Buffer enhancement of proton transfer in catalysis by human carbonic anhydrase III

Carbonic anhydrase modification for carbon management

Carbonic anhydrase modification (chemical and biological) is an attractive strategy for its diverse application to accelerate the absorption of CO₂ from a flue gas with improved activity and stability. This article reports various possibilities of CA modification using metal-ligand homologous chemistry, cross-linking agents, and residue- and group-specific and genetic modifications, and assesses their role in carbon management. Chemically modified carbonic anhydrase is able to improve the absorption of carbon dioxide from a gas stream into mediation compounds with enhanced sequestration and mineral formation. Genetically modified CA polypeptide can also increase carbon dioxide conversion. Chemical modification of CA can be categorized in terms of (i) residue-specific modification (involves protein-ligand interaction in terms of substitution/addition) and group-specific modifications (based on the functional groups of the target CA). For every sustainable change, there should be no/limited toxic or immunological response. In this review, several CA modification pathways and biocompatibility rules are proposed as a theoretical support for emerging research in this area.

Cellular Small Molecules Contribute to Twister Ribozyme Catalysis

The number of self-cleaving small ribozymes has increased sharply in recent years. Advances have been made in describing these ribozymes in terms of four catalytic strategies: α describes in-line attack, β describes neutralization of the nonbridging oxygens, γ describes activation of the nucleophile, and δ describes stabilization of the leaving group. Current literature presents the rapid self-cleavage of the twister ribozyme in terms of all four of these classic catalytic strategies. Herein, we describe the nonspecific contribution of small molecules to ribozyme catalysis. At biological pH, the rate of the wild-type twister ribozyme is enhanced up to 5-fold in the presence of moderate buffer concentrations, similar to the 3-5-fold effects reported previously for buffer catalysis for protein enzymes. We observe this catalytic enhancement not only with standard laboratory buffers, but also with diverse biological small molecules, including imidazole, amino acids, and amino sugars. Brønsted plots suggest that small molecules assist in proton transfer, most likely with δ catalysis. Cellular small molecules provide a simple way to overcome the limited functional diversity of RNA and have the potential to participate in the catalytic mechanisms of many ribozymes in vivo.

Mechanism for activation of triosephosphate isomerase by phosphite dianion: the role of a hydrophobic clamp

The role of the hydrophobic side chains of Ile-172 and Leu-232 in catalysis of the reversible isomerization of R-glyceraldehyde 3-phosphate (GAP) to dihydroxyacetone phosphate (DHAP) by triosephosphate isomerase (TIM) from Trypanosoma brucei brucei (Tbb) has been investigated. The I172A and L232A mutations result in 100- and 6-fold decreases in k(cat)/K(m) for the isomerization reaction, respectively. The effect of the mutations on the product distributions for the catalyzed reactions of GAP and of [1-(13)C]-glycolaldehyde ([1-(13)C]-GA) in D(2)O is reported. The 40% yield of DHAP from wild-type Tbb TIM-catalyzed isomerization of GAP with intramolecular transfer of hydrogen is found to decrease to 13% and to 4%, respectively, for the reactions catalyzed by the I172A and L232A mutants. Likewise, the 13% yield of [2-(13)C]-GA from isomerization of [1-(13)C]-GA in D(2)O is found to decrease to 2% and to 1%, respectively, for the reactions catalyzed by the I172A and L232A mutants. The decrease in the yield of the product of intramolecular transfer of hydrogen is consistent with a repositioning of groups at the active site that favors transfer of the substrate-derived hydrogen to the protein or the oxygen anion of the bound intermediate. The I172A and L232A mutations result in (a) a >10-fold decrease (I172A) and a 17-fold increase (L232A) in the second-order rate constant for the TIM-catalyzed reaction of [1-(13)C]-GA in D(2)O, (b) a 170-fold decrease (I172A) and 25-fold increase (L232A) in the third-order rate constant for phosphite dianion (HPO(3)(2-)) activation of the TIM-catalyzed reaction of GA in D(2)O, and (c) a 1.5-fold decrease (I172A) and a larger 16-fold decrease (L232A) in K(d) for activation of TIM by HPO(3)(2-) in D(2)O. The effects of the I172A mutation on the kinetic parameters for the wild-type TIM-catalyzed reactions of the whole substrate and substrate pieces are consistent with a decrease in the basicity of the carboxylate side chain of Glu-167 for the mutant enzyme. The data provide striking evidence that the L232A mutation leads to a ca. 1.7 kcal/mol stabilization of a catalytically active loop-closed form of TIM (E(C)) relative to an inactive open form (E(O)).

