Processivity and subcellular localization of glycogen synthase depend on a non-catalytic high affinity glycogen-binding site

Glycogen synthase, a central enzyme in glucose metabolism, catalyzes the successive addition of α-1,4-linked glucose residues to the non-reducing end of a growing glycogen molecule. A non-catalytic glycogen-binding site, identified by x-ray crystallography on the surface of the glycogen synthase from the archaeon Pyrococcus abyssi, has been found to be functionally conserved in the eukaryotic enzymes. The disruption of this binding site in both the archaeal and the human muscle glycogen synthases has a large impact when glycogen is the acceptor substrate. Instead, the catalytic efficiency remains essentially unchanged when small oligosaccharides are used as substrates. Mutants of the human muscle enzyme with reduced affinity for glycogen also show an altered intracellular distribution and a marked decrease in their capacity to drive glycogen accumulation in vivo. The presence of a high affinity glycogen-binding site away from the active center explains not only the long-recognized strong binding of glycogen synthase to glycogen but also the processivity and the intracellular localization of the enzyme. These observations demonstrate that the glycogen-binding site is a critical regulatory element responsible for the in vivo catalytic efficiency of GS.     

Modulation of heme orientation and binding by a single residue in catalase HPII of Escherichia coli

Heme-containing catalases have been extensively studied, revealing the roles of many residues, the existence of two heme orientations, flipped 180° relative to one another along the propionate-vinyl axis, and the presence of both heme b and heme d. The focus of this report is a residue, situated adjacent to the vinyl groups of the heme at the entrance of the lateral channel, with an unusual main chain geometry that is conserved in all catalase structures so far determined. In Escherichia coli catalase HPII, the residue is Ile274, and replacing it with Gly, Ala, and Val, found at the same location in other catalases, results in a reduction in catalytic efficiency, a reduced intensity of the Soret absorbance band, and a mixture of heme orientations and species. The reduced turnover rates and higher H(2)O(2) concentrations required to attain equivalent reaction velocities are explained in terms of less efficient containment of substrate H(2)O(2) in the heme cavity arising from easier escape through the more open entrance to the lateral channel created by the smaller side chains of Gly and Ala. Inserting a Cys at position 274 resulted in the heme being covalently linked to the protein through a Cys-vinyl bond that is hypersensitive to X-ray irradiation being largely degraded within seconds of exposure to the X-ray beam. Two heme orientations, flipped along the propionate-vinyl axis, are found in the Ala, Val, and Cys variants.     

Crystal structure of Brucella abortus deoxyxylulose-5-phosphate reductoisomerase-like (DRL) enzyme involved in isoprenoid biosynthesis

Most bacteria use the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway for the synthesis of their essential isoprenoid precursors. The absence of the MEP pathway in humans makes it a promising new target for the development of much needed new and safe antimicrobial drugs. However, bacteria show a remarkable metabolic plasticity for isoprenoid production. For example, the NADPH-dependent production of MEP from 1-deoxy-d-xylulose 5-phosphate in the first committed step of the MEP pathway is catalyzed by 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR) in most bacteria, whereas an unrelated DXR-like (DRL) protein was recently found to catalyze the same reaction in some organisms, including the emerging human and animal pathogens Bartonella and Brucella. Here, we report the x-ray crystal structures of the Brucella abortus DRL enzyme in its apo form and in complex with the broad-spectrum antibiotic fosmidomycin solved to 1.5 and 1.8 Å resolution, respectively. DRL is a dimer, with each polypeptide folding into three distinct domains starting with the NADPH-binding domain, in resemblance to the structure of bacterial DXR enzymes. Other than that, DRL and DXR show a low structural relationship, with a different disposition of the domains and a topologically unrelated C-terminal domain. In particular, the active site of DRL presents a unique arrangement, suggesting that the design of drugs that would selectively inhibit DRL-harboring pathogens without affecting beneficial or innocuous bacteria harboring DXR should be feasible. As a proof of concept, we identified two strong DXR inhibitors that have virtually no effect on DRL activity.     

Minor group human rhinovirus-receptor interactions: Geometry of multimodular attachment and basis of recognition

X-ray structures of human rhinovirus 2 (HRV2) in complex with soluble very-low-density lipoprotein receptors encompassing modules 1, 2, and 3 (V123) and five V3 modules arranged in tandem (V33333) demonstrates multi-modular binding around the virion’s five-fold axes. Occupancy was 60% for V123 and 100% for V33333 explaining the high-avidity of the interaction. Surface potentials of 3D-models of all minor group HRVs and K-type major group HRVs were compared; hydrophobic interactions between a conserved lysine in the viruses and a tryptophan in the receptor modules together with coulombic attraction via diffuse opposite surface potentials determine minor group HRV receptor specificity.     

