer molecules in the reaction medium.

The sulfhydryl groups of Cys 269 and Cys 272 are critical for the oligomeric state of chloroplast carbonic anhydrase from Pisum sativum

Chloroplast carbonic anhydrase is dependent on a reducing environment to retain its catalytic activity. To investigate the properties of the three accessible cysteine residues of pea carbonic anhydrase, mutants were made in which Ala or Ser substituted for C165, C269, and C272. The mutants at position 165 were found to be spectroscopically similarly to the wild-type. They have a high catalytic activity, and are also sensitive to oxidation. In contrast, both C269 and C272 were found to be critical both for the structure and for the catalytic activity. All mutants with substitutions at either of these two positions had to be co-overexpressed with GroES/EL chaperones to give soluble enzyme in Escherichia coli. The k(cat) values were decreased by 2 and 3 orders of magnitude for the C272A and C269A mutants, respectively, and the Km values were increased approximately 7 times. However, the binding of the inhibitor ethoxyzolamide was only slightly weakened. The near-UV CD spectra were found to be changed in both sign and intensity compared to that of the wild-type, and the far-UV spectra indicate some loss of alpha-helix structure. Moreover, the quaternary structure was changed from the wild-type octameric to tetrameric in these mutants. The results indicate that mutation of either of these cysteines causes minor structural changes around at least one of the two tryptophans of the subunit. Furthermore, the data demonstrate that C269 and C272 are involved in the interaction between subunits and are necessary for a proper structure at the tetramer-tetramer interface.

Novel zinc-containing ferredoxin family in thermoacidophilic archaea

The dicluster-type ferredoxins from the thermoacidophilic archaea such as Thermoplasma acidophilum and Sulfolobus sp. are known to contain an unusually long extension of unknown function in the N-terminal region. Recent x-ray structural analysis of the Sulfolobus ferredoxin has revealed the presence of a novel zinc center, which is coordinated by three histidine ligand residues in the N-terminal region and one aspartate in the ferredoxin core domain. We report here the quantitative metal analyses together with electron paramagnetic resonance and resonance Raman spectra of T. acidophilum ferredoxin, demonstrating the presence of a novel zinc center in addition to one [3Fe-4S] and one [4Fe-4S] cluster (Fe/Zn = 6.8 mol/mol). A phylogenetic tree constructed for several archaeal monocluster and dicluster type ferredoxins suggests that the zinc-containing ferredoxins of T. acidophilum and Sulfolobus sp. form an independent subgroup, which is more distantly related to the ferredoxins from the hyperthermophiles than those from the methanogenic archaea, indicating the existence of a novel group of ferredoxins, namely, a "zinc-containing ferredoxin family" in the thermoacidophilic archaea. Inspection of the N-terminal extension regions of the archaeal zinc-containing ferredoxins suggested strict conservation of three histidine and one aspartate residues as possible ligands to the novel zinc center.

Is cyanate a carbonic anhydrase substrate?

A study was undertaken to investigate whether diverse carbonic anhydrase (CA) isozymes (both native Zn as well as cobalt-substituted) are able to catalyze the hydrolysis of anions such as cyanide, cyanate, and thiocyanate. A controversy exists between the crystallographic and spectroscopic data of CA II-anion adducts. In the former case it has been shown that "metal poisons" such as CN- and CNO- are not directly coordinated to the active site Zn(II) ion whereas spectroscopic studies indicate otherwise. A theoretical study in the above systems did not resolve this controversy, since it was calculated that all three anions can act as CA substrates. In this paper we prove experimentally that none of them may act as substrates of CA and propose an explanation to the above controversy, discussing the mode of binding of small molecules within the enzyme active site.

A comparative CD study of carbonic anhydrase isoenzymes with different number of tryptophans: impact on calculation of secondary structure content

The CD spectra of human carbonic anhydrase I and II and bovine carbonic anhydrase III were recorded and analyzed. The 3D structures of these isoenzymes are known, showing very similar secondary structure and polypeptide-chain fold. The tryptophan content, however, differs between the isoenzymes, i.e., isoenzymes I, II, and III possess 6, 7, and 8 tryptophans, respectively. All of the tryptophans except the additional tryptophans in isoenzymes II and III, i.e., W245 and W47, are conserved. Despite the fact that X-ray structure determinations showed that the isoenzymes had highly similar secondary structure, the contents of alpha-helix and beta-sheet structure differed considerably when using different CD algorithms for estimation of the fractions of various secondary structural elements. This shows that aromatic amino acids also interfere in the wavelength region (far-UV) used to calculate the amount of secondary structure. Such interference is especially problematic when analyzing proteins like carbonic anhydrase, which consist mainly of beta-structure that gives rise to weak ellipticity bands, compared to the bands arising from alpha-helical structure.