A paradigm for enzyme-catalyzed proton transfer at carbon: triosephosphate isomerase

Triosephosphate isomerase (TIM) catalyzes the stereospecific 1,2-proton shift at dihydroxyacetone phosphate (DHAP) to give (R)-glyceraldehyde 3-phosphate through a pair of isomeric enzyme-bound cis-enediolate phosphate intermediates. The chemical transformations that occur at the active site of TIM were well understood by the early 1990s. The mechanism for enzyme-catalyzed isomerization is similar to that for the nonenzymatic reaction in water, but the origin of the catalytic rate acceleration is not understood. We review the results of experimental work that show that a substantial fraction of the large 12 kcal/mol intrinsic binding energy of the nonreacting phosphodianion fragment of TIM is utilized to activate the active site side chains for catalysis of proton transfer. Evidence is presented that this activation is due to a phosphodianion-driven conformational change, the most dramatic feature of which is closure of loop 6 over the dianion. The kinetic data are interpreted within the framework of a model in which activation is due to the stabilization by the phosphodianion of a rare, desolvated, loop-closed form of TIM. The dianion binding energy is proposed to drive the otherwise thermodynamically unfavorable desolvation of the solvent-exposed active site. This reduces the effective local dielectric constant of the active site, to enhance stabilizing electrostatic interactions between polar groups and the anionic transition state, and increases the basicity of the carboxylate side chain of Glu-165 that functions to deprotonate the bound carbon acid substrate. A rebuttal is presented to the recent proposal [Samanta, M., Murthy, M. R. N., Balaram, H., and Balaram, P. (2011) ChemBioChem 12, 1886-1895] that the cationic side chain of K12 functions as an active site electrophile to protonate the carbonyl oxygen of DHAP.

Transport activity of the sodium bicarbonate cotransporter NBCe1 is enhanced by different isoforms of carbonic anhydrase

Transport metabolons have been discussed between carbonic anhydrase II (CAII) and several membrane transporters. We have now studied different CA isoforms, expressed in Xenopus oocytes alone and together with the electrogenic sodium bicarbonate cotransporter 1 (NBCe1), to determine their catalytic activity and their ability to enhance NBCe1 transport activity. pH measurements in intact oocytes indicated similar activity of CAI, CAII and CAIII, while in vitro CAIII had no measurable activity and CAI only 30% of the activity of CAII. All three CA isoforms increased transport activity of NBCe1, as measured by the transport current and the rate of intracellular sodium rise in oocytes. Two CAII mutants, altered in their intramolecular proton pathway, CAII-H64A and CAII-Y7F, showed significant catalytic activity and also enhanced NBCe1 transport activity. The effect of CAI, CAII, and CAII mutants on NBCe1 activity could be reversed by blocking CA activity with ethoxyzolamide (EZA, 10 µM), while the effect of the less EZA-sensitive CAIII was not reversed. Our results indicate that different CA isoforms and mutants, even if they show little enzymatic activity in vitro, may display significant catalytic activity in intact cells, and that the ability of CA to enhance NBCe1 transport appears to depend primarily on its catalytic activity.

Structure of a dimeric fungal α-type carbonic anhydrase

The crystal structure of Aspergillus oryzae carbonic anhydrase (AoCA) was determined at 2.7Å resolution and it revealed a dimer, which only has precedents in the α class in two membrane and cancer-associated enzymes. α carbonic anhydrases are underrepresented in fungi compared to the β class, this being the first structural representative. The overall fold and zinc binding site resemble other well studied carbonic anhydrases. A major difference is that the histidine, thought to be the major proton shuttle residue in most mammalian enzymes, is replaced by a phenylalanine in AoCA. This finding poses intriguing questions as to the biological functions of fungal α carbonic anhydrases, which are promising candidates for biotechnological applications.

