Phytoremediation of an arsenic-contaminated site using Pteris vittata L.: a two-year study

A field study was conducted to determine the efficiency of Chinese brake fern (Pteris vittata L.), an arsenic hyperaccumulator, on removal of arsenic from soil at an arsenic-contaminated site. Chinese brake ferns were planted on a site previously used to treat wood with chromated copper arsenate (CCA). Arsenic concentrations in surface and profile soil samples were determined for 2000, 2001, and 2002. In both 2001 and 2002, senesced and senescing fronds only, as well as all fronds, were harvested. Frond arsenic concentrations were not significantly different between the three harvests. Compared to senesced fronds, live fronds resulted in the greatest amount of arsenic removal. There were no significant differences in soil arsenic concentrations between 2000, 2001, and 2002, primarily due to the extreme variability in soil arsenic concentrations. However, the mean surface soil arsenic was reduced from 190 to 140 mg kg(-1). Approximately 19.3 g of arsenic were removed from the soil by Chinese brake fern. Therefore, this fern is capable of accumulating arsenic from the CCA -contaminated site and may be competitive, in terms of cost, to conventional remediation systems. However, better agronomic practices are needed to enhance plant growth and arsenic uptake to obtain maximum soil arsenic removal and to minimize remediation time.

A new approach to detect QRS complexes based on a histogram and genetic algorithm

A new approach, based on a histogram and genetic algorithm (GA), has been developed to rapidly and reliably detect QRS complexes of the ECG curve. Here, the new approach is described and the principle of the histogram and GA is explained. The results of detection of QRS complexes using the new method are given. Additionally, as an example of its novel application, the P-wave and f-wave (when atrial fibrillation is absent and present, respectively) are extracted easily and efficiently from the ECG curve using the method.

Proton transfer in a Thr200His mutant of human carbonic anhydrase II

Human carbonic anhydrase II (HCA II) has a histidine at position 64 (His64) that donates a proton to the zinc-bound hydroxide in catalysis of the dehydration of bicarbonate. To examine the effect of the histidine location on proton shuttling, His64 was replaced with Ala and Thr200 replaced with histidine (H64A-T200H HCAII), effectively relocating the proton shuttle residue 2 A closer to the zinc-bound hydroxide compared to wild type HCA II. The crystal structure of H64A-T200H HCA II at 1.8 A resolution shows the side chain of His200 directly hydrogen-bonded with the zinc-bound solvent. Different proton transfer processes were observed at pH 6 and at pH 8 during the catalytic hydration-dehydration cycle, measured by mass spectrometry as the depletion of 18O from C18O2 by H64A-T200H HCA II. The process at pH 6.0 is attributed to proton transfer between the side chain of His200 and the zinc-bound hydroxide, in analogy with proton transfer involving His64 in wild-type HCA II. At pH 8.0 it is attributed to proton transfer between bicarbonate and the zinc-bound hydroxide, as supported by the dependence of the rate of proton transfer on bicarbonate concentration and on solvent hydrogen isotope effects. This study establishes that a histidine directly hydrogen-bonded to the zinc-bound hydroxide, can adopt the correct distance geometry to support proton transfer

Human Carbonic Anhydrase III: Structural and Kinetic Study of Catalysis and Proton Transfer,

The residue phenylalanine 198 (Phe 198) is a prominent cause of the lower activity of human carbonic anhydrase III (HCA III) compared with HCA II and other isozymes which have leucine at this site. We report the crystal structures of HCA III and the site-directed mutant F198L HCA III, both at 2.1 A resolution, and the enhancement of catalytic activity by exogenous proton donors containing imidazole rings. Both enzymes had a hexahistidine extension at the carboxy-terminal end, used to aid in purification, that was ordered in the crystal structures bound in the active site cavity of an adjacent symmetry-related enzyme. This observation allowed us to comment on a number of possible binding sites for imidazole and derivatives as exogenous proton donors/acceptors in catalysis by HCA III. Kinetic and structural evidence indicates that the phenyl side chain of Phe 198 in HCA III, about 5 A from the zinc, is a steric constriction in the active site, may cause altered interactions at the zinc-bound solvent, and is a binding site for the activation of catalysis by histidylhistidine. This suggests that sites of activation of the proton-transfer pathway in carbonic anhydrase are closer to the zinc than considered in previous studies.

