The crystal structure of the tungsten-containing aldehyde ferredoxin oxidoreductase (AOR) from Pyrococcus furiosus, a hyperthermophilic archaeon (formerly archaebacterium) that grows optimally at 100 degrees C, has been determined at 2.3 angstrom resolution by means of multiple isomorphous replacement and multiple crystal form averaging. AOR consists of two identical subunits, each containing an Fe4S4 cluster and a molybdopterin-based tungsten cofactor that is analogous to the molybdenum cofactor found in a large class of oxotransferases. Whereas the general features of the tungsten coordination in this cofactor were consistent with a previously proposed structure, each AOR subunit unexpectedly contained two molybdopterin molecules that coordinate a tungsten by a total of four sulfur ligands, and the pterin system was modified by an intramolecular cyclization that generated a three-ringed structure. In comparison to other proteins, the hyperthermophilic enzyme AOR has a relatively small solvent-exposed surface area, and a relatively large number of both ion pairs and buried atoms. These properties may contribute to the extreme thermostability of this enzyme.

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The structures of the oxidized and reduced forms of the rubredoxin from the archaebacterium, Pyrococcus furiosus, an organism that grows optimally at 100 °C, have been determined by X‐ray crystallography to a resolution of 1.8 å. Crystals of this rubredoxin grow in space group P212121 with room temperature cell dimensions a = 34.6 å, b = 35.5 å, and c = 44.4 å. Initial phases were determined by the method of molecular replacement using the oxidized form of the rubredoxin from the mesophilic eubacterium, Clostridium pasteurianum, as a starting model. The oxidized and reduced models of P. furiosus rubredoxin each contain 414 nonhydrogen protein atoms comprising 53 residues. The model of the oxidized form contains 61 solvent H2O oxygen atoms and has been refined with X‐PLOR and TNT to a final R = 0.178 with root mean square (rms) deviations from ideality in bond distances and bond angles of 0.014 å and 2.06°, respectively. The model of the reduced form contains 37 solvent H2O oxygen atoms and has been refined to R = 0.193 with rms deviations from ideality in bond lengths of 0.012 å and in bond angles of 1.95°. The overall structure of P. furiosus rubredoxin is similar to the structures of mesophilic rubredoxins, with the exception of a more extensive hydrogen‐bonding network in the β‐sheet region and multiple electrostatic interactions (salt bridge, hydrogen bonds) of the Glu 14 side chain with groups on three other residues (the amino‐terminal nitrogen of Ala 1; the indole nitrogen of Trp 3; and the amide nitrogen group of Phe 29). The influence of these and other features upon the thermostability of the P. furiosus protein is discussed.

The peptide hormone Urocortin3 (Ucn3) is abundantly expressed by mature beta cells, yet its physiological role is unknown. Here we demonstrate that Ucn3 is stored and co–released with insulin and potentiates glucose–stimulated somatostatin secretion via cognate receptor on delta cells. Further, we found that islets lacking endogenous Ucn3 demonstrate fewer delta cells, reduced somatostatin content, impaired somatostatin secretion and exaggerated insulin release, and that these defects are rectified by synthetic Ucn3 in vitro. Our observations indicate that the paracrine actions of Ucn3 activate a negative feedback loop that promotes somatostatin release to ensure the timely reduction of insulin secretion upon normalization of plasma glucose. Moreover, Ucn3 is markedly depleted from beta cells in mouse and macaque diabetes models and in human diabetic islets. This suggests that Ucn3 is a key contributor to stable glycemic control whose reduction during diabetes aggravates glycemic volatility and contributes to the pathophysiology of this disease.

tions is the same. However, off axis (8, > 0), they are substantially different. To make a true omnidirectional dielectric mirror. one wants to have a broad reflection band that survives for all angles and for both polarizations. The p-polarized band was thought to always have a hole or "window" at Brewster's angle, OB, where p-polarized light propagates from an nl to an n2 layer, and from an n2 to an n1 layer without any reflection (3) Once light gets into the multilayer film at OB-regardless of the frequency-it will propagate unimpeded: there is a hole in the reflection band at that angle. It is somewhat ironic that even Joannopoulos himself once implied that it was impossible to make a one-dimensional (ID) omnidirectional reflector in his own book with Meade and Winn (4) Their proof actually showed that there is no omnidirectional photonic band gap in an infinite ID, periodic, layered structure. However, all physical 1D structures are finite, and this offers the way out.The real reflection coefficient R12 at a n, to n7 interface is identicallv zero for Brewster's condition, O I B = arctan n2/nl. Now in the figure, we see that the overall incident angle O0 has a full domain [0, n/2]. However, the corresponding range of the refracted angle O1 is restricted by Snell's law (no sin O0 = n1 sin 01) to the interval [0, Olillax], where 011"" = arcsin nO/nl. If Olmhx < elB, then incident light from the outside can never couple to the Brewster window. That is the trick, and it is easy to do by making no/nl sufficiently large. Now we make sure that 11, and n2 are just right so that we run out of angle Oo before we run out of bandwidth, and voila, we have the

