Carlos Frazão

Structure of a Dioxygen Reduction Enzyme from Desulfovibrio Gigas

Desulfovibrio gigas is a strict anaerobe that contains a well-characterized metabolic pathway that enables it to survive transient contacts with oxygen. The terminal enzyme in this pathway, rubredoxin:oxygen oxidoreductase (ROO) reduces oxygen to water in a direct and safe way. The 2.5 Å resolution crystal structure of ROO shows that each monomer of this homodimeric enzyme consists of a novel combination of two domains, a flavodoxin-like domain and a Zn-β-lactamase-like domain that contains a di-iron center for dioxygen reduction. This is the first structure of a member of a superfamily of enzymes widespread in strict and facultative anaerobes, indicating its broad physiological significance.

Unravelling the dynamics of RNA degradation by ribonuclease II and its RNA-bound complex

RNA degradation is a determining factor in the control of gene expression. The maturation, turnover and quality control of RNA is performed by many different classes of ribonucleases. Ribonuclease II (RNase II) is a major exoribonuclease that intervenes in all of these fundamental processes; it can act independently or as a component of the exosome, an essential RNA-degrading multiprotein complex. RNase II-like enzymes are found in all three kingdoms of life, but there are no structural data for any of the proteins of this family. Here we report the X-ray crystallographic structures of both the ligand-free (at 2.44 Å resolution) and RNA-bound (at 2.74 Å resolution) forms of Escherichia coli RNase II. In contrast to sequence predictions, the structures show that RNase II is organized into four domains: two cold-shock domains, one RNB catalytic domain, which has an unprecedented αβ-fold, and one S1 domain. The enzyme establishes contacts with RNA in two distinct regions, the ‘anchor’ and the ‘catalytic’ regions, which act synergistically to provide catalysis. The active site is buried within the RNB catalytic domain, in a pocket formed by four conserved sequence motifs. The structure shows that the catalytic pocket is only accessible to single-stranded RNA, and explains the specificity for RNA versus DNA cleavage. It also explains the dynamic mechanism of RNA degradation by providing the structural basis for RNA translocation and enzyme processivity. We propose a reaction mechanism for exonucleolytic RNA degradation involving key conserved residues. Our three-dimensional model corroborates all existing biochemical data for RNase II, and elucidates the general basis for RNA degradation. Moreover, it reveals important structural features that can be extrapolated to other members of this family.

Crystal Structure of Cardosin A, a Glycosylated and Arg-Gly-Asp-containing Aspartic Proteinase from the Flowers of Cynara cardunculus L.*

Aspartic proteinases (AP) have been widely studied within the living world, but so far no plant AP have been structurally characterized. The refined cardosin A crystallographic structure includes two molecules, built up by two glycosylated peptide chains (31 and 15 kDa each). The fold of cardosin A is typical within the AP family. The glycosyl content is described by 19 sugar rings attached to Asn-67 and Asn-257. They are localized on the molecular surface away from the conserved active site and show a new glycan of the plant complex type. A hydrogen bond between Gln-126 and Manβ4 renders the monosaccharide oxygen O-2 sterically inaccessible to accept a xylosyl residue, therefore explaining the new type of the identified plant glycan. The Arg-Gly-Asp sequence, which has been shown to be involved in recognition of a putative cardosin A receptor, was found in a loop between two β-strands on the molecular surface opposite the active site cleft. Based on the crystal structure, a possible mechanism whereby cardosin A might be orientated at the cell surface of the style to interact with its putative receptor from pollen is proposed. The biological implications of these findings are also discussed.

