Maltose anomeric configuration

The mechanism of the action of beta-amylase involves interaction of the starch with imidazole and carboxyl groups of the enzyme, resulting in a glycosyl ester intermediate (fi form of the D-glucose residue) that is hydrolyzed by addition of water to the carboxyl group, thus releasing the maltose in the /3-anomeric configuration. 

The hydrolyses of phenyl a-maltoside and diverse substituted-phenyl a-malto-sides by B. subtilis a-amylase have been examined under conditions that produce only maltose and the corresponding phenols, so that the anomeric configuration of the released disaccharide could be determined. At the pH optimum (5.4) of the enzyme, maltose was released as the P-anomer cf. a previous report although some of the a-anomer was released at high pH values. The action of B. subtilis a-amylase on a- and p-maltose has been studied plots of the initial rate against the substrate concentration gave sigmoidal curves in both cases. ... 

In the nomenclature of oligosaccharides, trivial names are still used for a number of substances (e.g. maltose, cellobiose or trehalose. Figure 4.5). In the case of reducing disaccharides, systematic nomenclature takes as its basis the name of the monosaccharide with a free hemiacetal hydroxyl group, which is preceded by the name of the substituted monosaccharide and the respective anomeric configuration is indicated (a- or P-). Disaccharide maltose (Figure 4.5), formed by condensation of... 

Scheme 18. Coupling reaction via locked anomeric configuration for maltose and glucose moieties.

Pure m. (1,4-a-D-glucosido-glucose) exists in two anomeric configurations, which form, when dissolved in water, an equilibrium of a-m. p-m. 42 58. It crystallizes slowly from water, forming P-m.-monohydrate anhydrous maltose consists of both anomeric forms. 

Both maltose and cellobiose have a free anomeric hydroxyl group that is not involved in a glycoside bond The configuration at the free anomeric center is variable and may be either a or (3 Indeed two stereoisomeric forms of maltose have been iso lated one has its anomeric hydroxyl group m an equatorial orientation the other has an axial anomeric hydroxyl... 

The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the a or the /I configuration at this center. The axial and equatorial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. 

The (3-amylases catalyze starch hydrolysis by a mechanism that gives inversion of configuration at the anomeric center. All known (3-amylases have an exo-mechanism and act on the non-reducing ends of starch polymer chains or starch polymer-derived chains. There are two general classes of (3-amylases, those that are classically known as (3-amylases and produce (3-maltose, and those that are known as glucoamylases and produce (3-D-glucose. 

In the structure for maltose shown in Example 2.23, no configuration is given for the anomeric carbon of the glucose unit on the right the structure represents the state of maltose in solution—a mixture of a and (3 anomers. In crystalline maltose, the anomeric hydroxyl is a, and maltose can be described as a-D-glucosyl-(l — 4)-a-D-glucose. 

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