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Microorganisms that can grow at and above 100 degrees C were discovered a decade ago, and about 20 different genera are now known. These so-called hyperthermophiles are the most ancient of all extant life; all but two genera are classified as Archaea. All have been isolated from geothermal heated environments including deep-sea hydrothermal vents. This group includes some methanogenic and sulfate-reducing species, but the majority are strictly anaerobic heterotrophs that utilize complex peptide mixtures as sources of energy, carbon, and nitrogen. Only a few species are saccharolytic. Most of the hyperthermophiles absolutely depend on the reduction of elemental sulfur (S0) to H2S for significant growth, a property that severely limits their large-scale culture in conventional fermentation systems. Consequently, most physiological and metabolic studies have focused on those that can also grow in the absence of S0, including species of the Archaea, Pyrococcus and Thermococcus, and the bacterium Thermotoga. The fermentative pathways for the metabolism of both peptides and carbohydrates in the Archaea appear to depend upon enzymes that contain tungsten, an element seldom used in biological systems. The mechanisms of S0 reduction and energy conservation remain unclear. Enzymes purified from the S0-reducing hyperthermophiles include proteases, amylolytic-type enzymes, hydrogenases, redox proteins, various ferredoxin-linked oxidoreductases, dehydrogenases, and DNA polymerases, some of which are active up to 140 degrees C. However, complete amino acid sequences are known for only a handful of these proteins, and the three-dimensional structure of only one hyperthermophilic protein has been determined. Potential mechanisms by which proteins and various biological cofactors and organic intermediates are stabilized at extreme temperatures are only now beginning to emerge.

Previous studies have shown that the hyperthermophilic archaeon Pyrococcus furiosus contains four distinct cytoplasmic 2-ketoacid oxidoreductases (ORs) which differ in their substrate specificities, while the hyperthermophilic bacterium Thermotoga maritima contains only one, pyruvate ferredoxin oxidoreductase (POR). These enzymes catalyze the synthesis of the acyl (or aryl) coenzyme A derivative in a thiamine PP i -dependent oxidative decarboxylation reaction with reduction of ferredoxin. We report here on the molecular analysis of the POR (por) and 2-ketoisovalerate ferredoxin oxidoreductase (vor) genes from P. furiosus and of the POR gene from T. maritima, all of which comprise four different subunits. The operon organization for P. furiosus POR and VOR was porG-vorDAB-porDAB, wherein the ␥ subunit is shared by the two enzymes. The operon organization for T. maritima POR was porGDAB. The three enzymes were 46 to 53% identical at the amino acid level. Their ␦ subunits each contained two ferredoxin-type [4Fe-4S] cluster binding motifs (CXXCXXC XXXCP), while their ␤ subunits each contained four conserved cysteines in addition to a thiamine PP i -binding domain. Amino-terminal sequence comparisons show that POR, VOR, indolepyruvate OR, and 2-ketoglutarate OR of P. furiosus all belong to a phylogenetically homologous OR family. Moreover, the single-subunit pyruvate ORs from mesophilic and moderately thermophilic bacteria and from an amitochondriate eucaryote each contain four domains which are phylogenetically homologous to the four subunits of the hyperthermophilic ORs (27% sequence identity). Three of these subunits are also homologous to the dimeric POR from a mesophilic archaeon, Halobacterium halobium (21% identity). A model is proposed to account for the observed phenotypes based on genomic rearrangements of four ancestral OR subunits.The final oxidative step in the fermentation of carbohydrates in anaerobic microorganisms is typically catalyzed by pyruvate ferredoxin oxidoreductase (POR). This involves the oxidative decarboxylation of pyruvate with the participation of thiamine PP i (TPP) followed by the transfer of an acetyl moiety to coenzyme A (CoASH) for the synthesis of acetyl-CoA as shown in the following equation: CH 3 -CO-COOH ϩ CoASH 3 CH 3 -CO-SCoA ϩ CO 2 ϩ 2H ϩ ϩ 2e Ϫ (e Ϫ is an electron) (44, 45). The electrons from this reaction are transferred to low-potential ferredoxins or flavodoxins typically for ultimate disposal either as H 2 or in an organic compound.PORs have been purified from several anaerobic hyperthermophiles, which are organisms that grow at temperatures near and above 100ЊC (42). These include the sulfur-reducing archaea Pyrococcus furiosus (6) and Thermococcus litoralis (15), the sulfate-reducing archaeon Archaeoglobus fulgidus (28), and the bacterium Thermotoga maritima (7). All the hyperthermophilic PORs comprise four different subunits (␣, ␤, ␥, and ␦) with apparent molecular masses of approximately 43, 35, 23, and 12 kDa, respectively. We have also purified three other types ...