Voltage-Gated Electrocatalysis of Efficient and Selective Methane Oxidation by Tricopper Clusters under Ambient Conditions.

Selective methane oxidation is difficult chemistry. Here we describe a strategy for the electrocatalysis of selective methane oxidation by immobilizing tricopper catalysts on the cathodic surface. In the presence of dioxygen and methane, the activation of these catalysts above a threshold cathodic potential can initiate the dioxygen chemistry for O atom transfer to methane. The catalytic turnover is completed by facile electron injections into the tricopper catalysts from the electrode. This technology leads to dramatic enhancements in performance of the catalysts toward methane oxidation. Unprecedented turnover frequencies (>40 min-1) and high product throughputs (turnover numbers >30 000 in 12 h) are achieved for this challenging chemical transformation in water under ambient conditions. The technology is green and suitable for on-site direct conversion of methane into methanol.

Mechanism of Pyrroloquinoline Quinone-Dependent Hydride Transfer Chemistry from Spectroscopic and High-Resolution X-ray Structural Studies of the Methanol Dehydrogenase from Methylococcus capsulatus (Bath).

The active site of methanol dehydrogenase (MDH) contains a rare disulfide bridge between adjacent cysteine residues. As a vicinal disulfide, the structure is highly strained, suggesting it might work together with the pyrroloquinoline quinone (PQQ) prosthetic group and the Ca2+ ion in the catalytic turnover during methanol (CH3OH) oxidation. We purify MDH from Methylococcus capsulatus (Bath) with the disulfide bridge broken into two thiols. Spectroscopic and high-resolution X-ray crystallographic studies of this form of MDH indicate that the disulfide bridge is redox active. We observe an internal redox process within the holo-MDH that produces a disulfide radical anion concomitant with a companion PQQ radical, as evidenced by an optical absorption at 408 nm and a magnetically dipolar-coupled biradical in the EPR spectrum. These observations are corroborated by electron-density changes between the two cysteine sulfurs of the disulfide bridge as well as between the bound Ca2+ ion and the O5-C5 bond of the PQQ in the high-resolution X-ray structure. On the basis of these findings, we propose a mechanism for the controlled redistribution of the two electrons during hydride transfer from the CH3OH in the alcohol oxidation without formation of the reduced PQQ ethenediol, a biradical mechanism that allows for possible recovery of the hydride for transfer to an external NAD+ oxidant in the regeneration of the PQQ cofactor for multiple catalytic turnovers. In support of this mechanism, a steady-state level of the disulfide radical anion is observed during turnover of the MDH in the presence of CH3OH and NAD+.

Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics.

Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation. Here, we review the progress made over the past two to three decades toward delineating the structures and functions of the catalytic sites in two MMOs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). sMMO is a water-soluble three-component protein complex consisting of a hydroxylase with a nonheme diiron catalytic site; pMMO is a membrane-bound metalloenzyme with a unique tricopper cluster as the site of hydroxylation. The metal cluster in each of these MMOs harnesses O2 to functionalize the C-H bond using different chemistry. We highlight some of the common basic principles that they share. Finally, the development of functional models of the catalytic sites of MMOs is described. These efforts have culminated in the first successful biomimetic catalyst capable of efficient methane oxidation without overoxidation at room temperature.

A Carbon Electrode Functionalized by a Tricopper Cluster Complex: Overcoming Overpotential and Production of Hydrogen Peroxide in the Oxygen Reduction Reaction.

A study of the oxygen reduction reaction (ORR) on a screen printed carbon electrode surface mediated by the tricopper cluster complex Cu3 (7-N-Etppz(CH2 OH)) dispersed on electrochemically reduced carbon black, where 7-N-Etppz(CH2 OH) is the ligand 3,3'-(6-(hydroxymethyl)-1,4-diazepane-1,4-diyl)bis(1-(4-ethyl piperazin-1-yl)propan-2-ol), is described. Onset oxygen reduction potentials of about 0.92 V and about 0.77 V are observed at pH 13 and pH 7 vs. the reversible hydrogen electrode, which are comparable to the best values reported for any synthetic copper complex. Based on half-wave potentials (E1/2 ), the corresponding overpotentials are about 0.42 V and about 0.68 V, respectively. Kinetic studies indicate that the trinuclear copper catalyst can accomplish the 4 e- reduction of O2 efficiently and the ORR is accompanied by the production of only small amounts of H2 O2 . The involvement of the copper triad in the O2 activation process is also verified.

