Biomedical subjects

C M Bruns

Publications and source records attributed to C M Bruns.

8 recordsLinked to original sources

PubMed2001

Crystallographic and biochemical analyses of the metal-free Haemophilus influenzae Fe3+-binding protein.

The crystal structure of the iron-free (apo) form of the Haemophilus influenzae Fe(3+)-binding protein (hFbp) has been determined to 1.75 A resolution. Information from this structure complements that derived from the holo structure with respect to the delineation of the process of iron binding and release. A 21 degrees rotation separates the two structural domains when the apo form is compared with the holo conformer, indicating that upon release of iron, the protein undergoes a change in conformation by bending about the central beta-sheet hinge. A surprising finding in the apo-hFbp structure was that the ternary binding site anion, observed in the crystals as phosphate, remained bound. In solution, apo-hFbp bound phosphate with an affinity K(d) of 2.3 x 10(-3) M. The presence of this ternary binding site anion appears to arrange the C-terminal iron-binding residues conducive to complementary binding to Fe(3+), while residues in the N-terminal binding domain must undergo induced fit to accommodate the Fe(3+) ligand. These observations suggest a binding process, the first step of which is the binding of a synergistic anion such as phosphate to the C-terminal domain. Next, iron binds to the preordered half-site on the C-terminal domain. Finally, the presence of iron organizes the N-terminal half-site and closes the interdomain hinge. The use of the synergistic anion and this iron binding process results in an extremely high affinity of the Fe(3+)-binding proteins for Fe(3+) (nFbp K'(eff) = 2.4 x 10(18) M(-1)). This high-affinity ligand binding process is unique among the family of bacterial periplasmic binding proteins and has interesting implications in the mechanism of iron removal from the Fe(3+)-binding proteins during FbpABC-mediated iron transport across the cytoplasmic membrane.

Anions↗

PubMed1999

Human glutathione transferase A4-4 crystal structures and mutagenesis reveal the basis of high catalytic efficiency with toxic lipid peroxidation products.

The oxidation of lipids and cell membranes generates cytotoxic compounds implicated in the etiology of aging, cancer, atherosclerosis, neurodegenerative diseases, and other illnesses. Glutathione transferase (GST) A4-4 is a key component in the defense against the products of this oxidative stress because, unlike other Alpha class GSTs, GST A4-4 shows high catalytic activity with lipid peroxidation products such as 4-hydroxynon-2-enal (HNE). The crystal structure of human apo GST A4-4 unexpectedly possesses an ordered C-terminal alpha-helix, despite the absence of any ligand. The structure of human GST A4-4 in complex with the inhibitor S-(2-iodobenzyl) glutathione reveals key features of the electrophilic substrate-binding pocket which confer specificity toward HNE. Three structural modules form the binding site for electrophilic substrates and thereby govern substrate selectivity: the beta1-alpha1 loop, the end of the alpha4 helix, and the C-terminal alpha9 helix. A few residue changes in GST A4-4 result in alpha9 taking over a predominant role in ligand specificity from the N-terminal loop region important for GST A1-1. Thus, the C-terminal helix alpha9 in GST A4-4 provides pre-existing ligand complementarity rather than acting as a flexible cap as observed in other GST structures. Hydrophobic residues in the alpha9 helix, differing from those in the closely related GST A1-1, delineate a hydrophobic specificity canyon for the binding of lipid peroxidation products. The role of residue Tyr212 as a key catalytic residue, suggested by the crystal structure of the inhibitor complex, is confirmed by mutagenesis results. Tyr212 is positioned to interact with the aldehyde group of the substrate and polarize it for reaction. Tyr212 also coopts part of the binding cleft ordinarily formed by the N-terminal substrate recognition region in the homologous enzyme GST A1-1 to reveal an evolutionary swapping of function between different recognition elements. A structural model of catalysis is presented based on these results.

Aldehydes↗

PubMed1997

Structure of Haemophilus influenzae Fe(+3)-binding protein reveals convergent evolution within a superfamily.