Intramolecular proton shuttle supports not only catalytic but also noncatalytic function of carbonic anhydrase II

Carbonic anhydrases (CAs) catalyze the reversible hydration of CO(2) to HCO(3)(-) and H(+). The rate-limiting step in this reaction is the shuttle of protons between the catalytic center of the enzyme and the bulk solution. In carbonic anhydrase II (CAII), the fastest and most wide-spread isoform, this H(+) shuttle is facilitated by the side chain of His64, whereas CA isoforms such as carbonic anhydrase III (CAIII), which lack such a shuttle, have only low catalytic activity in vitro. By using heterologous protein expression in Xenopus oocytes, we tested the role of this intramolecular H(+) shuttle on CA activity in an intact cell. The data revealed that CAIII, shown in vitro to have ∼1,000-fold reduced activity as compared with CAII, displays significant catalytic activity in the intact cell. Furthermore, we tested the hypothesis that the H(+) shuttle in CAII itself can facilitate transport activity of the monocarboxylate transporters 1 and 4 (MCT1/4) independent of catalytic activity. Our results show that His64 is essential for the enhancement of lactate transport via MCT1/4, because a mutation of this residue to alanine (CAII-H64A) abolishes the CAII-induced increase in MCT1/4 activity. However, injection of 4-methylimidazole, which acts as an exogenous H(+) donor/acceptor, can restore the ability of CAII-H64A to enhance transport activity of MCT1/4. These findings support the hypothesis that the H(+) shuttle in CAII not only facilitates CAII catalytic activity but also can enhance activity of acid-/base-transporting proteins such as MCT1/4 in a direct, noncatalytic manner, possibly by acting as an "H(+)-collecting antenna."

Mechanism of proton transfer in the 3alpha-hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni

3alpha-hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni catalyzes the oxidation of androsterone with NAD(+) to form androstanedione and NADH with a concomitant releasing of protons to bulk solvent. To probe the proton transfer during the enzyme reaction, we used mutagenesis, chemical rescue, and kinetic isotope effects to investigate the release of protons. The kinetic isotope effects of (D)V and (D(2)O)V for wild-type enzyme are 1 and 2.1 at pL 10.4 (where L represents H, (2)H), respectively, and suggest a rate-limiting step in the intramolecular proton transfer. Substitution of alanine for Lys(159) changes the rate-limiting step to the hydride transfer, evidenced by an equal deuterium isotope effect of 1.8 on V(max) and V/K(androsterone) and no solvent kinetic isotope effect at saturating 3-(cyclohexylamino)propanesulfonic acid (CAPS). However, a value of 4.4 on V(max) is observed at 10 mm CAPS at pL 10.4, indicating a rate-limiting proton transfer. The rate of the proton transfer is blocked in the K159A and K159M mutants but can be rescued using exogenous proton acceptors, such as buffers, small primary amines, and azide. The Brønsted relationship between the log(V/K(d)(-base)Et) of the external amine (corrected for molecular size effects) and pK(a) is linear for the K159A mutant-catalyzed reaction at pH 10.4 (beta = 0.85 +/- 0.09) at 5 mm CAPS. These results show that proton transfer to the external base with a late transition state occurred in a rate-limiting step. Furthermore, a proton inventory on V/Et is bowl-shaped for both the wild-type and K159A mutant enzymes and indicates a two-proton transfer in the transition state from Tyr(155) to Lys(159) via 2'-OH of ribose.

A cluster of carboxylic groups in PsbO protein is involved in proton transfer from the water oxidizing complex of Photosystem II

The hypothesis presented here for proton transfer away from the water oxidation complex of Photosystem II (PSII) is supported by biochemical experiments on the isolated PsbO protein in solution, theoretical analyses of better understood proton transfer systems like bacteriorhodopsin and cytochrome oxidase, and the recently published 3D structure of PS II (Pdb entry 1S5L). We propose that a cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as a buffering network providing efficient acceptors of protons derived from substrate water molecules. The charge delocalization of the cluster ensures readiness to promptly accept the protons liberated from substrate water. Therefore protons generated at the catalytic centre of PSII need not be released into the thylakoid lumen as generally thought. The cluster is the beginning of a localized, fast proton transfer conduit on the lumenal side of the thylakoid membrane. Proton-dependent conformational changes of PsbO may play a role in the regulation of both supply of substrate water to the water oxidizing complex and the resultant proton transfer.