Crystal Structures of a Piscine Betanodavirus: Mechanisms of Capsid Assembly and Viral Infection.

Betanodaviruses cause massive mortality in marine fish species with viral nervous necrosis. The structure of a T = 3 Grouper nervous necrosis virus-like particle (GNNV-LP) is determined by the ab initio method with non-crystallographic symmetry averaging at 3.6 Å resolution. Each capsid protein (CP) shows three major domains: (i) the N-terminal arm, an inter-subunit extension at the inner surface; (ii) the shell domain (S-domain), a jelly-roll structure; and (iii) the protrusion domain (P-domain) formed by three-fold trimeric protrusions. In addition, we have determined structures of the T = 1 subviral particles (SVPs) of (i) the delta-P-domain mutant (residues 35-217) at 3.1 Å resolution; and (ii) the N-ARM deletion mutant (residues 35-338) at 7 Å resolution; and (iii) the structure of the individual P-domain (residues 214-338) at 1.2 Å resolution. The P-domain reveals a novel DxD motif asymmetrically coordinating two Ca2+ ions, and seems to play a prominent role in the calcium-mediated trimerization of the GNNV CPs during the initial capsid assembly process. The flexible N-ARM (N-terminal arginine-rich motif) appears to serve as a molecular switch for T = 1 or T = 3 assembly. Finally, we find that polyethylene glycol, which is incorporated into the P-domain during the crystallization process, enhances GNNV infection. The present structural studies together with the biological assays enhance our understanding of the role of the P-domain of GNNV in the capsid assembly and viral infection by this betanodavirus.

Inactivation of the particulate methane monooxygenase (pMMO) in Methylococcus capsulatus (Bath) by acetylene.

Acetylene (HCCH) has a long history as a mechanism-based enzyme inhibitor and is considered an active-site probe of the particulate methane monooxygenase (pMMO). Here, we report how HCCH inactivates pMMO in Methylococcus capsulatus (Bath) by using high-resolution mass spectrometry and computational simulation. High-resolution MALDI-TOF MS of intact pMMO complexes has allowed us to confirm that the enzyme oxidizes HCCH to the ketene (C2H2O) intermediate, which then forms an acetylation adduct with the transmembrane PmoC subunit. LC-MS/MS analysis of the peptides derived from in-gel proteolytic digestion of the protein subunit identifies K196 of PmoC as the site of acetylation. No evidence is obtained for chemical modification of the PmoA or PmoB subunit. The inactivation of pMMO by a single adduct in the transmembrane PmoC domain is intriguing given the complexity of the structural fold of this large membrane-protein complex as well as the complicated roles played by the various metal cofactors in the enzyme catalysis. Computational studies suggest that the entry of hydrophobic substrates to, and migration of products from, the catalytic site of pMMO are controlled tightly within the transmembrane domain. Support of these conclusions is provided by parallel experiments with two related alkynes: propyne (CH3CCH) and trifluoropropyne (CF3CCH). Finally, we discuss the implication of these findings to the location of the catalytic site in pMMO.

Metalloprotein design using genetic code expansion.

More than one third of all proteins are metalloproteins. They catalyze important reactions such as photosynthesis, nitrogen fixation and CO2 reduction. Metalloproteins such as the olfactory receptors also serve as highly elaborate sensors. Here we review recent developments in functional metalloprotein design using the genetic code expansion approach. We show that, through the site-specific incorporation of metal-chelating unnatural amino acids (UAAs), proton and electron transfer mediators, and UAAs bearing bioorthogonal reaction groups, small soluble proteins can recapitulate and expand the important functions of complex metalloproteins. Further developments along this route may result in cell factories and live-cell sensors with unprecedented efficiency and selectivity.

Crystal structures of vertebrate dihydropyrimidinase and complexes from Tetraodon nigroviridis with lysine carbamylation: metal and structural requirements for post-translational modification and function.