Essential Role of Proximal Histidine-Asparagine Interaction in Mammalian Peroxidases

In heme enzymes belonging to the peroxidase-cyclooxygenase superfamily the proximal histidine is in close interaction with a fully conserved asparagine. The crystal structure of a mixture of glycoforms of myeloperoxidase (MPO) purified from granules of human leukocytes prompted us to revise the orientation of this asparagine and the protonation status of the proximal histidine. The data we present contrast with previous MPO structures, but are strongly supported by molecular dynamics simulations. Moreover, comprehensive analysis of published lactoperoxidase structures suggest that the described proximal heme architecture is a general structural feature of animal heme peroxidases. Its importance is underlined by the fact that the MPO variant N421D, recombinantly expressed in mammalian cell lines, exhibited modified spectral properties and diminished catalytic activity compared with wild-type recombinant MPO. It completely lost its ability to oxidize chloride to hypochlorous acid, which is a characteristic feature of MPO and essential for its role in host defense. The presented crystal structure of MPO revealed further important differences compared with the published structures including the extent of glycosylation, interaction between light and heavy polypeptides, as well as heme to protein covalent bonds. These data are discussed with respect to biosynthesis and post-translational maturation of MPO as well as to its peculiar biochemical and biophysical properties.The majority of currently known heme peroxidases, ubiquitous in all kingdoms of life, are members of two superfamilies that arose independently. The superfamily of (archae)bacterial, fungal, and plant heme peroxidases (sometimes called “non-animal” peroxidase superfamily) is represented by catalase-peroxidases, ascorbate peroxidases, cytochrome c peroxidases, manganese and lignin peroxidases, and plant secretoric peroxidases (1, 2). The second superfamily (named the peroxidase-cyclooxygenase superfamily) was defined recently based on the reconstruction of the main evolutionary lines of mammalian heme peroxidases (3). This peroxidase-cyclooxygenase superfamily includes the mammalian peroxidases represented by myeloperoxidase (MPO),² eosinophil peroxidase (EPO), lactoperoxidase (LPO), and thyroid peroxidase (3).Both superfamilies differ greatly in their primary and tertiary structures and also, most strikingly, in the nature of the heme prosthetic group. Mature mammalian peroxidases are post-translationally modified with the heme covalently linked to the protein via autocatalytic formation of two ester bonds with highly conserved aspartate and glutamate residues (4–7). MPO is singular in having additionally a sulfonium ion linkage between the heme 2-vinyl group and a conserved methionine. The existence of these three covalent hemes to protein bonds has been correlated with the peculiar spectroscopic, redox, and catalytic properties of MPO (8, 9). Other, although more subtle, structural differences between the two heme peroxidase superfamilies concern the H-bonding partners of the essential proximal and distal histidines (4, 10).Closely related with these structural peculiarities is the physiological role and the nature of the substrates of these oxidoreductases. MPO, EPO, and LPO are functionally homologous enzymes involved in host defense. Myeloperoxidase is secreted at inflammatory sites from stimulated polymorphonuclear leukocytes and also monocytes (11), whereas EPO is released from activated eosinophils (12). Lactoperoxidase is found in mucosal surfaces and exocrine secretions such as milk, tears, and saliva (13). In contrast, thyroid peroxidase is involved in the biosynthesis of the thyroid hormones thyroxine and triiodothyronine (14). These four metalloproteins prefer small anionic molecules as electron donors, such as halides (chloride, bromide, and iodide), thiocyanate, and nitrite (8). The corresponding oxidation products (e.g. hypohalous acids or nitrogen dioxide) are (strong) halogenating and nitrating oxidants that play an important role in the innate immune defense system but also contribute to tissue injury in certain inflammatory diseases (15). Due to their important role in (patho)physiology, these enzymes are of strong interest for the pharmaceutical industry (15).X-ray structures of mammalian peroxidases have been published for one glycoform of canine (16) and human (4, 17) myeloperoxidase, as well as for caprine lactoperoxidase (5) (supplemental Table S1). Here, we present the crystal structure of a mixture of MPO glycoforms as obtained directly by purification from human leukocytes. The determined structure exhibits several significant differences compared with those reported in the literature. These variations include heme to protein linkages, sites, and extent of glycosylation as well as the interaction of the C terminus of the light with the N terminus of the heavy polypeptide. X-ray modeling, cross-checked by molecular dynamics simulations, revealed new insights in the interaction between proximal His³³⁶ and Asn⁴²¹. Specifically, the imidazole of His³³⁶ could be present as imidazolate. Its importance in maintaining the physical and catalytic properties of MPO is underlined by the fact that variant N421D exhibited modified spectral properties and completely lost its chlorination activity.     

