The sequence of equine muscle carbonic anhydrase

Refined structure of bovine carbonic anhydrase III at 2.0 A resolution

The three-dimensional structure of bovine carbonic anhydrase III (BCA III) from red skeletal muscle cells has been determined by molecular replacement methods. The structure has been refined at 2.0 A resolution by both constrained and restrained structure-factor least squares refinement. The current crystallographic R-value is 19.2% and 121 solvent molecules have so far been found associated with the protein. The structure is highly similar to the refined structure of human carbonic anhydrase II. Some differences in amino acid sequence and structure between the two isoenzymes are discussed. In BCA III, Lys 64 and Arg 91 (His 64 and Ile 91 in HCA II) are both pointing out from the active site cavity forming salt bridges with Glu 4 and Asp 72 (His 4 and Asp 72 in HCA II), respectively. However, Arg 67 and Phe 198 (Asn 67 and Leu 198 in HCA II) are oriented towards the zinc ion and significantly reduce the volume of the active site cavity. Phe 198 particularly reduces the size of the substrate binding region at the "deep water" position at the bottom of the cavity and we suggest that this is one of the major reasons for the differences in catalytic properties of isoenzyme III as compared to isozyme II.

Analyses of polypeptides in the liver of a novel mutant (LEC rats) to hereditary hepatitis and hepatoma by two-dimensional gel electrophoresis: identification of P29/6.8 as carbonic anhydrase III and triosephosphate isomerase

1. Total cellular proteins from the livers of 4-, 16- and 52-week-old hepatitis- and hepatoma-predisposed Long-Evans Cinnamon (LEC) rats were compared to those from the livers of the corresponding control rats [Long-Evans Agouti (LEA) rats] by two-dimensional gel electrophoresis. 2. A polypeptide, p50/7.2 (molecular weight x 10(-3)/isoelectric point) was only found in the LEC rats, and the p43/6.4 component was greater and the p51/6.8 component was less in the LEC rats than in the LEA rats during aging. 3. A polypeptide, p29/6.8, was dramatically greater in 4-week-old LEC rats than in 4-week-old LEA rats. 4. By sequencing and Western blotting analysis, the marked differences in the level of the p29/6.8 component were found to be due to carbonic anhydrase III.

Partial amino acid sequence of erythrocyte carbonic anhydrase from tiger shark

1. A partial primary structure (197 residues) of carbonic anhydrase from tiger shark (Galeocerdo cuvieri) erythrocytes has been determined. 2. The amino acid sequence of the enzyme is identical to those of human cytoplasmic carbonic anhydrases (CA I-III) by as much as 52-60%. 3. It is shown that tiger shark CA most closely resembles the CA II isoenzyme of amniotes. 4. Isoelectric focusing and inhibition studies on carbonic anhydrase from dogfish (Squalus acanthias) blood and muscle indicate the presence of the same isoenzyme in shark blood and muscle.

The carbonic anhydrases: widening perspectives on their evolution, expression and function

Now, some 55 years after its discovery in bovine red cells, carbonic anhydrase (CA), in all its varied forms, continues to challenge and intrigue physiologists, biochemists and molecular geneticists. This is so because of an increasing awareness of the many apparently diverse functions of the different CA isozymes encoded by this large multigene family, the continuing discovery of new CA, or CA-related, genes, and the extensive variation in their hormonal control, cellular expression and subcellular localization.

Comparative immunochemical studies of carbonic anhydrase III in horses and other mammalian species

1. Carbonic anhydrase III (CA-III) from different mammalian species (horse, cow, dog, cat, rat and rabbit) has been analyzed by the immunodiffusion technique with anti-equine CA-III serum. 2. Immunodiffusion demonstrated the absence of cross-reactivity between isozyme CA-I, CA-II, and CA-III. 3. Cross-reactions were observed between the CA-III from all the species examined except the rabbit. 4. Molecular weights and isoelectric points of CA-III from different species were determined by Western blotting.

Sequence of carbonic anhydrase II cDNA from chick retina

Sequences of three cDNA clones for carbonic anhydrase II (CA-II) from chick retina are presented. The longest cDNA clone encodes all but the first three amino acids of CA-II, and the encoded sequence generally agrees with published fragments of CA-II sequence from chick red blood cells. It is 70% identical to human CA-II; the active-site residues are conserved, but the chick protein has six extra cysteines. There is a long 3'-untranslated region which contains a second open reading frame, but this is not conserved. There appears to be a single CA-II gene in the chick. Some anomalies in cDNA synthesis and in Bal31 deletion are noted.

Carbonic anhydrases

Some of the current studies of carbonic anhydrases are directed to the genetic mechanisms underlying their synthesis. Determination of the structure of their genes will probably most readily resolve the question of whether the membrane bound forms of the enzyme represent products of additional loci other than those of the three well-known soluble forms. Extensions of our knowledge of the sequences of these isozymes as well as those from lower animals and from plants will make possible a more precise evaluation of the extent of the multigene aspects of these proteins and their evolutionary backgrounds. Studies of the interrelationships of the regulation of the transcriptional and translational processes of the well-known isozymes and in particular the effects of hormones will be of interest. Insights into modifications of the isozymes' synthetic processes occurring in various diseases should also be forth-coming from these studies. In addition to the above the applications of what are perhaps today somewhat classical methods of protein chemistry will be needed to explore the reasons for the changes in activity accompanying the sequence variations of the different isozymes, the decreases or increases in activity accompanying derivatizations of specific residues and the reasons for the differences in the activity of different inhibitors on the various isozymes. The broad specificity of these enzymes for different substrates and the ability of CA-III to hydrolyze various phenyl esters and in some cases to become derivatized also present problems in protein structural chemistry. In terms of the latter reactions, the meaning of the relationships of these activities to those of the protein ubiquitin, which is homologous to CA-III, needs clarification. It would appear that various of the protein structural studies will be aided by crystallographic investigations of not only CA-III but of various of its derivatives which undergo either increases or decreases in activity. The above areas of studies present a wide variety of problems for workers in various disciplines and backgrounds who are interested in the carbonic anhydrases.