Lysine carbamylation, a post-translational modification, facilitates metal coordination for specific enzymatic activities. We have determined structures of the vertebrate dihydropyrimidinase from Tetraodon nigroviridis (TnDhp) in various states: the apoenzyme as well as two forms of the holoenzyme with one and two metals at the catalytic site. The essential active-site structural requirements have been identified for the possible existence of four metal-mediated stages of lysine carbamylation. Only one metal is sufficient for stabilizing lysine carbamylation; however, the post-translational lysine carbamylation facilitates additional metal coordination for the regulation of specific enzymatic activities through controlling the conformations of two dynamic loops, Ala(69)-Arg(74) and Met(158)-Met(165), located in the tunnel for the substrate entrance. The substrate/product tunnel is in the "open form" in the apo-TnDhp, in the "intermediate state" in the monometal TnDhp, and in the "closed form" in the dimetal TnDhp structure, respectively. Structural comparison also suggests that the C-terminal tail plays a role in the enzymatic function through interactions with the Ala(69)-Arg(74) dynamic loop. In addition, the structures of the dimetal TnDhp in complexes with hydantoin, N-carbamyl-ß-alanine, and N-carbamyl-ß-amino isobutyrate as well as apo-TnDhp in complex with a product analog, N-(2-acetamido)-iminodiacetic acid, have been determined. These structural results illustrate how a protein exploits unique lysines and the metal distribution to accomplish lysine carbamylation as well as subsequent enzymatic functions.

Improved mass spectrometric analysis of membrane proteins based on rapid and versatile sample preparation on nanodiamond particles.

We have developed a novel streamlined sample preparation procedure for mass spectrometric (MS) analysis of membrane proteins using surface-oxidized nanodiamond particles. The platform consists of solid-phase extraction and elution of the membrane proteins on nanodiamonds, concentrating the membrane proteins on the nanodiamonds and separating out detergents, chaotropic agents, and salts, and other impurities that are often present at high concentrations in solubilized membrane preparations. In this manner, membrane-protein extracts are transformed into MS-ready samples in minutes. The protocol is not only fast, but also widely adaptable and highly effective for preparing generic membrane protein samples for both MALDI-MS studies of membrane-protein complexes and shotgun membrane proteomics studies. As proof of concept, we have demonstrated substantial improvements in the MALDI-MS analysis of the particulate methane monooxygenase (pMMO) complex, a three-subunit transmembrane protein solubilized in various detergent buffers. Enzymatic digestions of membrane proteins are also greatly facilitated since the proteins extracted on to the nanodiamonds are exposed on the surface of the nanoparticles rather than in SDS gels or in detergent solutions. We illustrate the effectiveness of nanodiamonds for SDS removal in the preparation of membrane proteins for MS analysis on the proteome level by examining the quality of the tryptic peptides prepared by on-surface nanodiamond digestion of an E. coli membrane fraction for shotgun proteomics.

Cotranslational protein folding within the ribosome tunnel influences trigger-factor recruitment.

In recent years, various folding zones within the ribosome tunnel have been identified and explored through x-ray, cryo-electron microscopy (cryo-EM), and molecular biology studies. Here, we generated ribosome-bound nascent polypeptide complexes (RNCs) with different polyalanine (poly-A) inserts or signal peptides from membrane/secretory proteins to explore the influence of nascent chain compaction in the Escherichia coli ribosome tunnel on chaperone recruitment. By employing time-resolved fluorescence resonance energy transfer and immunoblotting, we were able to show that the poly-A inserts embedded in the passage tunnel can form a compacted structure (presumably helix) and reduce the recruitment of Trigger Factor (TF) when the helical motif is located in the region near the tunnel exit. Similar experiments on nascent chains containing signal sequences that may form compacted structural motifs within the ribosome tunnel and lure the signal recognition particle (SRP) to the ribosome, provided additional evidence that short, compacted nascent chains interfere with TF binding. These findings shed light on the possible controlling mechanism of nascent chains within the tunnel that leads to chaperone recruitment, as well as the function of L23, the ribosomal protein that serves as docking sites for both TF and SRP, in cotranslational protein targeting.

The metal core structures in the recombinant Escherichia coli transcriptional factor SoxR.