The first crystal structure of the iron-transporter ferric ion-binding protein from Haemophilus influenzae (hFBP), at 1.6 A resolution, reveals the structural basis for iron uptake and transport required by several important bacterial pathogens. Paradoxically, although hFBP belongs to a protein superfamily which includes human transferrin, iron binding in hFBP and transferrin appears to have developed independently by convergent evolution. Structural comparison of hFBP with other prokaryotic periplasmic transport proteins and the eukaryotic transferrins suggests that these proteins are related by divergent evolution from an anion-binding common ancestor, not from an iron-binding ancestor. The iron binding site of hFBP incorporates a water and an exogenous phosphate ion as iron ligands and exhibits nearly ideal octahedral metal coordination. FBP is highly conserved, required for virulence, and is a nodal point for free iron uptake in several Gram-negative pathogenic bacteria, thus providing a potential target for broad-spectrum antibacterial drug design against human pathogens such as H. influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis.

Amino Acid Sequence↗

PubMed1995

Involvement of serine 96 in the catalytic mechanism of ferredoxin-NADP+ reductase: structure--function relationship as studied by site-directed mutagenesis and X-ray crystallography.

The crystal structure of ferredoxin-NADP+ reductase (FNR) suggests that Ser96 is directly involved in hydride transfer between the isoalloxazine moiety of FAD and the nicotinamide ring of NADP(H). To probe its role, Ser96 has been mutated to valine (S96V) and glycine (S96G). These mutations primarily affected the interaction of the nicotinamide ring with the flavin. Absorbance, fluorescence, and circular dichroism spectra and the crystal structure of FNR-S96V indicate that this mutant folds properly. FNR-S96V shows only 0.05% of wild-type activity, while the affinities for both ferredoxin and NADP+ are virtually unchanged. However, spectral perturbations induced by NADP+ binding to FNR-S96V strongly resemble those elicited by the binding of 2'-monophosphoadenosine-5'-diphosphoribose, a substrate analog lacking the nicotinamide ring, both to the mutant and wild-type enzymes. Rapid reaction studies on the valine mutant failed to detect charge-transfer intermediates during flavin reduction by NADPH. In addition, no semiquinone formation was seen during photoreduction of FNR-S96V. The three-dimensional structure of the valine mutant shows small, albeit definite, changes only in the isoalloxazine microenvironment. The glycine mutant of FNR displays behavior intermediate between that of wild-type enzyme and that of the valine mutant. It maintains ca. 2% of the wild-type activity as well as the ability to form the charge-transfer species between reduced FNR and NADP+. In photoreduction experiments, the same degree of flavin semiquinone stabilization was observed with FNR-S96G and with the wild-type enzyme. NADP+ binding to the glycine mutant was very similar to that observed in the case of the valine mutant.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1995

Refined crystal structure of spinach ferredoxin reductase at 1.7 A resolution: oxidized, reduced and 2'-phospho-5'-AMP bound states.

The crystal structure of spinach ferredoxin-NADP(+)-oxidoreductase (FNR), determined by multiple isomorphous replacement at 2.6 A resolution, has been refined at 1.7 A resolution to an R-factor of 17.9%. The structure of FNR bound to the competitive inhibitor 2'-phospho-5'-AMP (P-AMP) has also been refined at 1.7 A to an R-factor of 17.4% and dithionite-reduced/P-AMP-bound FNR has been refined at 2.0 A to an R-factor of 14.9%. The P-AMP-bound structure was used to construct a model for the binding of NADP+. Over 200 solvation sites were included in each structure, and many of the best defined solvation sites stabilize buried turns. A bulk solvent correction obviated the need for a low-resolution data cutoff. An acidic side-chain likely to be responsible for the low pH requirement for crystallization has been identified. Three large networks of the hydrophobic side-chains help define the FNR structure. One of these contains a large cavity far from the active site, which coincides with the lone site of sequence heterogeneity in FNR, and may provide a site for membrane attachment. The reduced structure shows that Ser96 moves toward atom N-5 of FAD and a water molecule moves toward atom N-1 of FAD, while the flavin moiety remains planar. Possible sources of a proton that must be picked up upon reduction are discussed.

Binding Sites↗

PubMed1994

Structure-function relations for ferredoxin reductase.