Proton transfer from exogenous donors in catalysis by human carbonic anhydrase II

In the site-specific mutant of human carbonic anhydrase in which the proton shuttle His64 is replaced with alanine, H64A HCA II, catalysis can be activated in a saturable manner by the proton donor 4-methylimidazole (4-MI). From 1H NMR relaxivities, we found 4-MI bound as a second-shell ligand of the tetrahedrally coordinated cobalt in Co(II)-substituted H64A HCA II, with 4-MI located about 4.5 A from the metal. Binding constants of 4-MI to H64A HCA II were estimated from: (1) NMR relaxation of the protons of 4-MI by Co(II)-H64A HCA II, (2) the visible absorption spectrum of Co(II)-H64A HCA II in the presence of 4-MI, (3) the inhibition by 4-MI of the catalytic hydration of CO2, and (4) from the catalyzed exchange of 18O between CO2 and water. These experiments along with previously reported crystallographic and catalytic data help identify a range of distances at which proton transfer is efficient in carbonic anhydrase II.

Structural and kinetic characterization of active-site histidine as a proton shuttle in catalysis by human carbonic anhydrase II

In the catalysis of the hydration of carbon dioxide and dehydration of bicarbonate by human carbonic anhydrase II (HCA II), a histidine residue (His64) shuttles protons between the zinc-bound solvent molecule and the bulk solution. To evaluate the effect of the position of the shuttle histidine and pH on proton shuttling, we have examined the catalysis and crystal structures of wild-type HCA II and two double mutants: H64A/N62H and H64A/N67H HCA II. His62 and His67 both have their side chains extending into the active-site cavity with distances from the zinc approximately equivalent to that of His64. Crystal structures were determined at pH 5.1-10.0, and the catalysis of the exchange of (18)O between CO(2) and water was assessed by mass spectrometry. Efficient proton shuttle exceeding a rate of 10(5) s(-)(1) was observed for histidine at positions 64 and 67; in contrast, relatively inefficient proton transfer at a rate near 10(3) s(-)(1) was observed for His62. The observation, in the crystal structures, of a completed hydrogen-bonded water chain between the histidine shuttle residue and the zinc-bound solvent does not appear to be required for efficient proton transfer. The data suggest that the number of intervening water molecules between the donor and acceptor supporting efficient proton transfer in HCA II is important, and furthermore suggest that a water bridge consisting of two intervening water molecules is consistent with efficient proton transfer.

Exchange of (18)O in the reaction of peroxynitrite with CO(2)

We have observed an exchange of (18)O in the reactions of CO(2) with peroxynitrite using membrane-inlet mass spectrometry and HPLC negative electrospray ionization mass spectrometry. The exchange appeared on addition of peroxynitrite to a solution containing (18)O-labeled CO(2) in equilibrium with bicarbonate. It was observed as a temporarily enhanced rate of depletion of (18)O from CO(2), a rate that was greater than the rate of (18)O depletion caused by the hydration/dehydration cycle of CO(2). In addition, we detected the appearance of mass peaks attributed to (18)O in product NO(3)(-). As a further measure of the (18)O exchange, there was a redistribution of (18)O such that the ratio of doubly to singly labeled CO(2) could not be described by the binomial expansion. This is not due to the hydration/dehydration cycle of CO(2) but most likely to recycling of CO(2) in the reaction with peroxynitrite. This (18)O exchange associated with the reactions of CO(2) and peroxynitrite may open a new methodology for studying this significant process.

Examination of the role of Gln-158 in the mechanism of CO2 hydration catalyzed by β-carbonic anhydrase from Arabidopsis thaliana

We have cloned and overexpressed a variant of Arabidopsis thaliana beta-carbonic anhydrase (Q158A) that deletes the functional equivalent of the backbone amide NH of Thr-199 in human alpha-carbonic anhydrase II. The latter residue is hypothesized to be important in catalyzing the rate of CO(2)(-) HCO (3)(-) interconversion in alpha-carbonic anhydrase but this hypothesis is not directly testable in that enzyme. Kinetic studies of a variant of the functionally equivalent residue in A. thaliana beta-carbonic anhydrase provide direct evidence for the role of this residue in beta-carbonic anhydrase. Namely, the mutation of Gln-158 to Ala results in a significant decrease in the maximal k(cat) (33% of wild type) at steady state and the maximal rate of CO(2)(-) HCO(2)(-) exchange at chemical equilibrium as measured by R(1)/[E] (7% of wild type), while leaving the maximal rate of H(+) transfer, as measured by k(cat) at steady state, or R(H(2)O)) at chemical equilibrium, largely unaffected.