The structure and function of carbonic anhydrase isozymes in the respiratory system of vertebrates

Carbonic anhydrase is a ubiquitous metalloenzyme that catalyzes the reversible hydration/dehydration of carbon dioxide. To date, 16 different CA isozymes have been identified in mammals, and several novel isozymes have also been identified in non-mammalian vertebrates. These isozymes are involved in many physiological processes; however, one of the most important roles is facilitating the transport and subsequent excretion of carbon dioxide. As such, CA isozymes are found at virtually every step of the process, including the metabolic site of CO(2) production (muscle), the circulating red blood cells, and the primary respiratory surface (gills/lungs). This review will examine the structural characteristics that are integral to CAs participation in respiration, as well as highlight the specific roles and tissues that the different CA isozymes are involved in.

Proton transfer within the active-site cavity of carbonic anhydrase III

The maximal turnover rate of CO2 hydration catalyzed by the carbonic anhydrases is limited by proton transfer steps from the zinc-bound water to solution, steps that regenerate the catalytically active zinc-bound hydroxide. Catalysis of CO2 hydration by wild-type human carbonic anhydrase III (HCA III) (k(cat) = 2 ms (-1)) is the least efficient among the carbonic anhydrases in its class, in part because it lacks an efficient proton shuttle residue. We have used site-directed mutagenesis to test positions within the active-site cavity of HCA III for their ability to carry out proton transfer by replacing various residues with histidine. Catalysis by wild-type HCA III and these six variants was determined from the initial velocity of hydration of CO2 measured by stopped-flow spectrophotometry and from the exchange of 18O between CO2 and H2O at chemical equilibrium by mass spectrometry. The results show that histidine at three positions (Lys64His, Arg67His and Phe131His) have the capacity to transfer protons during catalysis, enhancing maximal velocity of CO2 hydration and 18O exchange from 4- to 15-fold compared with wild-type HCA III. Histidine residues at the other three positions (Trp5His, Tyr7His, Phe20His) showed no firm evidence for proton transfer. These results are discussed in terms of the stereochemistry of the active-site cavity and possible proton transfer pathways.

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.

Carbonic anhydrase inhibitors. Arylsulfonylureido- and arylureido-substituted aromatic and heterocyclic sulfonamides: towards selective inhibitors of carbonic anhydrase isozyme I

Reaction of twenty aromatic/heterocyclic sulfonamides containing a free amino, imino, hydrazino or hydroxyl group, with tosyl isocyanate or 3,4-dichlorophenyl isocyanate afforded two series of derivatives containing arylsulfonylureido or diarylureido moieties in their molecule respectively. The new derivatives were assayed as inhibitors of three carbonic anhydrase (CA) isozymes, CA I, II (cytosolic forms) and IV (membrane-bound form). Potent inhibition was observed against all three isozymes but especially against CA I, which is generally 10-75 times less susceptible to inhibition by the classical sulfonamides in clinical use as compared to the other major red cell isozyme, CA II, or the membrane-bound one, CA IV. The derivatives obtained from tosyl isocyanate were generally more potent than the corresponding ones obtained from 3,4-dichlorophenyl isocyanate. This is the first reported example of selective inhibition of CA I and might lead to more selective drugs/diagnostic agents from this class of pharmacologically relevant compounds.

Intermolecular proton transfer in catalysis by carbonic anhydrase V

The dehydration of bicarbonate catalyzed by carbonic anhydrase is accompanied by the transfer of a proton from solution to the zinc-bound hydroxide. We have investigated the properties of proton transfer from donors in solution, mostly derivatives of imidazole and pyridine, to a truncated mutant of carbonic anhydrase V with replacements that render the active site cavity less sterically constrained, Tyr 64 →> Ala and Phe 65 →> Ala. Catalysis was measured by determining the rate of exchange of 18O between the CO2-HCO3- system and water, and rate constants for proton transfer were estimated as the rate-limiting step in the release of H218O from the enzyme to solution. Each proton donor enhanced catalytic activity in a saturable manner. The resulting rate constants for proton transfer when compared with the values of pKa of the donor and acceptor gave a Brønsted plot of high curvature. These data could also be described by Marcus theory which showed an intrinsic barrier for intermolecular proton transfer near 0.8 kcal/mol and a work term or thermodynamic contribution to the free energy of reaction near 10 kcal/mol. This low intrinsic kinetic barrier for proton transfer is very similar to nonenzymic bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. However, the significant thermodynamic contribution suggests appreciable involvement of solvent and active-site organization prior to proton transfer. These Marcus parameters are very similar to those describing intramolecular proton transfer from His 64 in carbonic anhydrase, suggesting similarities in the intra- and intermolecular proton transfer processes.Key words: carbonic anhydrase, proton transfer, Marcus theory, carbon dioxide.