Ferredoxin: NADP+ reductase is representative of a large family of flavoenzymes which catalyze the interchange of reducing equivalents between one-electron carriers and the two-electron-carrying nicotinamide dinucleotides. The structure of the enzyme from spinach is known at 1.7 A resolution and this structure, together with results of chemical modification and site-directed mutagenesis studies, gives insights into features of the structure that are important for function.

Amino Acid Sequence↗

PubMed1993

Structural prototypes for an extended family of flavoprotein reductases: comparison of phthalate dioxygenase reductase with ferredoxin reductase and ferredoxin.

The structure of phthalate dioxygenase reductase (PDR), a monomeric iron-sulfur flavoprotein that delivers electrons from NADH to phthalate dioxygenase, is compared to ferredoxin-NADP+ reductase (FNR) and ferredoxin, the proteins that reduce NADP+ in the final reaction of photosystem I. The folding patterns of the domains that bind flavin, NAD(P), and [2Fe-2S] are very similar in the two systems. Alignment of the X-ray structures of PDR and FNR substantiates the assignment of features that characterize a family of flavoprotein reductases whose members include cytochrome P-450 reductase, sulfite and nitrate reductases, and nitric oxide synthase. Hallmarks of this subfamily of flavoproteins, here termed the FNR family, are an antiparallel beta-barrel that binds the flavin prosthetic group, and a characteristic variant of the classic pyridine nucleotide-binding fold. Despite the similarities between FNR and PDR, attempts to model the structure of a dissociable FNR:ferredoxin complex by analogy with PDR reveal features that are at odds with chemical crosslinking studies (Zanetti, G., Morelli, D., Ronchi, S., Negri, A., Aliverti, A., & Curti, B., 1988, Biochemistry 27, 3753-3759). Differences in the binding sites for flavin and pyridine nucleotides determine the nucleotide specificities of FNR and PDR. The specificity of FNR for NADP+ arises primarily from substitutions in FNR that favor interactions with the 2' phosphate of NADP+. Variations in the conformation and sequences of the loop adjoining the flavin phosphate affect the selectivity for FAD versus FMN. The midpoint potentials for reduction of the flavin and [2Fe-2S] groups in PDR are higher than their counterparts in FNR and spinach ferredoxin, by about 120 mV and 260 mV, respectively. Comparisons of the structure of PDR with spinach FNR and with ferredoxin from Anabaena 7120, along with calculations of electrostatic potentials, suggest that local interactions, including hydrogen bonds, are the dominant contributors to these differences in potential.

Amino Acid Sequence↗

PubMedDate not supplied

Androgen excess fetal programming of female reproduction: a developmental aetiology for polycystic ovary syndrome?

The aetiology of polycystic ovary syndrome (PCOS) remains unknown. This familial syndrome is prevalent among reproductive-aged women and its inheritance indicates a dominant regulatory gene with incomplete penetrance. However, promising candidate genes have proven unreliable as markers for the PCOS phenotype. This lack of genetic linkage may represent both extreme heterogeneity of PCOS and difficulty in establishing a universally accepted PCOS diagnosis. Nevertheless, hyperandrogenism is one of the most consistently expressed PCOS traits. Animal models that mimic fetal androgen excess may thus provide unique insight into the origins of the PCOS syndrome. Many female mammals exposed to androgen excess in utero or during early post-natal life typically show masculinized and defeminized behaviour, ovulatory dysfunction and virilized genitalia, although behavioural and ovulatory dysfunction can coexist without virilized genitalia based upon the timing of androgen excess. One animal model shows particular relevance to PCOS: the prenatally androgenized female rhesus monkey. Females exposed to androgen excess early in gestation exhibit hyperandrogenism, oligomenorrhoea and enlarged, polyfollicular ovaries, in addition to LH hypersecretion, impaired embryo development, insulin resistance accompanying abdominal obesity, impaired insulin response to glucose and hyperlipidaemia. Female monkeys exposed to androgen excess late in gestation mimic these programmed changes, except for LH and insulin secretion defects. In utero androgen excess may thus variably perturb multiple organ system programming and thereby provide a single, fetal origin for a heterogeneous adult syndrome.

Androgens↗