Alkalization of larval mosquito midgut and the role of carbonic anhydrase in different species of mosquitoes

We have previously demonstrated the involvement of carbonic anhydrase (CA) in the alkalization mechanism of the Aedes aegypti larval midgut. In this study, we used Hansson's histochemistry to examine the distribution of the enzyme in the midgut of six different species of mosquito larvae (Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, Culex nigripalpus, Ochlerotatus taeniorhynchus, Anopheles quadrimaculatus). Additionally, we quantitated CA content in the gastric caeca, anterior and posterior midgut of fourth instar larvae from these species using the 18O isotope exchange method coupled to mass spectrometry. We also tested the effect of CA inhibitors such as methazolamide and acetazolamide in the alkalization of the midgut for these species. Our results indicate that CA is present in the larval midgut of the species studied and that it appears to be associated with the posterior midgut and gastric caeca in some species and with the anterior midgut in others. CA inhibitors appear to have a profound effect on the alkalization mechanism of the midgut with lethal consequences for most of the species tested.

Activation of carbonic anhydrase II by active-site incorporation of histidine analogs

The hydration of CO2 catalyzed by human carbonic anhydrase II (HCA II) is accompanied by proton transfer from the zinc-bound water of the enzyme to solution. We have replaced the proton shuttling residue His 64 with Ala and placed cysteine residues within the active-site cavity by mutating sites Trp 5, Asn 62, Ile 91, and Phe 131. These mutants were modified at the single inserted cysteine with imidazole analogs to introduce new potential shuttle groups. Catalysis by these modified mutants was determined by stopped-flow and 18O-exchange methods. Specificity in proton transfer was demonstrated; only modifications of the Cys 131-containing mutant showed enhancement in the proton transfer step of catalysis compared with unmodified Cys 131-containing mutant. Modifications at other sites resulted in up to 3-fold enhancement in rates of CO2 hydration, with apparent second-order rate constants near 350 microM(-1) s(-1). These are among the largest values of kcat/Km observed for a carbonic anhydrase.

Impact of endoscopic ultrasound combined with fine-needle aspiration biopsy in the management of esophageal cancer

BACKGROUND AND STUDY AIMS

Endoscopic ultrasound (EUS) in combination with fine-needle aspiration biopsy (FNA) is a highly accurate method for the preoperative staging of esophageal cancer. Its impact on medical decision-making and the cost of care is unknown. This prospective case series was undertaken in order to determine the impact of EUS in combination with FNA on patients' choice of therapy and on the cost of care.

PATIENTS AND METHODS

Sixty consecutive patients with esophageal cancer, referred for preoperative EUS staging in a large tertiary-care academic medical center, were enrolled. The accuracy of EUS, the impact of EUS-based staging on the patients' choice of therapy, and costs were studied.

RESULTS

The accuracy rates for EUS combined with FNA in tumor and lymph-node staging were 83 % and 89 %, respectively. Twenty-five patients (42 %) had EUS stage I and II and were candidates for curative surgery. Twenty-eight patients (47 %) had stage III, and seven (12 %) had stage IV. All patients with stage I had surgery, while all patients with stage IV had medical therapy. The majority (62 %) of patients with stage II had surgery, while only a minority (25 %) of patients with stage III had surgery. Thirty-six patients (60 %) underwent medical therapy. Patients' medical decisions in favor of surgical or medical therapy correlated strongly with the results of their EUS staging ( P = 0.005), but not with age, sex, or referring physicians (surgeons vs. nonsurgeons). EUS-guided therapy potentially decreased the cost of care by $ 740 424 ($ 12 340/patient) by reducing the number of thoracotomies.

CONCLUSIONS

Patients' decisions regarding therapy correlated with their overall tumor staging, suggesting that the information provided by EUS played a significant role in patients' decision-making. The use of EUS in combination with FNA reduces the cost of managing patients with esophageal cancer.