X-ray absorption, circular dichroism, and EPR spectroscopy were employed to investigate the metal-core structures in the Escherichia coli transcriptional factor SoxR under reduced, oxidized, and nitrosylated conditions. The spectroscopic data revealed that the coordination environments of the metal active centers varied only very slightly between the reduced and oxidized states, similar to most other proteins containing iron-sulfur clusters. Upon nitrosylation of oxidized SoxR, however, we observed a low-temperature EPR spectrum characteristic of a protein dinitrosyl iron complex (DNIC), with an intensity corresponding to about two DNICs per iron sulfur cluster in the protein, according to spin quantification relative to a low-molecular-weight DNIC standard. In addition, there was no evidence for dichroic spectral features in the responsive region of the nitrosyl iron complexes, as well as for Fe-Fe back-scattering in the fitting of the Fe extended X-ray absorption fine structure (EXAFS) spectrum. Instead the Fe EXAFS spectrum of the nitrosylated SoxR core exhibited the same first- and second-shell coordination environments characteristic of modeled small molecular DNICs, indicating that each of the [2 Fe-2 S] cores in the homodimeric SoxR was dissociated into two individual DNICs. Similar nitrosylation of the reduced mixed-valence SoxR for 1 min led to degradation of the iron-sulfur clusters to give several iron species, including one with EPR signals characteristic of a reduced Roussin's red ester (rRRE), a diamagnetic species, presumably Roussin's red ester (RRE), and a small amount of DNIC. We also undertook in vivo time-course studies of E. coli cells containing recombinant SoxR after rapid purging of the cells with exogenous NO gas. Rapid freeze-quenched EPR experiments demonstrated rapid formation of the SoxR rRRE species, followed by fast breakup of this precursor intermediate to form the stable protein-bound DNIC species. Accordingly, under nitrosative stress, we believe that the response of SoxR to NO could depend on the intracellular redox state of E. coli, the central modulator of which could be exploited to deduce the appropriate mechanism to sense the presence of NO for physiological regulation.

Dioxygen activation of a trinuclear Cu(I)Cu(I)Cu(I) cluster capable of mediating facile oxidation of organic substrates: competition between O-atom transfer and abortive intercomplex reduction.

The dioxygen activation of a series of Cu(I)Cu(I)Cu(I) complexes based on the ligands (L) 3,3'-(1,4-diazepane- 1,4-diyl)bis(1-{[2-(dimethylamino)ethyl](methyl)amino}propan-2-ol)(7-Me) or 3,3'-(1,4-diazepane-1,4-diyl)bis(1-{[2-(diethylamino)ethyl](ethyl)amino}propan-2-ol)(7-Et) forms an intermediate capable of mediating facile O-atom transfer to simple organic substrates at room temperature. To elucidate the dioxygen chemistry, we have examined the reactions of 7-Me, 7-Et, and 3,3'-(1,4-diazepane-1,4-diyl)bis[1-(4-methylpiperazin-1-yl)propan-2-ol] (7-N-Meppz) with dioxygen at -80, -55, and -35 °C in propionitrile (EtCN) by UV-visible, 77 K EPR, and X-ray absorption spectroscopy, and 7-N-Meppz and 7-Me with dioxygen at room temperature in acetonitrile (MeCN) by diode array spectrophotometry. At both -80 and -55 °C, the mixing of the starting [Cu(I)Cu(I)Cu(I)(L)](1+) complex (1) with O(2)-saturated propionitrile (EtCN) led to a bright green solution consisting of two paramagnetic species: the green dioxygen adduct [Cu(II)Cu(II)(µ-η(2):η(2)-peroxo)Cu(II)(L)](2+) (2) and the blue [Cu(II)Cu(II)(µ-O)Cu(II)(L)](2+) species (3). These observations are consistent with the initial formation of [Cu(II)Cu(II)(µ-O)(2)Cu(III)(L)](1+)(4), followed by rapid abortion of this highly reactive species by intercluster electron transfer from a second molecule of complex 1 to give the blue species 3 and subsequent oxygenation of the partially oxidized [Cu(II)Cu(I)Cu(I)(L)](2+)(5) to form the green dioxygen adduct 2. Assignment of 2 to [Cu(II)Cu(II)(µ-η(2):η(2)-peroxo)Cu(II)(L)](2+) is consistent with its reactivity with water to give H(2)O(2) and the blue species 3, as well as its propensity to be photoreduced in the X-ray beam during X-ray absorption experiments at room temperature. In light of these observations, the development of an oxidation catalyst based on the tricopper system requires consideration of the following design criteria: 1) rapid dioxygen chemistry; 2) facile O-atom transfer from the activated cluster to substrate; and 3) a suitable reductant to rapidly regenerate complex 1 to accomplish efficient catalytic turnover.