Molecular basis of the oxygen exchange from CO2 catalyzed by carbonic anhydrase III from bovine skeletal muscle

The exchange of 18O from CO2 to H2O in aqueous solution is caused by the hydration-dehydration cycle and is catalyzed by the carbonic anhydrases. In our previous studies of 18O exchange at chemical equilibrium catalyzed by isozymes I and II of carbonic anhydrase, we observed simple first-order depletion of 18O from CO2 with the 18O distribution among the species C18O18O, C16O18O, and C16O16O described by the binomial expansion (i.e., a random distribution of 18O). Using membrane-inlet mass spectrometry, we have measured 18O exchange between CO2 and H2O catalyzed by native zinc-containing and cobalt(II)-substituted carbonic anhydrase III from bovine skeletal muscle near pH 7.5. The distributions of 18O in CO2 deviate from the binomial expansion and are accompanied by biphasic 18O-exchange patterns; moreover, we observed regions in which 18O loss from CO2 was faster than 18O loss from HCO3-. These data are interpreted in terms of a model that includes 18O loss from an enzyme-substrate or intermediate complex. We conclude that more than one 18O can be lost from CO2 per encounter with the active site of isozyme III, a process that requires scrambling of oxygens in a bicarbonate-enzyme complex and cycling between intermediate complexes. This suggests that the rate of dissociation of H2(18)O (or 18OH-) from isozyme III is comparable to or faster than substrate and product dissociation.

Activation of bovine muscle carbonic anhydrase by modification of thiol groups

Two of the five cysteine residues in bovine muscle carbonic anhydrase (isoenzyme III) react rapidly and stoichiometrically with Ellman's reagent without effect on the CO2 hydration activity. These residues, which can be alkylated with iodoacetamide, have been identified as Cys-183 and Cys-188. Treatment of the enzyme with a large excess of Ellman's reagent results in additional derivatization of thiol groups and a 180% increase of the CO2 hydration activity. The cysteine residues associated with this activation are Cys-66 as well as Cys-203 and/or Cys-206. Activation has also been achieved with 2,2'-dithiodipyridine (120%) and with methyl methanethiosulfonate (approx. 400%), whereas no activation could be obtained with iodoacetamide, iodoacetate or N-ethylmaleimide.

Acylation and carbamylation of equine muscle carbonic anhydrase (CA-III) upon reaction with p-nitrophenyl esters and carbamoyl phosphate

Equine muscle carbonic anhydrase (CA-III) behaves like ubiquitin in undergoing extensive acylation of N epsilon-lysine residues upon reacting with p-nitrophenyl esters. The enzyme undergoes extensive carbamoylation of lysine residues when reacted with carbamoyl phosphate. The modification of from 6 to 7 lysine residues results in the production of a series of more anodic electrophoretic components. The derivatization of the lysine residues leads to a marked decrease in the enzyme's ability to hydrate CO2. The equine CA-III possesses both acid and alkaline phosphatase activities in contrast to the rabbit which possesses only the former type.

Nucleotide sequence and derived amino acid sequence of a cDNA encoding human muscle carbonic anhydrase

We report the nucleotide (nt) sequence of a full length cDNA clone, pCA15, which encodes the human muscle-specific carbonic anhydrase, CAIII. pCA15 identifies a 1.7-kb mRNA, which is present at high levels in skeletal muscle, at much lower levels in cardiac and smooth muscle and which appears to be developmentally regulated. The CAIII mRNA is distinguished by a 887-nt long 3'-untranslated region, containing two AAUAAA signal sequences and is longer than either of the mRNAs encoding the erythrocyte CAs, CAI and CAII, which each have relatively shorter 3'-untranslated regions, 360 and 670 nt long, respectively. The derived amino acid (aa) sequence for human CAIII shows 85% homology with ox CAIII, 62% homology with human CAII and 54% with human CAI when simple pairwise aa comparisons are made. We describe an allelic variation at a TaqI restriction site for CAIII which occurs at high frequency in the European population.

Purification and some properties of carbonic anhydrase from bovine skeletal muscle

Procedures for the purification of bovine muscle carbonic anhydrase (isoenzyme III) are described. The purified enzyme has a molecular weight near 29,000 and contains one Zn2+ ion per molecule. The sedimentation coefficient, s(0)20,w, is 2.8 X 10(-13) s, the isoelectric pH is 8.5, and A280(0.1%) = 2.07 cm-1. The CO2 hydration activity, expressed as kcat/Km, is about 1.5% of that of human isoenzyme I (or B) and about 0.3% of that of human isoenzyme II (or C) at pH 8 and 25 degrees C. The activity is nearly independent of pH between pH 6.0 and 8.6. The muscle enzyme is weakly inhibited by the sulfonamide inhibitor, acetazolamide, whereas some anions, particularly sulfide and cyanate, are efficient inhibitors. Bovine carbonic anhydrase III contains five thiol groups, two of which react readily with Ellman's reagent without effect on the catalytic activity. A reinvestigation of the amino acid sequences of cysteine-containing tryptic peptides has shown that cysteine residues occur at sequence positions 66, 183, 188, 203, and 206.