Root-parasitic nematodes enhance soil microbial activities and nitrogen mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes ( Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg(-1) soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.

The refined atomic structure of carbonic anhydrase II at 1.05 A resolution: implications of chemical rescue of proton transfer

Using synchrotron radiation and a CCD detector, X-ray data have been collected at 100 K for the His64Ala mutant of human carbonic anhydrase II complexed with 4-methylimidazole (4-MI) to a maximal 1.05 A resolution, allowing full anisotropic least-squares refinement. The refined model has a conventional R factor of 15.7% for all reflections. The C(alpha) coordinates of the model presented here have an r.m.s. deviation of 0.10 A relative to the previously determined structure at 1.6 A resolution. Several amino-acid residues (six of the 255 observed) have been identified with multiple rotamer side-chain conformations. C, N and O atoms can be differentiated with selective electron-density map contouring. The estimated standard deviations for all main-chain non-H atom bond lengths and angles are 0.013 and 0.030 A, respectively, based on unrestrained full-matrix least-squares refinement. This structure gives detailed information about the tetrahedrally arranged zinc ion coordinated by three histidine N atoms (His94 N(epsilon 2), His96 N(epsilon2) and His119 N(delta1)) and a water/hydroxide, the multiple binding sites of the proton chemical rescue molecule 4-MI and the solvent networks linking the zinc-bound water/hydroxide and 4-MI molecules. This structure presents the highest resolution structure of a carbonic anhydrase isozyme so far determined and adds to the understanding of proton-transfer processes.

Chemical rescue of proton transfer in catalysis by carbonic anhydrases in the beta- and gamma-class

Catalysis of the dehydration of HCO(3)(-) by carbonic anhydrase requires proton transfer from solution to the zinc-bound hydroxide. Carbonic anhydrases in each of the alpha, beta, and gamma classes, examples of convergent evolution, appear to have a side chain extending into the active site cavity that acts as a proton shuttle to facilitate this proton transfer, with His 64 being the most prominent example in the alpha class. We have investigated chemical rescue of mutants in two of these classes in which a proton shuttle has been replaced with a residue that does not transfer protons: H216N carbonic anhydrase from Arabidopsis thaliana (beta class) and E84A carbonic anhydrase from the archeon Methanosarcina thermophila (gamma class). A series of structurally homologous imidazole and pyridine buffers were used as proton acceptors in the activation of CO(2) hydration at steady state and as proton donors of the exchange of (18)O between CO(2) and water at chemical equilibrium. Free energy plots of the rate constants for this intermolecular proton transfer as a function of the difference in pK(a) of donor and acceptor showed extensive curvature, indicating a small intrinsic kinetic barrier for the proton transfers. Application of Marcus rate theory allowed quantitative estimates of the intrinsic kinetic barrier which were near 0.3 kcal/mol with work functions in the range of 7-11 kcal/mol for mutants in the beta and gamma class, similar to results obtained for mutants of carbonic anhydrase in the alpha class. The low values of the intrinsic kinetic barrier for all three classes of carbonic anhydrase reflect proton transfer processes that are consistent with a model of very rapid proton transfer through a flexible matrix of hydrogen-bonded solvent structures sequestered within the active sites of the carbonic anhydrases.

Kinetic analysis of multiple proton shuttles in the active site of human carbonic anhydrase

We have prepared a site-specific mutant of human carbonic anhydrase (HCA) II with histidine residues at positions 7 and 64 in the active site cavity. Using a different isozyme, we have placed histidine residues in HCA III at positions 64 and 67 and in another mutant at positions 64 and 7. Each of these histidine residues can act as a proton transfer group in catalysis when it is the only nonliganding histidine in the active site cavity, except His(7) in HCA III. Using an (18)O exchange method to measure rate constants for intramolecular proton transfer, we have found that inserting two histidine residues into the active site cavity of either isozyme II or III of carbonic anhydrase results in rates of proton transfer to the zinc-bound hydroxide that are antagonistic or suppressive with respect to the corresponding single mutants. The crystal structure of Y7H HCA II, which contains both His(7) and His(64) within the active site cavity, shows the conformation of the side chain of His(64) moved from its position in the wild type and hydrogen-bonded through an intervening water molecule with the side chain of His(7). This suggests a cause of decreased proton transfer in catalysis.

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.