Crystal structure of the secretory form of membrane-associated human carbonic anhydrase IV at 2.8-A resolution

It has recently been demonstrated that the C-terminal deletion mutant of recombinant human carbonic anhydrase IV (G267X CA IV) converts the normally glycosylphosphatidylinositol-anchored enzyme into a soluble secretory form which has the same catalytic properties as the membrane-associated enzyme purified from human tissues. We have determined the three-dimensional structure of the secretory form of human CA IV by x-ray crystallographic methods to a resolution of 2.8 A. Although the zinc binding site and the hydrophobic substrate binding pocket of CA IV are generally similar to those of other mammalian isozymes, unique structural differences are found elsewhere in the active site. Two disufide linkages, Cys-6-Cys-11G and Cys-23-Cys-203, stabilize the conformation of the N-terminal domain. The latter disulfide additionally stabilizes an active site loop containing a cis-peptide linkage between Pro-201 and Thr-202 (this loop contains catalytic residue Thr-199). On the opposite side of the active site, the Val-131-Asp-136 segment adopts an extended loop conformation instead of an alpha-helix conformation as found in other isozymes. Finally, the C terminus is surrounded by a substantial electropositive surface potential, which is likely to stabilize the interaction of CA IV with the negatively charged phospholipid headgroups of the membrane. These structural features are unique to CA IV and provide a framework for the design of sulfonamide inhibitors selective for this particular isozyme.

Characterization of heterologously produced carbonic anhydrase from Methanosarcina thermophila

The gene encoding carbonic anhydrase from Methanosarcina thermophila was hyperexpressed in Escherichia coli, and the heterologously produced enzyme was purified 14-fold to apparent homogeneity. The enzyme purified from E. coli has properties (specific activity, inhibitor sensitivity, and thermostability) similar to those of the authentic enzyme isolated from M. thermophila; however, a discrepancy in molecular mass suggests that the carbonic anhydrase is posttranslationally modified in either E. coli or M. thermophila. Both the authentic and heterologously produced enzymes were stable to heating at 55 degrees C for 15 min but were inactivated at higher temperatures. No esterase activity was detected with p-nitrophenylacetate as the substrate. Plasma emission spectroscopy revealed approximately 0.6 Zn per subunit. As judged from the estimated native molecular mass, the enzyme is either a trimer or a tetramer. Western blot (immunoblot) analysis of cell extract proteins from M. thermophila indicates that the levels of carbonic anhydrase are regulated in response to the growth substrate, with protein levels higher in acetate than in methanol- or trimethylamine-grown cells.

The phosphatase activity of carbonic anhydrase III is reversibly regulated by glutathiolation

Carbonic anhydrase isozyme III (CAIII) is unique among the carbonic anhydrases because it demonstrates phosphatase activity. CAIII forms a disulfide link between glutathione and two of its five cysteine residues, a process termed S-glutathiolation. Glutathiolation of CAIII occurs in vivo and is increased during aging and under acute oxidative stress. We show that glutathiolation serves to reversibly regulate the phosphatase activity of CAIII. Glutathiolation of Cys-186 is required for phosphatase activity, while glutathiolation of Cys-181 blocks activity. Phosphotyrosine is the preferred substrate, although phosphoserine and phosphothreonine can also be cleaved. Thus, glutathiolation is a reversible covalent modification that can regulate CAIII, a phosphatase that may function in the cellular response to oxidative stress.

