Front Matter

(Rich et al., 1995; Woudenberg et al., 1996; Kim et al., 1998; Ferrer et al., 1999). On

the other hand, the acylation position in comparable monosaccharides is the same for

both hydrolases, the 6-OH group. Recently, Kitagawa et al. (1999) examined the

protease-catalyzed transesterifications between hexoses and divinyladipate. In di-

methylformamide hexoses such as D-glucose, D-mannose, D-galactose and a-

methyl-D-galactoside were acylated with divinyladipate by alkaline protease

fromStreptomycessp. to give corresponding 6–O-vinyl adipoyl sugars. Surpris-

ingly, when the denaturing co-solvent DMSO was added to the solvent, galactose

was selectively acylated at only the C 2 position.

17.3 Glycosidase-catalyzed glycosylation of fatty

alcohols with sugars

Alkyl-glycosides have the advantage over sugar fatty acid esters as they are more

stable in alkaline conditions. However, in contrast to the above-mentioned lipase-

catalyzed processes, their production by suitable enzymes is more problematic at

present.

In general, it has been shown that alkyl-glycosides can be synthesized from al-

cohols and carbohydrates using glycosidases as catalysts. Under physiological con-

ditions, glycosidases hydrolyze glycosidic bonds. The synthesis of alkyl-glycosides

can be carried out either by reverse hydrolysis or by transglycosylation reaction

involving alcohol as a glycosyl acceptor. The difference between the two means

of synthesis depends on the nature of the glycosyl donor. In reverse hydrolysis,

the reaction is performed with a monosaccharide (e.g., glucose), while in transgly-

cosylation a glycoside (e.g., disaccharide) is used as the glycosyl donor (Figure 5).

The first is an equilibrium-controlled, and the latter a kinetically controlled ap-

proach. Generally, the transglycosylation is much faster and leads to higher yields

than that of the reverse hydrolysis. Both reactions can be conducted either in mono-

phasic or biphasic systems consisting of: (i) more or less water; and (ii) the liquid

alcohol to be glycosylated (in most cases no additional organic solvent is used).

In general, both initial velocity and yield decrease when the carbon chain length of

alcohol increases. Only in rare studies were alcohols longer than C 8 suitable lipid

reactands. Overviews relating to glycosidase-catalyzed glycolipid syntheses are

shown in Tables 6 and 7. The dominant enzyme studied by most authors was the

almondb-glucosidase (EC 3.2.1.21). Often, two-phase systems based on aqueous

buffer and the alcohol were used. Several studies have been conducted to investigate

the effect of operating conditions, mainly water content, on the synthesis reaction.

When studying this in detail, Ismail et al. (1999a,b) recently found that in the case of

a monophasic system (butanol phase), addition of water led to a significant increase

in product concentration. The highest concentration was obtained at 17 % (v/v) of

water in a transglycosylation system using lactose,n-butanol and theb-galactosidase

(EC 3.2.1.23) fromAspergillus oryzaeforn-butylgalactoside synthesis. In the bipha-

sic system, the reaction performance was lower than that in the monophasic system,

probably due to the shift of equilibrium towards the hydrolysis and the synthesis of

376 17 Enzymatic Synthesis and Modification of Glycolipids