Structure of amidase from Pseudomonas aeruginosa showing a trapped acyl transfer reaction intermediate state

Microbial amidases belong to the thiol nitrilases family and have potential biotechnological applications in chemical and pharmaceutical industries as well as in bioremediation. The amidase from Pseudomonas aeruginosa isa6 × 38-kDa enzyme that catalyzes the hydrolysis of a small range of short aliphatic amides. The hereby reported high resolution crystallographic structure shows that each amidase monomer is formed by a globular four-layer αββα sandwich domain with an additional 81-residue long C-terminal segment. This wraps arm-in-arm with a homologous C-terminal chain of another monomer, producing a strongly packed dimer. In the crystal, the biological active homo-hexameric amidase is built grouping three such dimers around a crystallographic 3-fold axis. The structure also elucidates the structural basis for the enzyme activity, with the nitrilases catalytic triad at the bottom of a 13-Å deep, funnel-shaped pocket, accessible from the solvent through a narrow neck with 3-Å diameter. An acyl transfer intermediate, resulting from the purification protocol, was found bound to the amidase nucleophilic agent, Cys166. These results suggest that some pocket defining residues should undergo conformational shifts to allow substrates and products to access and leave the catalytic pocket, for turnover to occur.

Ab initio structure solution of a dimeric cytochrome c 3 from Desulfovibrio gigas containing disulfide bridges

Abstract The 1.2 Å resolution crystal structure of the 29 kDa di-tetrahaem cytochrome c3 from the sulfate reducing bacterium Desulfovibrio gigas was solved by ab initio methods, making this the largest molecule to be solved by this procedure. The actual refined model of the cysteine-linked dimeric molecule reveals that this molecule is very similar to the non-covalently linked symmetrical dimer of the di-tetrahaem cytochrome c3 from Desulfomicrobium norvegicum. Each monomer has the typical polypeptide fold, haem arrangement and iron coordination found for the tetrahaem cytochrome c3 molecules. The interface between the covalently linked monomers in the asymmetric unit of the crystal shows a pseudo two-fold arrangement, disturbed from symmetry by crystal packing forces. The fact that D. gigas contains a dimeric tetrahaem cytochrome with solvent accessible disulfide bridges and that this cytochrome specifically couples hydrogen oxidation to thiosulfate reduction in bacterial extracts provides an interesting aspect related to disulfide exchange reactions in this microorganism.

Functional control of the binuclear metal site in the metallo‐β‐lactamase‐like fold by subtle amino acid replacements

At present there are three protein families that share a common structural domain, the αβ/βα fold of class B β‐lactamases: zinc β‐lactamases, glyoxalases II, and A‐type flavoproteins. A detailed inspection of their superimposed structures was undertaken and showed that although these proteins contain binuclear metal sites in spatially equivalent positions, there are some subtle differences within the first ligand sphere that determine a distinct composition of metals. Although zinc β‐lactamases contain either a mono or a di‐zinc center, the catalytically active form of glyoxalase II contains a mixed iron–zinc binuclear center, whereas A‐type flavoproteins contain a di‐iron site. These variations on the type of metal site found within a common fold are correlated with the subtle variations in the nature of the ligating amino acid residues and are discussed in terms of the different reactions catalyzed by each of the protein families. Correlation of these observations with sequence data results in the definition of a sequence motif that comprises the possible binuclear metal site ligands in this broad family. The evolution of the proteins sharing this common fold and factors modulating reactivity are also discussed.

Structural Studies on Flavodiiron Proteins

Crystallographic studies on flavodiiron proteins (FDPs) have revealed that the common sequence core ( approximately 400 residues) that defines this protein family comprises two structural domains. The N-terminal domain (of approximately 250 residues) displays a metallo-beta-lactamase-like-fold, being indeed structurally homologous to beta-lactamases and glyoxalases, despite the poor sequence similarity. Whereas beta-lactamases have mono- or dizinc sites and glyoxalases a mixed iron-zinc site, the lactamase domain of FDPs harbors a nonheme diiron center with carboxylate and histidine residues as ligands, assigned as the active site of NO and/or O(2) reduction. The C-terminal domain of FDPs is characterized by a flavodoxin-like fold, homologous to short-chain flavodoxins, and harbors a flavin mononucleotide moiety, stabilized by van der Waals interactions and a number of hydrogen bonds. Structures of FDPs obtained in different conditions and oxidation states display some heterogeneities, mostly at the diiron site, but still fail to provide unequivocal evidence for some pending questions regarding the substrate activation mechanism of FDPs, namely the preference for either substrate (NO or oxygen) observed in different members of this protein family. More structural studies are therefore required to achieve a deeper understanding on these matters.

