Biology Reference
In-Depth Information
Cellobiose phosphorylase catalyzes the phosphorolysis of
cellobiose with inorganic phosphate by an inversion mechanism
to produce Glc-1-P and glucose [14]. This enzyme recognizes the
β
-anomeric hydroxy group at the reducing end of the cellobiose
moiety (Fig. 3.5). Therefore, cellobiose phosphorylase does not
phosphorolyze cellooligosaccharide larger than cellobiose. The
specificity of acceptor structure in the reverse reaction has also
been investigated. Consequently, it was found that this enzyme
has a strict specificity for the recognition of the hydroxy groups at
positions
-1, 3, and 4 of the glucose but not of the hydroxy groups
at positions 2 and 6 [15]. Cellodextrin phosphorylase is an enzyme
that catalyzes the reversible phosphorolysis of cellooligosaccharides
larger than cellobiose to produce Glc-1-P [16]. The regiospecificity
of this enzyme is identical to that of cellobiose phosphorylase,
whereas specificity with regard to the DPs of the acceptor is quite
different from each other. Therefore, cellooligosaccharides have
been synthesized by the cellodextrin phosphorylase-catalyzed
chain elongation using various cellobiose acceptors and Glc-1-P
donor (Fig. 3.6) [17]. When cellobiose was used as a glycosyl
acceptor, various cellooligosaccharides ranging from water-soluble
products to crystalline precipitates were obtained, depending on the
concentration of the acceptor. The NMR analysis of the crystalline
precipitate indicated an average DP of ca. 8. The precipitates
showed the diffraction diagrams of low-molecular-weight cellulose
II. The cellodextrin phosphorylase-catalyzed synthesis of the
cellooligosaccharides substituted at their reducing end was also
achieved using various cellobiose derivatives and analogues as a
glycosyl acceptor. When the cellodextrin phosphorylase-catalyzed
reaction was performed using Glc-1-P as a glycosyl donor and
glucose as a glycosyl acceptor, cellooligosaccharides with an average
DP of 9 were produced and the products formed highly crystalline
cellulose II [18]. Although glucose had been believed to not act as
the glycosyl acceptor for the cellodextrin phosphorylase catalysis, a
significant amount of insoluble cellulose was precipitated without
the accumulation of soluble cellooligosaccharides in this enzymatic
reaction system using the glucose acceptor. This result was explained
in terms of the large difference in the acceptor reactivity between
glucose and cellooligosaccharides as the general acceptor of the
cellodextrin phosphorylase catalysis.
β
