More than 20 types of carbohydrate epimerase have been reported to date. Distinctly different enzymes recognize the C1, C2, C3, C4, C5 or C6 position of carbohydrate substrates. They include, for instance, aldose 1-epimerase, N-acetyl-D-glucosamine 2-epimerase, UDP-N-acetylglucosamine 2-epimerase, cellobiose 2-epimerase, D-ribulose-5-phosphate 3-epimerase, L-ribulose-5-phosphate 4-epimerase, UDP-galactose 4-epimerase, dTDP-6-deoxy-D-xylo-4-hexulose 3,5-epimerase, GDP-mannose 3,5-epimerase and ADP-L-glycero-D-mannoheptose 6-epimerase. Their biological properties, catalytic mechanisms and tertiary structures are very diverse. This review focuses on the catalysis, structures and applications of carbohydrate epimerases that have recently been characterized in detail.
The non-Newtonian behavior and storage modulus of an alginate isolated from Nemacystus decipiens which had been harvested in Uruma City (Okinawa, Japan) in aqueous solutions were investigated. Gelation occurred at a concentration of 0.5% of the alginate on addition of a mixed solution of NaCl (0.1%), KCl (0.1%) and CaCl2 (0.1%) at room temperature (25°C). The flow curves, at 35°C, of the alginate showed plastic behavior at various concentrations. The storage modulus of the alginate increased with an increase in concentration and decreased gradually with an increase in temperature up to 50°C, which was estimated to be a transition temperature; then it, on the contrary, increased gradually with a further increase in the temperature and showed the maximum value at a temperature of 60, 70 and 65°C at 0.3, 0.5 and 0.7%, respectively. The storage modulus of the alginate showed a large value even in the presence of urea (4.0 M). The storage modulus of the alginate showed a very large value at the pH range of 6.0-7.2, but very small at pH 4.0. However, a comparable large storage modulus was observed in the alkaline range (pH 9.0-11.0). Intra- and intermolecular Ca2+-bridges may play a dominant role in the gel formation of alginate molecules in the presence of Ca2+ ions with ionic bonding and electrostatic forces of attraction. The hydrophobic interaction may take place on the L-guluronic acid-rich homopolymeric blocks of the alginate molecules in the high temperature range.
1. Structural and physicochemical characteristics of endosperm starches of rice cultivars bred in Japan. Properties of rice starches were investigated using 4 types of endosperm starches, waxy, low amylose (AM), medium AM and high AM starches of rice cultivars bred in Japan. There was a highly negative relationship between the real AM contents and the peak viscosity measured by Rapid Visco Analyser (RVA) for non-waxy starches. In the group of high AM starches, there were two types of rice starch samples with similar apparent AM contents (about 30%), different contents of super-long chains (SLC) in amylopectin (AP), and different values in peak viscosity and setback of starches measured by RVA. Since there was a highly positive relationship between SLC contents and setback, the SLC in AP seems to have a great influence on the setback of starch. Both the side-chain length distribution within the short-chain range of AP and the SLC content of AP greatly influence the gelatinization temperature of rice starches. The SLC in AP contributed to neither swelling nor dissolution of rice starches in water at 75°C. 2. Structure and properties of endosperm starches from cultivated rice of Asia and other countries. Starch granules were prepared from mature grains of 75 cultivars of rice originating in Asia and other countries. Non-waxy cultivars of the indica and Chinese indica varieties had higher contents of apparent AM and SLC in AP. HPAEC-PAD of isoamylase-debranched starches showed that the starches of non-waxy cultivars of indica and Chinese indica had decreased amounts of branch chains with DP 6-12 (Fr.A). The Fr.A contents correlated negatively with the peak temperature of gelatinization of the rice starches. There was a high positive relationship between amounts of Waxy (Wx) protein and SLC contents in starch. This appears to show that Wx protein is concerned with synthesis of SLC. We proposed a scheme of the relationship between the SLC and the Fr.A contents of AP as a classification of endosperm starches of rice. 3. Effects of variations in starch synthase on starch properties. We evaluated the effects of functional variation in three starch synthases in rice granule-bound starch synthase I (GBSSI, wx), starch synthase I (SSI, SSI), and starch synthase IIa (SSIIa, alk) between indica cultivar Kasalath and japonica cultivar Nipponbare on starch properties. We used three near-isogenic lines-NIL (Wxa), NIL (SSIK), and NIL (Alk)-containing chromosomal segments of Kasalath on a Nipponbare genetic background. The Wxa allele explained most of the difference in AM content between the two cultivars, and decreased the peak viscosity and breakdown to less than half of those of Nipponbare. NIL (Alk) showed very similar profiles to those of Kasalath in having fewer short chains of DP 6-11 and more mid-length chains of DP 13-23 for the chain length distributions of amylopectins among the lines. Differential scanning calorimetry revealed faster retrogradation of the once-gelatinized starch in NIL (Alk).