Location of the cross-ß structure in prion fibrils: A search by seeding and electron spin resonance spectroscopy.

Prion diseases are transmissible fatal neurodegenerative disorders spreading between humans and other mammals. The pathogenic agent, prion, is a protease-resistant, ß-sheet-rich protein aggregate, converted from a membrane protein called PrPC . PrPSc is the misfolded form of PrPC and undergoes self-propagation to form the infectious amyloids. Since the key hallmark of prion disease is amyloid formation, identifying and studying which segments are involved in the amyloid core can provide molecular details about prion diseases. It has been known that the prion protein could also form non-infectious fibrils in the presence of denaturants. In this study, we employed a combination of site-directed nitroxide spin-labeling, fibril seeding, and electron spin resonance (ESR) spectroscopy to identify the structure of the in vitro-prepared full-length mouse prion fibrils. It is shown that in the in vitro amyloidogenesis, the formation of the amyloid core is linked to an α-to-ß structural transformation involving the segment 160-224, which contains strand 2, helix 2, and helix 3. This method is particularly suitable for examining the hetero-seeded amyloid fibril structure, as the unlabeled seeds are invisible by ESR spectroscopy. It can be applied to study the structures of different strains of infectious prions or other amyloid fibrils in the future.

Structural insights into the enzyme catalysis from comparison of three forms of dissimilatory sulphite reductase from Desulfovibrio gigas.

The crystal structures of two active forms of dissimilatory sulphite reductase (Dsr) from Desulfovibrio gigas, Dsr-I and Dsr-II, are compared at 1.76 and 2.05 Å resolution respectively. The dimeric α2ß2γ2 structure of Dsr-I contains eight [4Fe-4S] clusters, two saddle-shaped sirohaems and two flat sirohydrochlorins. In Dsr-II, the [4Fe-4S] cluster associated with the sirohaem in Dsr-I is replaced by a [3Fe-4S] cluster. Electron paramagnetic resonance (EPR) of the active Dsr-I and Dsr-II confirm the co-factor structures, whereas EPR of a third but inactive form, Dsr-III, suggests that the sirohaem has been demetallated in addition to its associated [4Fe-4S] cluster replaced by a [3Fe-4S] centre. In Dsr-I and Dsr-II, the sirohydrochlorin is located in a putative substrate channel connected to the sirohaem. The γ-subunit C-terminus is inserted into a positively charged channel formed between the α- and ß-subunits, with its conserved terminal Cys104 side-chain covalently linked to the CHA atom of the sirohaem in Dsr-I. In Dsr-II, the thioether bond is broken, and the Cys104 side-chain moves closer to the bound sulphite at the sirohaem pocket. These different forms of Dsr offer structural insights into a mechanism of sulphite reduction that can lead to S3O6(2-), S2O3(2-) and S2-.

Tuning the regio- and stereoselectivity of C-H activation in n-octanes by cytochrome P450 BM-3 with fluorine substituents: evidence for interactions between a C-F bond and aromatic π systems.

We employed the water-soluble cytochrome P450 BM-3 to study the activity and regiospecificity of oxidation of fluorinated n-octanes. Three mutations, A74G, F87V, and L188Q, were introduced into P450 BM-3 to allow the system to undergo n-octane oxidation. In addition, the alanine at residue 328 was replaced with a phenylalanine to introduce an aromatic residue into the hydrophobic pocket to examine whether or not van der Waals interactions between a C-F substituent in the substrate and the polarizable π system of the phenylalanine may be used to steer the positioning of the substrate within the active-site pocket of the enzyme and control the regioselectivity and stereoselectivity of hydroxylation. Interestingly, not only was the regioselectivity controlled when the fluorine substituent was judiciously positioned in the substrate, but the electron input into the iron-heme group became tightly coupled to the formation of product, essentially without abortive side reactions. Remarkable enhancement of the coupling efficiency between electron input and product formation was observed for a range of fluorinated octanes in the enzyme even without the A328F mutation, presumably because of interactions of the C-F substituent with the π system of the porphyrin macrocycle within the active-site pocket. Evidently, tightening the protein domain containing the heme pocket tunes the distribution of accessible enzyme conformations and the associated protein dynamics that activate the iron porphyrin for substrate hydroxylation to allow the reactions mediated by the high-valent Fe(IV)=O to become kinetically more commensurate with electron transfer from the flavin adenine dinucleotide (FAD)/flavin mononucleotide (FMN) reductase. These observations lend compelling evidence to support significant van der Waals interactions between the CF(2) group and aromatic π systems within the heme pocket when the fluorinated octane substrate is bound.