Insertion scanning mutagenesis of subunit a of the F1F0 ATP synthase near His245 and implications on gating of the proton channel

Subunit a of the E. coli F1F0 ATP synthase was probed by insertion scanning mutagenesis in a region between residues Glu219 and His245. A series of single amino acid insertions, of both alanine and aspartic acid, were constructed after the following residues: 225, 229, 233, 238, 243, and 245. The mutants were tested for growth yield, binding of F1 to membranes, dicyclohexylcarbodiimide sensitivity of ATPase activity, ATP-driven proton translocation, and passive proton permeability of membranes stripped of F1. Significant loss of function was seen only with insertions after positions 238 and 243. In contrast, both insertions after residue 225 and the alanine insertion after residue 245 were nearly identical in function to the wild type. The other insertions showed an intermediate loss of function. Missense mutations of His245 to serine and cysteine were nonfunctional, while the W241C mutant showed nearly normal ATPase function. Replacement of Leu162 by histidine failed to suppress the 245 mutants, but chemical rescue of H245S was partially successful using acetate. An interaction between Trp241 and His245 may be involved in gating a "half-channel" from the periplasmic surface of F0 to Asp61 of subunit a.

Increase of local hydrogen ion gradient near bilayer lipid membrane under the conditions of catalysis of proton transfer across the interface

The measurements of pH profiles in the unstirred layers (USLs) near planar bilayer lipid membranes (BLM) were applied for the evaluation of the hydrogen ion fluxes which were induced by nigericin in the presence of potassium ion gradients. It was shown that at high concentrations of KCl the increase in the concentration of citrate buffer caused an anomalous effect, namely, an increase in the local pH shifts in the USLs. The hydrogen ion flux rose 50 times upon the increase in the citrate concentration from 1 mM to 20 mM. Phosphate stimulated the flux 7 times under these conditions. In agreement with our previous results, at low KCl concentrations, when the process is limited by the K+-nigericin interaction, an increase in the buffer concentration led to a reduction of the local pH shifts, under these conditions the usual concentration dependence was observed. The data obtained favor the model implying the existence of the kinetic barrier for proton transfer at the membrane-water interface.

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.

Mitochondrial carbonic anhydrase (isozyme V) in mouse and rat: cDNA cloning, expression, subcellular localization, processing, and tissue distribution

When the human cDNA, isolated on the basis of homology to the murine carbonic anhydrase (CA) "Y" was expressed in COS cells, the human CA was targeted to and processed in mitochondria, as expected for CA-V. However, tissue distribution reported for the corresponding mouse CA Y mRNA was much more limited than that reported for the distribution of CA-V immunostaining in rat tissues. To determine whether the murine cDNA actually encodes a mitochondrial CA activity and to compare the tissue distribution of the homologous murine and rat gene products, we used reverse transcription-PCR to reisolate the murine CA-V candidate cDNA and used the murine cDNA probe to isolate the homologous rat cDNA. We compared the two cDNA sequences, the activities they expressed after transfection of COS cells, and the sites of N-terminal processing of expressed products. In addition, we used antibodies to the C-terminal peptides predicted from each cDNA to compare distribution of CA-V in mouse and rat tissues and to identify CA-Vs in mitochondria isolated from mouse and rat liver. From these studies, we conclude that both mouse and rat CA-V candidate cDNAs encode active CAs that are targeted to and processed in mitochondria and that there are real differences in tissue distribution of CA-V between mouse and rat. However, the findings that are M(r) of CA-V in rat tissues is smaller than that previously reported and that the tissue distribution also differs lead us to conclude that the antibody used in prior reports most likely misidentified another antigen in rat tissues as CA-V.