Proton transfer by histidine 67 in site-directed mutants of human carbonic anhydrase III

The ability of a histidine residue at position 67 in human carbonic anhydrase III to transfer protons in the catalytic pathway for the hydration of CO2 was investigated for a series of site-specific mutants. Wild-type carbonic anhydrase III has an arginine at this position with the C alpha of residue 67 about 9.4 A from the zinc. The active-site cavity contains no other residues capable of facile proton transfer. Rate constants for proton transfer from His 67 to the zinc-bound hydroxide were determined from the rate constants for the exchange of 18O between CO2 and water measured by mass spectrometry. A range of values for the pKa of zinc-bound water was achieved by replacement of phenylalanine with leucine and aspartate at position 198 adjacent to the zinc. Application of Marcus rate theory showed that intramolecular proton transfer involving His 67 had an intrinsic energy barrier of 1.3 +/- 0.3 kcal/mol and a thermodynamic work function for a preceding unfavorable equilibrium of 10.9 +/- 0.1 kcal/mol. We previously showed that proton transfer from histidine 64 in carbonic anhydrase III could be described by Marcus rate theory [Silverman, D. N., Tu, C. K., Chen, X., Tanhauser, S. M., Kresge, A. J., & Laipis, P. J. (1993) Biochemistry 32, 10757-10762]. In comparison, proton transfer from His 67 must overcome a more unfavorable preceding equilibrium (a larger work function) that probably represents an energy requirement for proper alignment of donor and acceptor groups plus the intervening hydrogen-bonded water. Once this alignment is achieved, the intrinsic energy barrier appears the same for His 67 or His 64.

Carbonic anhydrase III. Oxidative modification in vivo and loss of phosphatase activity during aging

Oxidative modification of DNA, lipids, and proteins occurs as a consequence of reaction with free radicals and activated oxygen. Oxidative modification of total cellular proteins has been described under many pathologic and experimental conditions, but no specific proteins have been identified as in vivo targets for oxidative modification. Utilizing an immunochemical method for detection of oxidatively modified proteins, we identified a protein in rat liver that was highly oxidized. It was purified to homogeneity and identified as carbonic anhydrase, isozyme III. Its characteristics match those previously described for a protein that was lost during aging of the rat, senescence marker protein-1. Carbonic anhydrase III was purified from rats aged 2, 10, and 18 months, and the proteins were characterized. All three preparations were highly oxidatively modified as assessed by their carbonyl content. The enzyme has three known catalytic activities, and the specific activities for carbon dioxide hydration and for ester hydrolysis decreased during aging by approximately 30%. However, the third activity, that of a phosphatase, was virtually lost during aging. While the physiologic role of carbonic anhydrase III is unknown, we suggest that it functions in an oxidizing environment, which leads to its own oxidative modification.

Structural basis of inhibitor affinity to variants of human carbonic anhydrase II

The activities and structures of certain L198 variants of human carbonic anhydrase II (CAII) have been reported recently [Krebs, J. F., Rana, F., Dluhy, R. A., & Fierke, C. A. (1993) Biochemistry 32, 4496-4505; Nair, S. K., & Christianson, D. W. (1993) Biochemistry 32, 4506-4514]. In order to understand the structural basis of enzyme-inhibitor affinity, we now report the dissociation rate and equilibrium constants for acetazolamide and dansylamide binding to 13 variants of CAII containing substituted amino acids at position 198. These data indicate that inhibitor affinity is modulated by the hydrophobicity and charge of the 198 side chain. Furthermore, we have determined crystal structures of L198R, L198E, and L198F CAIIs complexed with the transition state analog acetazolamide. The substituted benzyl side chain of L198F CAII does not occlude the substrate association pocket, and it is therefore not surprising that this substitution has minimal effects on catalytic properties and inhibitor binding. Nevertheless, the F198 side chain undergoes a significant conformation change in order to accommodate the binding of acetazolamide; the same behavior is observed for the engineered side chain of L198R CAII. In contrast, the engineered side chain of L198E CAII does not alter its conformation upon inhibitor binding. We conclude that the mobility and hydrophobicity or residue 198 side chains affect enzyme-inhibitor (and enzyme-substrate) affinity, and these structure-function relationships are important for understanding the behavior of carbonic anhydrase isozyme III, which bears a wild-type F198 side chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Solvent hydrogen isotope effects and anion inhibition of CO2 hydration catalysed by carbonic anhydrase from Pisum sativum