Kinetic characterization of wild-type and proton transfer-impaired variants of beta-carbonic anhydrase from Arabidopsis thaliana

We have cloned and overexpressed a truncated, recombinant form of beta-carbonic anhydrase from Arabidopsis thaliana. The wild-type enzyme and two site-directed variants, H216N and Y212F, have been kinetically characterized both at steady state by stopped-flow spectrophotometry and at chemical equilibrium by (18)O isotope exchange methods. The wild-type enzyme has a maximal k(cat) for CO2 hydration of 320 ms(-1) and is rate limited by proton transfer involving two residues with apparent pK(a) values of 6.0 and 8.7. The mutant enzyme H216N has a maximal k(cat) at high pH that is 43% that of wild type, but is only 5% that of wild type at pH 7.0. (18)O exchange studies reveal that the effect of the mutations H216N or Y212F is primarily on proton transfer steps in the catalytic mechanism and not in the rate of CO2-HCO3- exchange. These results suggest that residues His-216 and Tyr-212 are both important for efficient proton transfer in A. thaliana carbonic anhydrase.

Crystallization and preliminary X-ray analysis of human carbonic anhydrase III

Carbonic anhydrases catalyze the interconversion of carbon dioxide to bicarbonate. Human carbonic anhydrase isozyme III with a C-terminal hexahistidine tag was overexpressed in Eschericha coli, purified and crystallized. Diffraction data (93.4% completeness) were collected to 2.2 A resolution on an in-house R-AXIS IV++ image-plate system with Osmic mirrors and a Rigaku HU-H3R CU rotating-anode generator operating at 50 kV and 100 mA. A 60 degrees sweep of data were collected from a single crystal with a crystal-to-detector distance of 150 mm and a 0.5 degrees oscillation angle per frame using an exposure of 60 s per frame at 293 K. The crystals were shown to conform to the Laue hexagonal crystal system P6, with unit-cell parameters a = 44.7, c = 222.5 A and a scaling R(sym) of 0.087 for 11 962 unique reflections. Using the known crystal structure of the rat form of carbonic anhydrase isozyme III, a molecular-replacement model was built. This model was used for rotation and translation searches and uniquely defined the space group as P6(5). Rigid-body refinement of the model was used to generate an initial phased electron-density map with an R(work) of 31.17%.

Chemical rescue in catalysis by human carbonic anhydrases II and III

The maximal velocity of catalysis of CO(2) hydration by human carbonic anhydrase II (HCA II) requires proton transfer from zinc-bound water to solution assisted by His 64. The catalytic activity of a site-specific mutant of HCA II in which His 64 is replaced with Ala (H64A HCA II) can be rescued by exogenous proton donors/acceptors, usually derivatives of imidazole and pyridine. X-ray crystallography has identified Trp 5 as a binding site of the rescue agent 4-methylimidazole (4-MI) on H64A HCA II. This binding site overlaps with the "out" position in which His 64 in wild-type HCA II points away from the zinc. Activation by 4-MI as proton donor/acceptor in catalysis was determined in the dehydration direction using (18)O exchange between CO(2) and water and in the hydration direction by stopped-flow spectrophotometry. Replacement of Trp 5 by Ala, Leu, or Phe in H64A HCA II had no significant effect on enhancement by 4-MI of maximal rate constants for proton transfer in catalysis to levels near 10(5) s(-1). This high activity for chemical rescue indicates that the binding site of 4-MI at Trp 5 in H64A HCA II appears to be a nonproductive binding site, although it is possible that a similarly effective pathway for proton transfer exists in the mutants lacking Trp 5. Moreover, the data suggest that the out position of His 64 considered alone is not active in proton transfer in HCA II. In contrast to isozyme II, the replacement of Trp 5 by Ala in HCA III abolished chemical rescue of k(cat) by imidazole but left k(cat)/K(m) for hydration unchanged. This demonstrates that Trp 5 contributes to the predominant productive binding site for imidazole, with a maximal level for the rate constant of proton transfer near 10(4) s(-1). This difference in the susceptibility of CA II and III to chemical rescue may be related to the more sterically constrained and electrostatically positive nature of the active site cavity of CA III compared with CA II. The possibility of nonproductive binding sites for exogenous proton donors offers an explanation for the unusually low value of the intrinsic kinetic barrier obtained by application of Marcus theory to chemical rescue of H64A HCA II.

