Role of Trp19 and Tyr200 in catalysis by the γ-class carbonic anhydrase from Methanosarcina thermophila.

Although widely distributed in Nature, only two γ class carbonic anhydrases are reported besides the founding member (Cam). Although roles for active-site residues important for catalysis have been identified in Cam, second shell residues have not been investigated. Two residues (Trp19 and Tyr200), positioned distant from the catalytic metal, were investigated by structural and kinetic analyses of replacement variants. Steady-state k(cat)/K(m) and k(cat) values decreased 3- to 10-fold for the Trp19 variants whereas the Y200 variants showed up to a 5-fold increase in k(cat). Rate constants for proton transfer decreased up to 10-fold for the Trp19 variants, and an increase of ~2-fold for Y200F. The pK(a) values for the proton donor decreased 1-2 pH units for Trp19 and Y200 variants. The variant structures revealed a loop composed of residues 62-64 that occupies a different conformation than previously reported. The results show that, although Trp19 and Y200 are non-essential, they contribute to an extended active-site structure distant from the catalytic metal that fine tunes catalysis. Trp19 is important for both CO(2)/bicarbonate interconversion, and the proton transfer step of catalysis.

Structural and kinetic effects on changes in the CO(2) binding pocket of human carbonic anhydrase II.

This work examines the effect of perturbing the position of bound CO(2) in the active site of human carbonic anhydrase II (HCA II) on catalysis. Variants of HCA II in which Val143 was replaced with hydrophobic residues Ile, Leu, and Ala were examined. The efficiency of catalysis in the hydration of CO(2) for these variants was characterized by (18)O exchange mass spectrometry, and their structures were determined by X-ray crystallography at 1.7-1.5 Šresolution. The most hydrophobic substitutions, V143I and V143L, showed decreases in the level of catalysis, as much as 20-fold, while the replacement by the smaller V143A mutation showed an only moderate 2-fold decrease in activity. Structural data for all three variants show no significant change in the overall position of amino acid side chains in the active site compared with the wild type. However, V143A HCA II showed additional ordered water molecules in the active site compared to the number for the wild type. To further investigate the decrease in the catalytic efficiency of V143I HCA II, an X-ray crystallographic CO(2) entrapment experiment was performed to 0.93 Šresolution. This structure revealed an unexpected shift in the CO(2) substrate toward the zinc-bound solvent, placing it ~0.3 Ǻ closer than previously observed in the wild type in conjunction with the observed dual occupancy of the product bicarbonate, presumably formed during the acquisition of data. These data suggest that the Ile substitution at position 143 reduced the catalytic efficiency, which is likely due to steric crowding resulting in destabilization of the transition state for conversion of CO(2) into bicarbonate and a decreased product dissociation rate.

The Escherichia coli clamp loader can actively pry open the ß-sliding clamp.

Clamp loaders load ring-shaped sliding clamps onto DNA. Once loaded onto DNA, sliding clamps bind to DNA polymerases to increase the processivity of DNA synthesis. To load clamps onto DNA, an open clamp loader-clamp complex must form. An unresolved question is whether clamp loaders capture clamps that have transiently opened or whether clamp loaders bind closed clamps and actively open clamps. A simple fluorescence-based clamp opening assay was developed to address this question and to determine how ATP binding contributes to clamp opening. A direct comparison of real time binding and opening reactions revealed that the Escherichia coli γ complex binds ß first and then opens the clamp. Mutation of conserved "arginine fingers" in the γ complex that interact with bound ATP decreased clamp opening activity showing that arginine fingers make an important contribution to the ATP-induced conformational changes that allow the clamp loader to pry open the clamp.

Structure and catalysis by carbonic anhydrase II: role of active-site tryptophan 5.

The tryptophan residue Trp5, highly conserved in the α class of carbonic anhydrases including human carbonic anhydrase II (HCA II), is positioned at the entrance of the active site cavity and forms a π-stacking interaction with the imidazole ring of the proton shuttle His64 in its outward orientation. We have observed that replacement of Trp5 in HCA II caused significant structural changes, as determined by X-ray diffraction, in the conformation of 11 residues at the N-terminus and in the orientation of the proton shuttle residue His64. Most significantly, two variants W5H and W5E HCA II had His64 predominantly outward in orientation, while W5F and wild type showed the superposition of both outward and inward orientations in crystal structures. Although Trp5 influences the orientation of the proton shuttle His64, this orientation had no significant effect on the rate constant for proton transfer near 1µs(-1), determined by exchange of (18)O between CO(2) and water measured by mass spectrometry. The apparent values of the pK(a) of the zinc-bound water and the proton shuttle residue suggest that different active-site conformations influence the two stages of catalysis, the proton transfer stage and the interconversion of CO(2) and bicarbonate.

