Membrane-bound quinoprotein glucose dehydrogenase (GDH) in Escherichia coli donates electrons directly to ubiquinone during the oxidation of D-glucose as a substrate, and these electrons are subsequently transferred to ubiquinol oxidase in the respiratory chain. To determine whether the specific ubiquinone-reacting site of GDH resides in the N-terminal transmembrane domain or in the large C-terminal periplasmic catalytic domain (cGDH), we constructed a fusion protein between the signal sequence of ␤-lactamase and cGDH. This truncated GDH was found to complement a GDH gene-disrupted strain in vivo. The signal sequence of the fused protein was shown to be cleaved off, and the remaining cGDH was shown to be recovered in the membrane fraction, suggesting that cGDH has a membraneinteracting site that is responsible for binding to membrane, like peripheral proteins. Kinetic analysis and reconstitution experiments revealed that cGDH has ubiquinone reductase activity nearly equivalent to that of the wild-type GDH. Thus, it is likely that the C-terminal periplasmic domain of GDH possesses a ubiquinonereacting site and transfers electrons directly to ubiquinone.Membrane-bound GDH 1 in Escherichia coli is a PQQ-containing quinoprotein that catalyzes a direct oxidation of Dglucose to D-gluconate in the periplasm and concomitantly transfers electrons to ubiquinol oxidase through ubiquinone in the respiratory chain (1-3). GDH is an 88-kDa monomeric protein with five transmembrane segments at the N-terminal portion (residues 1-140), which ensure a strong anchorage of the protein to the inner membrane (4, 5). The remaining large C-terminal portion (residues 141-796) has a catalytic domain including PQQ-(6, 7) and Ca 2ϩ or Mg 2ϩ -binding sites (8, 9) that is located in the periplasmic side. A model structure of GDH based on the x-ray crystallographic structure of the ␣-subunit of MDH in Methylobacterium extorquens has been proposed (10), and the putative structure of the PQQ-binding catalytic site has been further confirmed and characterized by mutagenic analysis of several amino acid residues around PQQ (11-15).The ubiquinone-reacting site in GDH has also been analyzed. Friedrich et al. (16) proposed that the ubiquinone-reacting site may be located at the N-terminal transmembrane domain of Acinetobacter calcoaceticus GDH in which Arg-91 and Asp-93 may be involved in interaction with ubiquinone. The topological model of the N-terminal transmembrane domain of E. coli GDH has shown that the corresponding amino acid residues, Arg-93 and Asp-95, are located near the membrane surface of the periplasmic side (5). Furthermore, using depth-dependent fluorescent ubiquinone analogues, Miyoshi et al. (17) demonstrated that the ubiquinone-reduction site of GDH is located close to the membrane surface rather than in the hydrophobic interior. X-ray crystallographic structures of cytochrome bo in E. coli (18,19) and cytochrome bc 1 complex (Q o and Q i centers) in bovine heart mitochondria have recently been determined (20, 21), and it has b...

When structures having inclined surfaces, such as cone-type and inclined-pile structures, are constructed in coastal and offshore cold regions, sea ice forces must be considered in their design. In order to estimate these forces, the relationships of the coefficients of static and kinetic friction between sea ice and construction materials must be evaluated. The authors have been conducting, for four years, experiments on the coefficients of friction between sea ice and various commonly used offshore construction materials such as concrete and steel. This paper summarizes the results of this study. The coefficients of friction have been found to be affected by the following: (i) relative velocity (i.e., velocity of construction material relative to sea ice); (ii) sea ice temperature; and (iii) surface roughness of construction material. They have been found to be relatively unaffected by the following: (i) contact area, (ii) normal stress, (iii) growth direction of sea ice, and (iv) water at the sea ice-material interface.

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