| STRUCTURE |
By working out the puzzling structure of a hydrogenase--an enzyme that makes molecular hydrogen from two electrons and two protons--researchers have provided a magnifying glass through which the details of nature's biomechanisms can be examined. The study not only solves a long-standing structural mystery, but it also may offer insights into the design of industrially useful catalysts. Shaped roughly like a mushroom, hydrogenase catalyzes hydrogen formation by transferring electrons from iron-sulfur clusters in the stem to an iron-sulfur cluster (the H cluster) in the cap, where reactions with protons can occur. The iron-rich hydrogenase comes from the soil bacterium Clostridium pasteurianum. Its X-ray crystal structure was determined by a team of investigators. Although the enzyme was discovered in the 1930s, the arrangement of its metal centers has remained an enigma. According to the Utah State group, spectroscopic and biochemical studies previously indicated that the enzyme's iron and sulfur atoms form five distinct clusters. On the basis of those studies, four of the clusters appeared similar in their bonding to others found in metal-containing enzyme systems. But the structure of the fifth cluster--the one believed to be at the center of catalytic activity and therefore named the hydrogen or H cluster--was apparently unlike any other characterized iron cluster. "But even considering indications that the H cluster has a unique arrangement, the structure we determined was still somewhat unexpected," Peters asserts. "It hadn't been observed before in a biological system." An enlarged view of the H cluster shows a four-iron, four-sulfur unit bound to four cysteine sulfur atoms, one of which serves as a bridge to a two-iron subcluster in which the metal atoms are bound to CO or CN ligands. Orange spheres represent sulfur atoms; large red, iron; small red, oxygen; and white, carbon. The unusual H cluster is composed of a grouping of four iron and four sulfur atoms bridged to an unprecedented two-iron center. In the two-atom subcluster, each iron atom is six-coordinate and each is bound to carbonyl or cyanide ligands. (The similar electron density of CO and CN precludes distinguishing those diatomic ligands crystallographically.) Authors liken the overall protein structure to a mushroom. The mushroom's cap, which includes the active-site iron-sulfur cluster, accounts for roughly two-thirds of the 60-kilodalton enzyme. The enzyme holds the active-site cluster in position via covalent bonds to four cysteine sulfur atoms. One of those sulfur atoms also serves as a bridge between the four-iron and two-iron portions of the H cluster. The remainder of the protein and the four accessory clusters form the stem of the mushroom. Authors proposes that as the anaerobic bacterium goes about its metabolic business of oxidizing sugars and evolving hydrogen, electrons from a pyruvate oxidation step travel through the stem in a series of electron-transfer reactions. This series of electron "handoffs" between iron-sulfur clusters eventually lands the electrons at the H cluster. At that site, two protons (derived from water) and two electrons combine (reversibly) to form dihydrogen. "Knowing the crystal structure enables you to propose a plausible mechanism. "but there's no guarantee that that mechanism is completely accurate." The real importance of the structural information, he says, is that "it provides a basis from which to propose additional experiments to probe the mechanism." And it is through these additional studies that advances toward possible applications can be made. Hydrogen is increasingly discussed as a future fuel source because of its clean-burning and renewable character. So, he adds, biological production of hydrogen represents a potential energy reserve that might be tapped through new understanding of biomechanisms. |
| UPDATE | 12.98 |
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