Sulfonamides incorporating 1,3,5-triazine moieties selectively and potently inhibit carbonic anhydrase transmembrane isoforms IX, XII and XIV over cytosolic isoforms I and II: Solution and X-ray crystallographic studies.

Reaction of cyanuryl chloride with d,l-amino acids and amino alcohols afforded a new series of triazinyl-substituted benzenesulfonamides incorporating amino acyl/hydroxyalkyl-amino moieties. Inhibition studies of physiologically relevant human carbonic anhydrase (CA, EC 4.2.1.1) isoforms, such as CA I, II, IX, XII and XIV with these compounds are reported. They showed moderate-weak inhibition of the cytosolic, offtarget isozymes CA I and II, but many of them were low nanomolar inhibitors of the transmembrane, tumor-associated CA IX and XII (and also of CA XIV). The X-ray crystal structure of two of these compounds in adduct with CA II allowed us to understand the features associated with this strong inhibitory properties and possibly also their selectivity. Two of these compounds were also investigated for the inhibition of other human isoforms, that is, hCA IV, VA, VB, VI, VII and XIII, as well as inhibitors of the fungal pathogenic CAs Nce103 (Candida albicans) and Can2 (Cryptococcus neoformans), showing interesting activity. The 1,3,5-triazinyl-substituted benzenesulfonamides constitute thus a class of compounds with great potential for obtaining inhibitors targeting both α-class mammalian, tumor-associated, and ß-class from pathogenic organisms CAs.

Emerging from pseudo-symmetry: the redetermination of human carbonic anhydrase II in monoclinic P2(1) with a doubled a axis.

The crystal structure of human carbonic anhydrase II in the monoclinic P2(1) space group with a doubled a axis from that of the usually observed unit cell has recently been reported, with one of the two molecules in the asymmetric unit demonstrating rotational disorder [Robbins et al. (2010), Acta Cryst. D66, 628-634]. The structure has been redetermined, with the coordinates of both pseudo-symmetrically related molecules in the crystallographic asymmetric unit translated by x' = x +/- 1/4, and no rotational disorder is observed. This corresponds to a different choice of how the four molecules in the unit cell should be grouped into pairs that represent a single asymmetric unit.

Structure of a monoclinic polymorph of human carbonic anhydrase II with a doubled a axis.

The crystal structure of human carbonic anhydrase II with a doubled a axis from that of the usually observed monoclinic unit cell has been determined and refined to 1.4 A resolution. The diffraction data with h = 2n + 1 were systematically weaker than those with h = 2n. Consequently, the scaling of the data, structure solution and refinement were challenging. The two molecules comprising the asymmetric unit are related by a noncrystallographic translation of (1/2) along a, but one of the molecules has two alternate positions related by a rotation of approximately 2 degrees. This rotation axis is located near the edge of the central beta-sheet, causing a maximum distance disparity of 1.7 A between equivalent atoms on the diametrically opposite side of the molecule. The crystal-packing contacts are similar to two sequential combined unit cells along a of the previously determined monoclinic unit cell. Abnormally high final R(cryst) and R(free) values (20.2% and 23.7%, respectively) are not unusual for structures containing pseudo-translational symmetry and probably result from poor signal to noise in the weak h-odd data.

Structure of the unbound form of HIV-1 subtype A protease: comparison with unbound forms of proteases from other HIV subtypes.

The crystal structure of the unbound form of HIV-1 subtype A protease (PR) has been determined to 1.7 A resolution and refined as a homodimer in the hexagonal space group P6(1) to an R(cryst) of 20.5%. The structure is similar in overall shape and fold to the previously determined subtype B, C and F PRs. The major differences lie in the conformation of the flap region. The flaps in the crystal structures of the unbound subtype B and C PRs, which were crystallized in tetragonal space groups, are either semi-open or wide open. In the present structure of subtype A PR the flaps are found in the closed position, a conformation that would be more anticipated in the structure of HIV protease complexed with an inhibitor. The amino-acid differences between the subtypes and their respective crystal space groups are discussed in terms of the differences in the flap conformations.

Carbonic anhydrase inhibitors. The X-ray crystal structure of human isoform II in adduct with an adamantyl analogue of acetazolamide resides in a less utilized binding pocket than most hydrophobic inhibitors.