Structure of acetylglutamate kinase,a key enzyme for arginine biosynthesis and a prototype for the amino acid kinase enzyme family,during catalysis

N-Acetyl-L-glutamate kinase (NAGK), a member of the amino acid kinase family, catalyzes the second and frequently controlling step of arginine synthesis. The Escherichia coli NAGK crystal structure to 1.5 A resolution reveals a 258-residue subunit homodimer nucleated by a central 16-stranded molecular open beta sheet sandwiched between alpha helices. In each subunit, AMPPNP, as an alphabetagamma-phosphate-Mg2+ complex, binds along the sheet C edge, and N-acetyl-L-glutamate binds near the dyadic axis with its gamma-COO- aligned at short distance from the gamma-phosphoryl, indicating associative phosphoryl transfer assisted by: (1) Mg2+ complexation; (2) the positive charges on Lys8, Lys217, and on two helix dipoles; and (3) by hydrogen bonding with the y-phosphate. The structural resemblance with carbamate kinase and the alignment of the sequences suggest that NAGK is a structural and functional prototype for the amino acid kinase family, which differs from other acylphosphate-making devices represented by phosphoglycerate kinase, acetate kinase, and biotin carboxylase.     

Three-dimensional structure of catalase from Penicillium vitale at 2.0 A resolution

The three-dimensional structure analysis of crystalline fungal catalase from Penicillium vitale has been extended to 2.0 A resolution. The crystals belong to space group P3(1)21, with the unit cell parameters of a = b = 144.4 A and c = 133.8 A. The asymmetric unit contains half a tetrameric molecule of 222 symmetry. Each subunit is a single polypeptide chain of approximately 670 amino acid residues and binds one heme group. The amino acid sequence has been tentatively determined by computer graphics model building (using the FRODO system) and comparison with the known sequence of beef liver catalase. The atomic model has been refined by the Hendrickson & Konnert (1981) restrained least-squares program against 68,000 reflections between 5 A and 2 A resolution. The final R-factor is 0.31 after 24 refinement cycles. The secondary and tertiary structure of the catalase has been analyzed.     

Crystal structure of a human rhinovirus neutralizing antibody complexed with a peptide derived from viral capsid protein VP2.

The three-dimensional structure of the complex between the Fab fragment of an anti-human rhinovirus neutralizing antibody (8F5) and a cross-reactive synthetic peptide from the viral capsid protein VP2 has been determined at 2.5 A resolution by crystallographic methods. The refinement is presently at an R factor of 0.18 and the antigen-binding site and viral peptide are well defined. The peptide antigen adopts a compact fold by two tight turns and interacts through hydrogen bonds, some with ionic character, and van der Waals contacts with antibody residues from the six hypervariable loops as well as several framework amino acids. The conformation adopted by the peptide is closely related to the corresponding region of the viral protein VP2 on the surface of human rhinovirus 1A whose three-dimensional structure is known. Implications for the cross-reactivity between peptides and the viral capsid are discussed. The peptide-antibody interactions, together with the analysis of mutant viruses that escape neutralization by 8F5 suggest two different mechanisms for viral escape. The comparison between the complexed and uncomplexed antibody structures shows important conformational rearrangements, especially in the hypervariable loops of the heavy chain. Thus, it constitutes a clear example of the 'induced fit' molecular recognition mechanism.     

Crystallization and preliminary X-ray diffraction studies of the Lb proteinase from foot-and-mouth disease virus.

Different crystal forms of the C23A mutant from the leader proteinase of foot-and-mouth disease virus were obtained by the hanging drop vapor diffusion technique, using MgCl2 and PEG 6000 as precipitants. Well-developed crystals, with cubic morphology growing to approximately 1.0 mm3 in size, presented a large unit cell parameter of 274.5 A and diffracted to, at most, 5 A resolution. A second type of crystal had a tetragonal appearance and these were obtained in droplets soaked in a silica gel matrix. These crystals, with an approximate size of 0.3 X 0.3 X 0.7 mm3, diffracted to approximately 4.0 A resolution, but presented a strong anisotropic mosaicity around the longest crystal axis. Crystals with a needlelike morphology and reaching sizes of about 0.2 X 0.3 X 1.2 mm3 diffracted beyond 3.5 A resolution and were stable to X-ray radiation for approximately one day when using a conventional source at room temperature. These crystals are orthorhombic with space group I222 (or I2(1)2(1)2(1)) and unit cell dimensions a = 65.9 A, b = 104.3 A, and c = 124.0 A, and appear well suited for high-resolution studies. Density packing considerations are consistent with the presence of two molecules in the asymmetric unit and a solvent content of approximately 54%.