Zirconia-ceria solid solutions of tetragonal symmetry (t'-ZrO,) containing 30 to 65 mol% ceria were prepared by annealing c'-ZrO, at 627°C samples sintered at 1660-1760"C, where c'-ZrO, is defined as a cubic or tetragonal phase whose axial ratio c / a (tetragonality) is equal to 1. The lattice parameters and the cube root of the unit cell volumes of the annealed samples increased linearly with the ceria content but, on the other hand, the axial ratio cla (tetragonality) decreased to 1.000 at 70 mol%. During the annealing of each sample, the cell volume decreased because the residual Ce3+ ions were oxidized to the smaller Ce4+. The temperature-time-transformation (TTT) diagram of the 1:l solid solution was investigated for the ~' 4 ' diffusionless phase transition and the c'-(t + c) diffusional reaction.The d -t ' transformation was found to behave as a thermally activated process with an activation energy estimated to be 113 kJ/mol. This value may indicate that c'-t' transformation is controlled by oxygen diffusion accompanied by dimensional changes of the cerium ions.

Diffusionless tetragonal (t') u cubic (c') phase transformation in 65-mol%-Ce0,-ZrO2 was investigated around the c'-t' equilibrium temperature Ti'" using powder X-ray diffraction, where c' was defined as a tetragonal or cubic phase with an axial ratio of unity. The (400) peak profile of t'-ZrO, broadened at 860" and 900°C. This indicated that the t'-ZrO, transformed partially into c'-ZrO,. The (400) peak profile of c'-ZrO, also broadened at 860" and 9OO0C, which indicated that the c'-ZrO, partially transformed to t'-ZrO,. The finish point of t ' -d transformation T:' " was investigated by annealing the t'-ZrO, containing 30-65 mol% CeO, at various temperatures. The T:'"' line existed in the vicinity of the cubic solubility limit within the t + c two-phase region. This Tf-" location, which should have been located in the vicinity of Ti"", could not be explained by a simple regular solution model. However, it was described successfully by a thermodynamic model based on Landau's phenomenologic theory.

The Clostridium paraputrificum chiB gene, encoding chitinase B (ChiB), consists of an open reading frame of 2,493 nucleotides and encodes 831 amino acids with a deduced molecular weight of 90,020. The deduced ChiB is a modular enzyme composed of a family 18 catalytic domain responsible for chitinase activity, two reiterated domains of unknown function, and a chitin-binding domain (CBD). The reiterated domains are similar to the repeating units of cadherin proteins but not to fibronectin type III domains, and therefore they are referred to as cadherin-like domains. ChiB was purified from the periplasm fraction of Escherichia coli harboring the chiB gene. The molecular weight of the purified ChiB (87,000) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, was in good agreement with the value (86,578) calculated from the deduced amino acid sequence excluding the signal peptide. ChiB was active toward chitin from crab shells, colloidal chitin, glycol chitin, and 4-methylumbelliferyl ␤-D-N,N-diacetylchitobioside [4-MU-(GlcNAc) 2 ]. The pH and temperature optima of the enzyme were 6.0 and 45°C, respectively. The K m and V max values for 4-MU-(GlcNAc) 2 were estimated to be 6.3 M and 46 mol/min/mg, respectively. SDS-PAGE, zymogram, and Western blot analyses using antiserum raised against purified ChiB suggested that ChiB was one of the major chitinase species in the culture supernatant of C. paraputrificum. Deletion analysis showed clearly that the CBD of ChiB plays an important role in hydrolysis of native chitin but not processed chitin such as colloidal chitin.Chitinase (EC 3.2.1.14) is a glycosyl hydrolase that catalyzes the degradation of chitin, an insoluble linear ␤-1,4-linked polymer of N-acetylglucosamine. Chitinases are present in a wide range of organisms, including bacteria, insects, viruses, plants, and animals, and play important physiological and ecological roles. On the basis of amino acid sequence homology, chitinases are divided into two unrelated families, families 18 and 19 of glycosyl hydrolases (15). Family 18 includes chitinases from bacteria, fungi, viruses, and animals and chitinases classified in class III or V from plants. On the other hand, family 19 includes almost exclusively plant chitinases classified in classes I, II, and IV but also a bacterial chitinase, Streptomyces griseus HUT 6037 chitinase C (32). To date, various chitinases were isolated from aerobic microorganisms such as Bacillus circulans (1,(55)(56)(57), Serratia marcescens (6,13,16,20), an Aeromonas sp. (41,(51)(52)(53), an Alteromonas sp. (48), Streptomyces plicatus (37), Streptomyces olivaceoviridis (3, 36, 38), and Janthiobacterium lividum (12). Several genes encoding chitinases were cloned in Escherichia coli and characterized in detail along with their translated products (9, 12, 20, 36-38, 41, 43, 49, 56, 57). From these studies, chitinases were found to comprise two or more discrete domains, while the function of each domain has not yet been elucidated. Watanabe et al. reported that B. ...

No abstract

The L-rhamnose isomerase gene (L-rhi) encoding for L-rhamnose isomerase (L-RhI) from Bacillus pallidus Y25, a facultative thermophilic bacterium, was cloned and overexpressed in Escherichia coli with a cooperation of the 6xHis sequence at a C-terminal of the protein. The open reading frame of L-rhi consisted of 1,236 nucleotides encoding 412 amino acid residues with a calculated molecular mass of 47,636 Da, showing a good agreement with the native enzyme. Mass-produced L-RhI was achieved in a large quantity (470 mg/l broth) as a soluble protein. The recombinant enzyme was purified to homogeneity by a single step purification using a Ni-NTA affinity column chromatography. The purified recombinant L-RhI exhibited maximum activity at 65 degrees C (pH 7.0) under assay conditions, while 90% of the initial enzyme activity could be retained after incubation at 60 degrees C for 60 min. The apparent affinity (K(m)) and catalytic efficiency (k(cat)/K(m)) for L-rhamnose (at 65 degrees C) were 4.89 mM and 8.36 x 10(5) M(-1) min(-1), respectively. The enzyme demonstrated relatively low levels of amino acid sequence similarity (42 and 12%), higher thermostability, and different substrate specificity to those of E. coli and Pseudomonas stutzeri, respectively. The enzyme has a good catalyzing activity at 50 degrees C, for D: -allose, L-mannose, D-ribulose, and L-talose from D-psicose, L-fructose, D-ribose and L-tagatose with a conversion yield of 35, 25, 16 and 10%, respectively, without a contamination of by-products. These findings indicated that the recombinant L-RhI from B. pallidus is appropriate for use as a new source of rare sugar producing enzyme on a mass scale production.