We investigated the inhibitory activity of several 1,3,4-thiadiazole-sulfonamides against all catalytically active CA (EC 4.2.1.1), CA I-XV. The tail derivatizing the 5-position in the 1,3,4-thiadiazole-2-sulfonamide scaffold was observed to be critical as an inhibitory determinant of these compounds. The high resolution X-ray crystal structure of hCA II in complex with 5-(1-adamantylcarboxamido)-1,3,4-thiadiazole-2-sulfonamide, showed the adamantyl moiety of the inhibitor residing in a less utilized binding pocket than that of most hydrophobic inhibitors, lined by the amino acid residues Ile91, Val121 and Phe131. This binding site may explain the diverse inhibition profiles of 5-carboxamide- and sufonamide-derivatized 1,3,4-thiadiazole-2-sulfonamides and offers a hot spot for designing isoform selective inhibitors, considering that residues 91 and 131 are highly variable among the 13 catalytically active isoforms.

Coumarinyl-substituted sulfonamides strongly inhibit several human carbonic anhydrase isoforms: solution and crystallographic investigations.

We investigated a series of coumarinyl-substituted aromatic sulfonamides as inhibitors of four carbonic anhydrase (CA, EC 4.2.1.1) isoforms with medical applications, the cytosolic hCA I, and II, and the transmembrane, tumor-associated hCA IX and XII. Compounds incorporating 7-methoxy-coumarin-4-yl-acetamide-tails and benzenesulfonamide and benzene-1,3-disulfonamide scaffolds showed medium potency inhibition of hCA I (KIs of 73-131 nM), effective hCA II inhibition (KIs of 9.1-36 nM) and less effective hCA IX and XII inhibition (KIs of 55-128 nM). Only one compound, the derivatized 4-amino-6-trifluoromethyl-benzene-1,3-disulfonamide with the coumarinyl tail, showed effective inhibition of the transmembrane isoforms, with KIs of 5.9-14.2 nM, although it was less effective as hCA I and II inhibitor (KIs of 36-120 nM). An X-ray crystal structure of hCA II in complex with 4-(7-methoxy-coumarin-4-yl-acetamido)-benzenesulfonamide (KI of 9.1 nM against hCA II) showed the intact inhibitor coordinated to the zinc ion from the enzyme active site by the sulfonamide moiety, and participating in a edge-to-face stacking with Phe131, in addition to other hydrophobic and hydrophilic interactions with water molecules and amino acid residues from the active site. Thus, sulfonamides incorporating coumarin rings have a distinct inhibition mechanism compared to the coumarins, and may lead to compounds with interesting inhibition profiles against various alpha-CAs found in mammals or parasites, such as Plasmodium falciparum.

Structural insights into the extracytoplasmic thiamine-binding lipoprotein p37 of Mycoplasma hyorhinis.

The Mycoplasma hyorhinis protein p37 has been implicated in tumorigenic transformation for more than 20 years. Though there are many speculations as to its function, based solely on sequence homology, the issue has remained unresolved. Presented here is the 1.6-A-resolution refined crystal structure of M. hyorhinis p37, renamed the extracytoplasmic thiamine-binding lipoprotein (Cypl). The structure shows thiamine pyrophosphate (TPP) and two calcium ions are bound to Cypl and give the first insights into possible functions of the Cypl-like family of proteins. Sequence alignments of Cypl-like proteins between several different species of mycoplasma show that the thiamine-binding site is likely conserved and structural alignments reveal the similarity of Cypl to various binding proteins. While the experimentally determined function of Cypl remains unknown, the structure shows that the protein is a TPP-binding protein, opening up many avenues for future mechanistic studies and making Cypl a possible target for combating mycoplasma infections and tumorigenic transformation.

Recombinant plasmepsin 1 from the human malaria parasite plasmodium falciparum: enzymatic characterization, active site inhibitor design, and structural analysis.