The aim of this study is to investigate the effect of temperature during grain filling on the molecular structure of rice starch from Hokkaido cultivars, and to develop new methods for eating quality evaluation of rice. The molecular structures and some properties of rice starches from Hokkaido cultivars, Hoshinoyume, Kirara 397 and Aya, were examined and compared with Akitakomachi. Iodine affinity of amylopectin from Hoshinoyume and Kirara 397 was 2.5 times higher than for Aya and Akitakomachi. The amount of amylopectin super-long chain from Hoshinoyume and Kirara 397 was 2.3-3.5 times higher than for Aya and Akitakomachi. The influence of grain-filling temperature on the super-long chain (LC) amount and unit-chain distribution of amylopectin was examined by the fluorescent labeling/HPSEC method. The increase of LC amount per 1°C was calculated to be 0.542%/°C, 0.152%/°C, and 0.037%/°C in the temperature range (average temperature of day and night) of 19-23°C, 23-27°C and 27-31°C, respectively, indicating that the low temperature increased the LC amount. An automatic analyzer was developed to obtain the iodine absorption spectrum ranging from 400 to 900 nm. The difference between the minimum and final viscosities in RVA (Y) is shown by an equation, Y=(29.7X-27.1)/(0.73X-0.72), where X is the ratio of absorbance at 400-600 nm (Fr.I) to that at 600-900 nm (Fr.II). Thus, the λmax and the Fr.I/Fr.II ratio can be used as efficient eating-quality selection indexes in rice breeding. The unit-chain distribution of amylopectin from japonica waxy rice cultivars with different hardness of rice cake was analyzed by the fluorescent labeling/HPSEC method; the relation of the unit-chain distribution to the hardness was then examined. Waxy rice producing soft cake, such as Hakuchoumochi, has shorter unit-chains (A+B1), and fewer long unit-chains (B2+B3), than that producing hard cake, such as Koganemochi. A negative correlation was found between the molar ratio of (A+B1)/(B2+B3) and the hardening of rice cake. Waxy rice having a molar ratio of 11 or below appeared to be suitable for processing use, and 11.5 or higher for principal food use. Thus, the molar ratio (A+B1)/(B2+B3) of amylopectin unit-chains can be one of the useful evaluation/selection indexes for the breeding of waxy rice.
This paper describes the studies on structure-function relationships of hemicellulases. Plant polysaccharides are the most abundant biomass in nature. However, only limited kinds of polysaccharides are used today. To use biomass more effectively, studies on structure-function relationships of hemicellulases are important because it will be possible to expand their use if desirable modification of these polysaccharides can be performed. The side chain in the substrate recognition mechanism of the glycoside hydrolase family 10 xylanase from Streptomyces olivaceoviridis, the determination of substrate specificity by the structure, and importance of the interactions of the N- and C-terminal of the enzyme for enzyme stability were elucidated on basis of the crystal structure. In a polysaccharide thickener degrading enzymes study, we succeeded in the cloning of exo-β-1,3-galactanase and endo-1,6-β-galactanase genes for the first time. The existence of β-L-arabinopyranosidase was elucidated and the comparison of the structure of this protein with that of α-D-galactosidase showed a single amino acid substitution (aspartic acid to glutamic acid) in the catalytic pocket of β-L-arabinopyranosidase, and a space for the hydroxymethyl group on the C-5 carbon of D-galactose bound to α-galactosidase was changed in β-L-arabinopyranosidase. Mutagenesis study revealed that the residue is critical for modulating the enzyme activity.