Abstract
This paper from the 1994 award address of the Society is composed of following four research topics that were carried out at the Bioconversion Laboratory, National Food Research Institute, during 1986 and 1991. A summary of these is as follows . Cellulase system of Cellvibrio gilvus was studied at enzymatic and DNA levels . A unique xylanase which hydrolyzed a series of pNP-cellooligosaccharides into pNP and respective cellooligosaccharides was purified. Shotgun cloning of the cellulase system using 4-methylumberriferyl cellobioside led to the isolation of a fl-glucosidase gene . The G+C content of the gene was very high (69.4%) and four amino acids (Ala, Arg, Gly, Pro) comprised 37% of the total amino acids . However, the deduced amino acid sequence has high homology with those of other β-glucosidases . Extensive treatment of wheat bran hemicellulose by Cellulase Onozuka RS (a commercial crude enzyme preparation from Trichoderma viride) produced an oligosaccharide fraction with a yield of 30%. This fraction was selectively utilized by Bifidobacteria, and its administration to rats increased the percentage of Bifidobacteria in the intestinal microflora from 1% to 40%. Structure of the main component of this oligosaccharide fraction was determined to be β-D-Xylp- (1°C→4) [α-L -Araf- (1→2) ] [α-L-Araf- (1→3) ] -β-D-Xylp- (1→4) -β-D-Xylp- (1→4) -D-Xvlp. Cellobiose phosphorylase was purified from C. gilvus cells and its catalytic properties were studied in detail. Different from sucrose phosphorylase, the enzyme was found to follow the sequential mechanism. It selectively utilized f3-glucose as an acceptor to form cellobiose . Inhibition observed with glucose at higher concentrations was extensively analyzed and a model for competitive substrate inhibition was proposed. The enzyme can utilize several glucose derivatives at its C-2 or C-6 positions . By the combined action of sucrose phosphorylase, xylose isomerase and cellobiose phosphorylase, sucrose was effectively (75% yield) converted into cellobiose under the presence of small amount of inorganic phosphate. Sucrose phosphorylase was purified from Leuconostoc mesenteroides cells and its transfer reactions were studied. The enzyme showed the highest specificity to xylulose as an acceptor to form α-D-glucopyranosyl- (1→2) β-D-xylulofuranoside (Glc-Xul). More than 90% of xylulose was converted into Glc-Xul when 300 mivi sucrose and 100 mii xylulose were incubated with the enzyme. Glc Xul is 70% as sweet as sucrose and has physicochemical properties close to those of sucrose. However, it was neither utilized by dextransucrase to form dextran nor by Streptococcus sobrinus cells to produce acid. This sugar is expected to be a sucrose substitute that is resistant to dental plaque formation.