A mutated form of truncated proplasmepsin 1 (proPfPM1) from the human malaria parasite Plasmodium falciparum, proPfPM1 K110pN, was generated and overexpressed in Escherichia coli. The automaturation process was carried out at pH 4.0 and 4.5, and the optimal catalytic pH of the resulting mature PfPM1 was determined to be pH 5.5. This mature PfPM1 showed comparable binding affinity to peptide substrates and inhibitors with the naturally occurring form isolated from parasites. The S3-S3' subsite preferences of the recombinant mature PfPM1 were explored using combinatorial chemistry based peptide libraries. On the basis of the results, a peptidomimetic inhibitor (compound 1) was designed and yielded 5-fold selectivity for binding to PfPM1 versus the homologous human cathepsin D (hcatD). The 2.8 A structure of the PfPM2-compound 1 complex is reported. Modeling studies were conducted using a series of peptidomimetic inhibitors (compounds 1-6, Table 3) and three plasmepsins: the crystal structure of PfPM2, and homology derived models of PfPM1 and PfPM4.

Crystallographic evidence for noncoplanar catalytic aspartic acids in plasmepsin II resides in the Protein Data Bank.

The carboxylate atoms of the two catalytic aspartic acid residues in aspartic proteases are nearly coplanar and in the uncomplexed form share an in-plane nucleophilic water molecule that is central to the mechanism of these enzymes. This note reports that while reviewing the electron-density maps derived from the deposited data for uncomplexed plasmepsin II from Plasmodium falciparum [Asojo et al. (2003), J. Mol. Biol. 327, 173-181; PDB code 1lf4], it was discovered that the aspartic acid residues in this structure should in fact be distinctly noncoplanar. The crystallographic model from the deposited coordinates has been re-refined against the 1.9 A resolution published diffraction data to an R(cryst) of 21.2% and an R(free) of 22.2%. The catalytic water molecule is present, but the plane of the carboxylate group of Asp214 is rotated by 66 degrees from its original position.

High-resolution structure of human carbonic anhydrase II complexed with acetazolamide reveals insights into inhibitor drug design.

The crystal structure of human carbonic anhydrase II (CA II) complexed with the inhibitor acetazolamide (AZM) has been determined at 1.1 A resolution and refined to an R(cryst) of 11.2% and an R(free) of 14.7%. As observed in previous CA II-inhibitor complexes, AZM binds directly to the zinc and makes several key interactions with active-site residues. The high-resolution data also showed a glycerol molecule adjacent to the AZM in the active site and two additional AZMs that are adventitiously bound on the surface of the enzyme. The co-binding of AZM and glycerol in the active site demonstrate that given an appropriate ring orientation and substituents, an isozyme-specific CA inhibitor may be developed.

Structure determination of the cancer-associated Mycoplasma hyorhinis protein Mh-p37.

The crystal structure of the Mycoplasma hyorhinis protein Mh-p37 has been solved and refined to 1.9 A resolution. This is the first de novo structure to be determined using the recently described heavy-atom reagent [Beck et al. (2008), Acta Cryst. D64, 1179-1182] 5-amino-2,4,6-triiodoisophthalic acid (I3C), which contains three I atoms arranged in an equilateral triangle, by SIRAS methods. Data collection was performed in-house at room temperature. SHELXD and SHELXE were used to determine the I-atom positions and phase the native protein and PHENIX AutoBuild software was used to automatically fit the amino-acid sequence to the electron-density map. The structure was refined using SHELX97 to an R(cryst) of 18.6% and an R(free) of 24.0%. Mh-p37 is an alpha/beta protein with two well defined domains which are separated by a deep cleft. An unanticipated ligand bound in the center of the molecule at the base of the cleft has been modeled as thiamine pyrophosphate or vitamin B(1). Retrospective attempts to solve the crystal structure by Patterson search methods using either isomorphous or anomalous differences failed. Additionally, attempts to use proteins with the highest structural homology in the Protein Data Bank to phase the data by molecular replacement were unsuccessful, most likely in hindsight because of their poor structural agreement. Therefore, the I3C reagent offers an alternative, quick and inexpensive method for in-house phasing of de novo structures where other methods may not be successful.

Discovery and SAR of Novel 2,3-Dihydroimidazo[1,2-c]quinazoline PI3K Inhibitors: Identification of Copanlisib (BAY 80-6946).

The phosphoinositide 3-kinase (PI3K) pathway is aberrantly activated in many disease states, including tumor cells, either by growth factor receptor tyrosine kinases or by the genetic mutation and amplification of key pathway components. A variety of PI3K isoforms play differential roles in cancers. As such, the development of PI3K inhibitors from novel compound classes should lead to differential pharmacological and pharmacokinetic profiles and allow exploration in various indications, combinations, and dosing regimens. A screening effort aimed at the identification of PI3Kγ inhibitors for the treatment of inflammatory diseases led to the discovery of the novel 2,3-dihydroimidazo[1,2-c]quinazoline class of PI3K inhibitors. A subsequent lead optimization program targeting cancer therapy focused on inhibition of PI3Kα and PI3Kß. Herein, initial structure-activity relationship findings for this class and the optimization that led to the identification of copanlisib (BAY 80-6946) as a clinical candidate for the treatment of solid and hematological tumors are described.