Dioscorea phytocompounds enhance murine splenocyte proliferation ex vivo and improve regeneration of bone marrow cells in vivo.

Specific cytokines have been tested clinically for immunotherapy of cancers; however, cytotoxicity has often impaired their usefulness. Consequently, alternative approaches are increasingly desirable. Dioscorea spp. tuber is a widely used traditional Chinese medicinal herb claimed to confer immunostimulatory activity. In this study, we evaluated Dioscorea as an adjuvant therapy for use alongside chemotherapy for cancer. Phytocompounds from Dioscorea tubers were ethanol fractioned and used for ex vivo splenocyte proliferation assay or in vivo force-feeding of mice pre-treated with the chemotherapy agent 5-fluorouracil. Co-treatment with a 50-75% ethanol-partitioned fraction of the tuber extract of D. batatas (DsCE-II) and interleukin (IL)-2 resulted in a significantly higher rate of murine splenocyte cell proliferation ex vivo than treatment with DsCE-II or IL-2 alone. This DsCE-II fraction, which contains a polysaccharide with a high proportion of ß-1,4-linkage mannose (≥64%), also promoted the regeneration of specific progenitor cell populations in damaged bone marrow tissues of 5-fluorouracil-treated mice. Colony-forming unit (CFU) analyses demonstrated that the population of CFU-GM cells, but not CFU-GEMM or BFU-E cells, preferentially recovered to ~67% in the bone marrow of immune-suppressed mice fed with DsCE-II. DsCE-II efficacy level was ~85% of that obtained by subcutaneous administration of recombinant G-CSF proteins (5 µg kg(-1)) in mice tested in parallel. This study suggests that the DsCE-II fraction of D. batatas extract may be considered for further development as a dietary supplement for use alongside chemotherapy during cancer treatment.

Catalytic machinery of methane oxidation in particulate methane monooxygenase (pMMO).

In this focused review, we portray the recently reported 2.5 Å cyro-EM structure of the particulate methane monooxygenase (pMMO) from M. capsulatus (Bath). The structure of the functional holo-pMMO near atomic resolution has uncovered the sites of the copper cofactors including the location of the active site in the enzyme. The three coppers seen in the original X-ray crystal structures of the enzyme are now augmented by additional coppers in the transmembrane domain as well as in the water-exposed C-terminal subdomain of the PmoB subunit. The cryo-EM structure offers the first glimpse of the catalytic machinery capable of methane oxidation with high selectivity and efficiency. The findings are entirely consistent with the biochemical and biophysical findings previously reported in the literature, including the chemistry of hydrocarbon hydroxylation, regeneration of the catalyst for multiple turnovers, and the mechanism of aborting non-productive cycles to ensure kinetic competence.

Crystal structure of Adenylylsulfate reductase from Desulfovibrio gigas suggests a potential self-regulation mechanism involving the C terminus of the beta-subunit.