Chloroplast carbonic anhydrase from Pisum sativum has been studied to elucidate the catalytic mechanism and to test if the mechanism proposed for human carbonic anhydrase II is also valid for pea carbonic anhydrase. The catalytic activity was found to depend on the chemical nature of the buffer. Barbital buffer gives the highest turnover number at infinite buffer concentration and the lowest Km value with respect to the buffer, while the kinetic parameters obtained in the imidazole-type buffer, 1-methylimidazole, do not differ from those obtained using the biological-type buffer Mops. The anion inhibition of CO2 hydration was investigated using SCN- at pH 6-9. The binding of the anion was found to be pH dependent with the strongest interaction at low pH. We obtained an uncompetitive inhibition pattern at high pH and noncompetitive inhibition patterns at pH 7 and low pH. The catalytic mechanism was further tested by measurements of the solvent hydrogen isotope effects on the kinetic parameters for CO2 hydration. The observed effects were comparatively small with a kcat value of approximately 2 irrespective of the pH. The effect on kcat/Km and on Km changes when going from high pH to pH 7 and low pH. At high pH, the solvent isotope effect in Km is at least 3, giving a value below 1 for kcat/Km, while at pH 7 and low pH the major effect is found in kcat/Km with values of 2.6 and 2.9. The dependence of the CO2-hydration activity on the buffer concentration is in agreement with a ping-pong mechanism with buffer acting as a second substrate. This is analogous to the behaviour of human carbonic anhydrase II. The inhibition patterns and the observed isotope effects at high pH can also be explained within the framework of the catalytic mechanism for human carbonic anhydrase II, with a rate-determining and buffer-dependent part. The results are consistent with a mechanism involving a proton transfer that contributes to rate limitation. However, the isotope effects found at pH 7 and low pH indicate that some part of the mechanism has changed. Moreover, we cannot decide whether the mechanism for pea carbonic anhydrase involves an internal proton-shuttle group, or if the buffer molecule acts in a direct proton transfer from the zinc-coordinated water.

The structure of a complex between carbonic anhydrase II and a new inhibitor, trifluoromethane sulphonamide

It has recently been shown that aliphatic sulphonamides are good inhibitors of carbonic anhydrase (CA) provided that the pK of the sulphonamide is low. We have determined the structure of the complex between CAII and CF3SO2NH2 by X-ray crystallographic methods. The nitrogen of the sulphonamide is bound to the zinc ion of the enzyme in the usual manner. The other parts of the inhibitor show a different mode of binding from aromatic sulphonamides since the trifluoromethyl group is bound at the hydrophobic part of the active site instead of pointing out from the active site. It should be possible to design new inhibitors specific for the different isoenzymes, starting from the present structure.

Kinetic studies of pea carbonic anhydrase

Chloroplast carbonic anhydrase from Pisum sativum has been isolated. The kinetic properties of the enzyme have been studied and comparisons to the well characterised human carbonic anhydrase II made. Pea carbonic anhydrase was found to be dependent on a reducing agent in order to retain the catalytic activity. Oxidised, inactive, enzyme could be activated by the addition of a SH-agent. However, such activation gave only 60% of the activity of an enzyme kept in a reduced state all the time. The kinetics of CO2 hydration show an increase in kcat as well as in kcat/Km with pH, but the pH profile does not follow a simple titration curve. The pH dependence is more complicated and it seems as if there are several titratable groups affecting the activity. At pH 9 we obtain a turnover number of 4 x 10(5) s-1 and a kcat/Km value of 1.8 x 10(8) M-1 s-1 with reference to the subunit. We also find that the enzyme needs high concentrations of buffer to work at a maximal rate. Apparent Km values with respect to the total buffer concentration are found between 52-185 mM at neutral and high pH. At low pH the situation is complex with deviations from Michaelis-Menten kinetics. Chloroplast carbonic anhydrase from higher plants have been reported to have primary structures that are completely different from the enzyme from animals. In addition, we find the circular dichroic spectrum of pea carbonic anhydrase to be well distinguished from that of human carbonic anhydrase II. Despite those structural differences the kinetic parameters indicate that pea carbonic anhydrase is equally efficient as human carbonic anhydrase II in catalysing the hydration of CO2. However, the mechanism for proton transfer from the active site to the surrounding medium seems to differ between the two enzymes.