Enzymatic Characterization of Recombinant Food Vacuole Plasmepsin 4 from the Rodent Malaria Parasite Plasmodium berghei.

The rodent malaria parasite Plasmodium berghei is a practical model organism for experimental studies of human malaria. Plasmepsins are a class of aspartic proteinase isoforms that exert multiple pathological effects in malaria parasites. Plasmepsins residing in the food vacuole (FV) of the parasite hydrolyze hemoglobin in red blood cells. In this study, we cloned PbPM4, the FV plasmepsin gene of P. berghei that encoded an N-terminally truncated pro-segment and the mature enzyme from genomic DNA. We over-expressed this PbPM4 zymogen as inclusion bodies (IB) in Escherichia coli, and purified the protein following in vitro IB refolding. Auto-maturation of the PbPM4 zymogen to mature enzyme was carried out at pH 4.5, 5.0, and 5.5. Interestingly, we found that the PbPM4 zymogen exhibited catalytic activity regardless of the presence of the pro-segment. We determined the optimal catalytic conditions for PbPM4 and studied enzyme kinetics on substrates and inhibitors of aspartic proteinases. Using combinatorial chemistry-based peptide libraries, we studied the active site preferences of PbPM4 at subsites S1, S2, S3, S1', S2' and S3'. Based on these results, we designed and synthesized a selective peptidomimetic compound and tested its inhibition of PbPM4, seven FV plasmepsins from human malaria parasites, and human cathepsin D (hcatD). We showed that this compound exhibited a >10-fold selectivity to PbPM4 and human malaria parasite plasmepsin 4 orthologs versus hcatD. Data from this study furthesr our understanding of enzymatic characteristics of the plasmepsin family and provides leads for anti-malarial drug design.

Anticonvulsant 4-aminobenzenesulfonamide derivatives with branched-alkylamide moieties: X-ray crystallography and inhibition studies of human carbonic anhydrase isoforms I, II, VII, and XIV.

Aromatic amides comprising branched aliphatic carboxylic acids and 4-aminobenzenesulfonamide were evaluated for their inhibition of carbonic anhydrase (CA) isoforms. Of the most anticonvulsant-active compounds (2, 4, 13, 16, and 17), only 13, 16, and 17 were potent inhibitors of CAs VII and XIV. Compounds 9, 14, and 19 inhibited CA II, while 10 and 12 inhibited all isoforms. Structural studies suggest that differences in the active sites' hydrophobicity modulate the affinity of the inhibitors.

Selective hydrophobic pocket binding observed within the carbonic anhydrase II active site accommodate different 4-substituted-ureido-benzenesulfonamides and correlate to inhibitor potency.

4-Substituted-ureido benzenesulfonamides showing inhibitory activity against carbonic anhydrase (CA, EC 4.2.1.1) II between 3.3-226 nM were crystallized in complex with the enzyme. Hydrophobic interactions between the scaffold of the inhibitors in different hydrophobic pockets of the enzyme were observed, explaining the diverse inhibitory range of these derivatives.

High-resolution structure of unbound human immunodeficiency virus 1 subtype C protease: implications of flap dynamics and drug resistance.

The X-ray crystal structure of the unbound state of human immunodeficiency virus 1 (HIV-1) subtype C protease (C PR) has been determined to 1.20 angstroms resolution in the tetragonal space group P4(1)2(1)2, with one monomer per asymmetric unit and unit-cell parameters a = 46.7, c = 100.8 angstroms, allowing full anisotropic least-squares refinement. The refined model has a conventional R factor of 14.1% for all reflections and estimated standard deviations in bond lengths and angles for all main-chain non-H atoms of 0.014 angstroms and 0.030 degrees , respectively. The structure is compared with three unbound subtype B proteases (B PRs) to identify structural changes arising from the naturally occurring polymorphisms and delineate their implications in antiretroviral drug resistance/susceptibility. The unbound C PR exhibits a larger distance between the tips of the flaps, a downward displacement of the 36-41 loop and an increased thermal stability of the 10s loop when compared with the B PR structures. The C PR structure presents the highest resolution of the unbound state of a non-subtype-B PR and adds to the understanding of flap dynamics and drug resistance.