Structure of dimeric cytochrome c3 from Desulfovibrio gigas at 1.2 Å resolution

The structure of dimeric cytochrome c(3) from the sulfate-reducing bacterium Desulfovibrio gigas, diDg, obtained by ab initio methods was further refined to 1.2 A resolution, giving final reliability factors of R(free) = 14.8% and R = 12.4%. This cytochrome is a dimer of tetraheme cytochrome c(3) molecules covalently linked by two solvent-accessible disulfide bridges, a characteristic unique to members of the cytochrome c(3) superfamily. Anisotropic analysis using the semi-rigid TLS method shows different behaviour for analogous loops in each monomer arising from their different packing environments. A detailed sequence and structural comparison with all other known cytochrome c(3) domains in single- and multi-domain cytochromes c(3) shows the presence of structurally conserved regions in this family, despite the high variability of the amino-acid sequence. An internal water molecule is conserved in a common structural arrangement in all c(3) tetraheme domains, indicating a probable electron-transfer pathway between hemes I and II. Unique features of diDg are an internal methionine residue close to heme I and to one of the axial ligands of heme III, where all other structures of the cytochrome c(3) superfamily have a phenylalanine, and a rather unusual CXXXCH heme-binding motif only found so far in this cytochrome.

Structure of Burkholderia cepacia UDP-Glucose Dehydrogenase (UGD) BceC and Role of Tyr10 in Final Hydrolysis of UGD Thioester Intermediate

Members of the Burkholderia cepacia complex (BCC) are serious respiratory pathogens in immunocompromised individuals and in patients with cystic fibrosis (CF). They are exceptionally resistant to many antimicrobial agents and have the capacity to spread between patients, leading to a decline in lung function and necrotizing pneumonia. BCC members often express a mucoid phenotype associated with the secretion of the exopolysaccharide (EPS) cepacian. There is much evidence supporting the fact that cepacian is a major virulence factor of BCC. UDP-glucose dehydrogenase (UGD) is responsible for the NAD-dependent 2-fold oxidation of UDP-glucose (UDP-Glc) to UDP-glucuronic acid (UDP-GlcA), which is a key step in cepacian biosynthesis. Here, we report the structure of BceC, determined at 1.75-Å resolution. Mutagenic studies were performed on the active sites of UGDs, and together with the crystallographic structures, they elucidate the molecular mechanism of this family of sugar nucleotide-modifying enzymes. Superposition with the structures of human and other bacterial UGDs showed an active site with high structural homology. This family contains a strictly conserved tyrosine residue (Y10 in BceC; shown in italics) within the glycine-rich motif (GXGYXG) of its N-terminal Rossmann-like domain. We constructed several BceC Y10 mutants, revealing only residual dehydrogenase activity and thus highlighting the importance of this conserved residue in the catalytic activity of BceC. Based on the literature of the UGD/GMD nucleotide sugar 6-dehydrogenase family and the kinetic and structural data we obtained for BceC, we determined Y10 as a key catalytic residue in a UGD rate-determining step, the final hydrolysis of the enzymatic thioester intermediate.

The Complex of Sphingomonas elodea ATCC 31461 Glucose-1-Phosphate Uridylyltransferase with Glucose-1-Phosphate Reveals a Novel Quaternary Structure, Unique among Nucleoside Diphosphate-Sugar Pyrophosphorylase Members

Gellan gum is a widely used commercial material, available in many different forms. Its economic importance has led to studies into the biosynthesis of exopolysaccharide gellan gum, which is industrially prepared in high yields using Sphingomonas elodea ATCC 31461. Glucose-1-phosphate uridylyltransferase mediates the reversible conversion of glucose-1-phosphate and UTP into UDP-glucose and pyrophosphate, which is a key step in the biosynthetic pathway of gellan gums. Here we present the X-ray crystal structure of the glucose-1-phosphate uridylyltransferase from S. elodea. The S. elodea enzyme shares strong monomeric similarity with glucose-1-phosphate thymidylyltransferase, several structures of which are known, although the quaternary structures of the active enzymes are rather different. A detailed comparison between S. elodea glucose-1-phosphate uridylyltransferase and available thymidylyltransferases is described and shows remarkable structural similarities, despite the low sequence identities between the two divergent groups of proteins.