Human mitochondrial carbonic anhydrase: cDNA cloning, expression, subcellular localization, and mapping to chromosome 16

A full-length cDNA clone encoding human mitochondrial carbonic anhydrase (CA), CA V, was isolated from a human liver cDNA library. The 1123-bp cDNA includes a 55-bp 5' untranslated region, a 915-bp open reading frame, and a 153-bp 3' untranslated region. Expression of the cDNA in COS cells produced active enzyme. The 34-kDa precursor and 30-kDa mature form of CA V were identified on Western blots of COS-cell homogenates by a CA V-specific antibody raised to a synthetic peptide corresponding to the C-terminal 17 aa of CA V. Both 34-kDa and 30-kDa bands were also present in mitochondria isolated from transfected COS cells, whereas only the 30-kDa band was present in mitochondria isolated from normal human liver. The N-terminal sequence determined directly on the 30-kDa soluble CA purified from transfected COS cells indicated that processing of the precursor to mature human CA V involves removal of a 38-aa mitochondrial leader sequence. The 267-aa sequence deduced for mature human CA V shows 30-49% similarity to amino acid sequences of previously characterized human CAs (CA I-CA VII) and 76% similarity to the corresponding amino acid sequence deduced from the mouse cDNA. PCR analysis of DNAs from human-rodent somatic cell hybrids localized the gene for CA V to human chromosome 16, the same chromosome to which CA VII has previously been mapped.

A T7 expression vector optimized for site-directed mutagenesis using oligodeoxyribonucleotide cassettes

Site-directed mutagenesis is widely used to examine structure/function relationships in proteins. We have designed a bacterial expression vector series which is optimized for efficient site-directed mutagenesis and subsequent protein synthesis without intervening subcloning steps. The vectors, derived from the T7 expression vectors of Studier and his collaborators [Studier et al., Methods Enzymol. 185 (1990) 60-89], are small and have a bacteriophage f1 origin of replication for production of single-stranded (ss) DNA. Both single-site mutants [using ssDNA and mutating oligodeoxyribonucleotides (oligos)] and cassette mutants (mutagenesis of a short region by inserting double-stranded oligos into unique restriction sites) are rapidly synthesized and expressed with these vectors. Vector construction and use are detailed with examples showing the expression of the sequences encoding human carbonic anhydrases II and III. Production levels of greater than 60 mg of protein per liter of culture have been obtained.

Specific activation of a tyrosine----glycine mutant of delta 5-3-ketosteroid isomerase by phenols

A key unknown still to be explored concerning the mechanism of delta 5-3-ketosteroid isomerase from Pseudomonas testosteroni is the extent of the proton transfer between tyrosine-14 of the enzyme and the C-3 carbonyl oxygen of the steroid substrate. This report is a preliminary study of a system we are developing to allow us eventually to use a Brønsted analysis to measure this transfer. We describe the construction of an expression vector and tyrosine-14----glycine-14 mutant of the enzyme and its specific activation, in the manner of chemical rescue, by a variety of phenolic compounds. We suggest that the binding region of phenol is very tight and that the level of activation may be a result of steric constraints as well as of differences in the pKa' of the phenol.

Proton dissociation from nigericin at the membrane-water interface, the rate-limiting step of K+/H+ exchange on the bilayer lipid membrane

The rate of K+/H+ exchange through bilayer lipid membranes (BLM) induced by nigericin was measured by the method of pH gradient offset according to Antonenko, Yu.N. and Yaguzhinsky L.S. [(1990) Biochim. Biophys. Acta 1026, 236-240]. It was shown that under the conditions of high potassium ion concentration the rate of nigericin-mediated K+/H+ exchange increased with an increase in the concentrations of such buffer compounds as citric acid and MES. The concentration dependence was different for citrate and MES. The buffer concentration effect was absent at low potassium ion concentrations. Citrate increased the rate of K+/H+ exchange being added to the side of BLM where the K+ concentration was higher and had no effect at the opposite side. At high KCl and citrate concentrations, the rate of K+/H+ exchange was about 6 times lower in D2O when compared to H2O solutions. It is concluded that under certain experimental conditions the overall rate of the K+/H+ exchange induced by nigericin is determined by the rate of proton dissociation from nigericin at the membrane-water interface.