Carbonic anhydrase in the midgut of larvalAedes aegypti: cloning, localization and inhibition

The larval mosquito midgut exhibits one of the highest pH values known in a biological system. While the pH inside the posterior midgut and gastric caeca ranges between 7.0 and 8.0, the pH inside the anterior midgut is close to 11.0. Alkalization is likely to involve bicarbonate/carbonate ions. These ions are produced in vivo by the enzymatic action of carbonic anhydrase. The purpose of this study was to investigate the role of this enzyme in the alkalization mechanism, to establish its presence and localization in the midgut of larval Aedes aegypti and to clone and characterize its cDNA. Here, we report the physiological demonstration of the involvement of carbonic anhydrase in midgut alkalization. Histochemistry and in situ hybridization showed that the enzyme appears to be localized throughout the midgut, although preferentially in the gastric caeca and posterior regions with specific cellular heterogeneity. Furthermore, we report the cloning and localization of the first carbonic anhydrase from mosquito larval midgut. A cDNA clone from Aedes aegypti larval midgut revealed sequence homology to α-carbonic anhydrases from vertebrates. Bioinformatics indicates the presence of at least six carbonic anhydrases or closely related genes in the genome of another dipteran, the fruit fly Drosophila melanogaster. Molecular analyses suggest that the larval mosquito may also possess multiple forms.

Crystal structure of F65A/Y131C-methylimidazole carbonic anhydrase V reveals architectural features of an engineered proton shuttle

The crystal structure of F65A/Y131C murine alpha-carbonic anhydrase V (CAV), covalently modified at cysteine residues with 4-chloromethylimidazole, is reported at 1.88 A resolution. This modification introduces a methylimidazole (MI) group at residue C131 in the active site with important consequences. F65A/Y131C-MI CAV exhibits an up to 3-fold enhancement of catalytic activity over that of wild-type CAV [Earnhardt, J. N., Wright, S. K., Qian, M., Tu, C., Laipis, P. J., Viola, R. E., and Silverman, D. N. (1999) Arch. Biochem. Biophys. 361, 264-270]. In this modified CAV variant, C131-MI acts as a proton shuttle, facilitating the deprotonation of a zinc-bound water molecule to regenerate the nucleophilic zinc-bound hydroxide ion. A network of three hydrogen-bonded water molecules, across which proton transfer likely proceeds, bridges the zinc-bound water molecule and the C131-MI imidazole group. The structure of F65A/Y131C-MI CAV is compared to structures of Y64H/F65A murine CAV, wild-type human alpha-carbonic anhydrase II, and the gamma-carbonic anhydrase from Methanosarcina thermophilain an effort to outline common features of catalytic proton shuttles.

Role of arginine 59 in the gamma-class carbonic anhydrases

The functional role of the highly conserved active site Arg 59 in the prototype of the gamma-class carbonic anhydrase Cam (carbonic anhydrase from Methanosarcina thermophila) was investigated. Variants (R59A, -C, -E, -H, -K, -M, and -Q) were prepared by site-directed mutagenesis and characterized by size exclusion chromatography (SEC), circular dichroism (CD) spectroscopy, and stopped-flow kinetic analyses. CD spectra indicated similar secondary structures for the wild type and the R59A and -K variants, independent of nondenaturing concentrations of guanidine hydrochloride (GdnHCl). SEC indicated that all variants purified as homotrimers like the wild type. SEC also revealed that the R59A and -K variants unfolded at > or = 1.5 M GdnHCl, compared to 3.0 M GdnHCl for the wild type. These results indicate that Arg 59 contributes to the thermodynamic stability of the Cam trimer. The R59K variant had k(cat) and k(cat)/K(m) values that were 8 and 5% of the wild-type values, respectively, while all other variants had k(cat) and k(cat)/K(m) values 10-100-fold lower than those of the wild type. The R59A, -C, -E, -M, and -Q variants exhibited 4-63-fold increases in k(cat) and 9-120-fold increases in k(cat)/K(m) upon addition of 100 mM GdnHCl, with the largest increases observed for the R59A variant, which was comparable to the R59K variant. The kinetic results indicate that a positive charge at position 59 is essential for the CO(2) hydration step of the overall catalytic mechanism.