Adenylylsulfate reductase (adenosine 5'-phosphosulfate [APS] reductase [APSR]) plays a key role in catalyzing APS to sulfite in dissimilatory sulfate reduction. Here, we report the crystal structure of APSR from Desulfovibrio gigas at 3.1-A resolution. Different from the alpha(2)beta(2)-heterotetramer of the Archaeoglobus fulgidus, the overall structure of APSR from D. gigas comprises six alphabeta-heterodimers that form a hexameric structure. The flavin adenine dinucleotide is noncovalently attached to the alpha-subunit, and two [4Fe-4S] clusters are enveloped by cluster-binding motifs. The substrate-binding channel in D. gigas is wider than that in A. fulgidus because of shifts in the loop (amino acid 326 to 332) and the alpha-helix (amino acid 289 to 299) in the alpha-subunit. The positively charged residue Arg160 in the structure of D. gigas likely replaces the role of Arg83 in that of A. fulgidus for the recognition of substrates. The C-terminal segment of the beta-subunit wraps around the alpha-subunit to form a functional unit, with the C-terminal loop inserted into the active-site channel of the alpha-subunit from another alphabeta-heterodimer. Electrostatic interactions between the substrate-binding residue Arg282 in the alpha-subunit and Asp159 in the C terminus of the beta-subunit affect the binding of the substrate. Alignment of APSR sequences from D. gigas and A. fulgidus shows the largest differences toward the C termini of the beta-subunits, and structural comparison reveals notable differences at the C termini, activity sites, and other regions. The disulfide comprising Cys156 to Cys162 stabilizes the C-terminal loop of the beta-subunit and is crucial for oligomerization. Dynamic light scattering and ultracentrifugation measurements reveal multiple forms of APSR upon the addition of AMP, indicating that AMP binding dissociates the inactive hexamer into functional dimers, presumably by switching the C terminus of the beta-subunit away from the active site. The crystal structure of APSR, together with its oligomerization properties, suggests that APSR from sulfate-reducing bacteria might self-regulate its activity through the C terminus of the beta-subunit.

Tuning the conformational properties of the prion peptide.

Previously, we disclosed that O-linked glycosylation of Ser-132 or Ser-135 could dramatically change the amyloidogenic property of the hamster prion peptide (sequence 108-144). This peptide, which corresponds to the flexible loop and the first beta-strand in the structure of the prion protein, is a random coil when it is initially dissolved in buffer, but amyloid fibrils are formed with time. Thus, it offers a convenient model system to observe and compare how different chemical modifications and sequence mutations alter the amyloidogenic property of the peptide within a reasonable experimental time frame. In our earlier study, aside from uncovering a site-specificity of the glycosylation on the fibrillogenesis, different effects of alpha-GalNAc and beta-GlcNAc were observed. In this work, we explore further how different sugar configurations affect the conformational property of the polypeptide chain. We compare the effects of O-linked glycosylation by the common sugars alpha-GalNAc, beta-GlcNAc with their non-native analogs beta-GalNAc, alpha-GlcNAc in an effort to uncover the origin of the sugar-specificity on the fibril formation. We find that the anomeric configuration of the sugar is the most important factor affecting the fibrillogenesis. Sugars with the glycosidic bond in the alpha-configuration at Ser-135 have a dramatic inhibitory effect on the structural conversion of the glycosylated peptide. Because O-glycosylation of Ser-135 with alpha-linked sugars also promote the formation of three slowly converting conformations at the site of glycosylation, we surmise that the amyloidogenic property of the peptide is related to its conformational flexibility, and the proclivity of this region of the peptide to undergo the structural conversion from the random coil to form the beta-structure. Upon O-glycosylation with an alpha-linked sugar, this conversion is inhibited and the nucleation of fibril formation is largely retarded. Consistent with this scenario, Arg-136 is the residue most affected in the TOCSY NMR spectra of the glycosylated peptides, other than the serine site modified. In addition, when Arg-136 is substituted by Gly, a mutation that should provide higher structural flexibility in this part of the peptide, the amyloidogenic property of the peptide is greatly enhanced, and the inhibition effect of glycosylation is largely diminished. These results are consistent with Ser-135 and Arg-136 being part of the kink region involved in the structural conversion.

Insight into the substrate length restriction of M32 carboxypeptidases: characterization of two distinct subfamilies.

M32 carboxypeptidases are a distinct family of HEXXH metalloproteases whose structures exhibit a narrow substrate groove that is blocked at one end. Structural alignments with other HEXXH metalloprotease-peptide complexes suggested an orientation in which the substrate is directed towards the back of the groove. This led us to hypothesize, and subsequently confirm that the maximum substrate length for M32 carboxypeptidases is restricted. Structural and sequence analyses implicate a highly conserved Arg at the back of the groove as being critical for this length restriction. However, the Thermus thermophilus and Bacillus subtilis M32 members lack this conserved Arg. Herein, we present the biochemical and structural characterization of these two proteins. Our findings support the important role of the conserved Arg in maintaining the length restriction, and reveal a proline-rich loop as an alternate blocking strategy. Based on our results, we propose that M32 carboxypeptidases from Bacilli belong to a separate subfamily.