Hydrolysis of complex carbohydrates

FIGURE 22.1 The structure amylopectin, the major carbohydrate found in starch. 

Glycosidase-catalyzed hydrolysis of a glycoside bond in a polysaccharide can occur with either inversion or retention of stereochemistry at the anomeric center. Both mechanisms probably proceed through a short-lived oxonium ion (RaO ) intermediate in which one face of the carbohydrate is effectively shielded by the leaving group, leaving the other face open to nucleophilic 

FIGURE 22.2 General mechanism of polysaccharide hydrolysis by glycosidases. 

The ten-step glycolysis pathway for catabolizing glucose to two molecules of pyruvate. The individual steps are described in the text. Co to this book s student companion site at www.cengage com/chemistry/ mcmurry to explore an interactive version of this figure. 

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Roasted and ground coffee is packed into columns and percolated by the passage of hot water under pressure at about 340°F. The use of pressurized boiling water not only brings about the dissolution of the most readily water-soluble coffee extractables, it also causes the solubilization and possibly the hydrolysis of complex carbohydrates. Water-soluble compounds of a smaller molecular weight are produced. The extent of possible... 

Hydrolysis of starch as in tenderization of peas Hydrolysis of complex carbohydrate cell walls Hydrolysis of cellulose during drying of beans Removal of graininess of pears peeling of apricots, tomatoes... 

GUM is a general term for thick, mucilagenous excretions from various plants. It consists of complex carbohydrates which yield sugars on hydrolysis. The gums either dissolve... 

Assay methods are available for most of the carbohydrates ranging from glycogen, starch, and cellulose to the monosaccharides and their monophosphates. Concentration determination of complex carbohydrates usually depends on their enzymatic hydrolysis to monosaccharides and determination of monosaccharide concentration via an enzymatic reaction. For example, starch is hydrolyzed to glucose by glucoamylase, and the... 

The indium-mediated Barbier reaction has certainly become one of the most popular reactions for creating a carbon-carbon bond under aqueous conditions and has led to spectacular developments in recent years. Compared to other metals, indium is resistant to oxidation, hydrolysis, and has a very low first ionization potential (5.79 eV, in contrast to the second one which is quite normal) which confers on it a remarkable reactivity in Barbier-type reactions. In 1991, Li and co-workers reported the first allylation of aldehydes and ketones mediated by indium in water without any additives or special activation [133]. In particular, the use of indium allowed reactions with acid-sensitive compounds [134] or the preparation of complex carbohydrates such as deaminated... 

Wood contains from 8 to 10 per cent of a gum which is a mixture of complex carbohydrates. Wood gum yields xylose on hydrolysis and, like other substances which contain pentosans, is. converted into furfural on boiling with hydrochloric acid. The formation of furfural, it will be recalled, is a test for a pentose (365). 

A feasible way of introducing acid-stable linkages into carbohydrates is N-deacetylation. This can be achieved with hydrazine.59,70,71 The use of sodium hydroxide-sodium benzenethioxide in aqueous dimethyl sulfoxide for this purpose has also been described72 The difference in the acid hydrolysis of N-acetylhexosamine-con-taining carbohydrates before and after N-deacetylation was used in the study of complex glycoprotein saccharides from human erythrocyte membranes.73-75 Methylation analysis of the glycopeptides prepared... 

The reaction mechanism of a-amylases is referred to as retaining, which means that the stereochemistry at the cleaved bond of the carbohydrate is retained. Hydrolysis of the glycosidic bond is mediated by an acid hydrolysis mechanism, which is in turn mediated by Aspl97 and Glu233 in pig pancreatic amylase. These interactions have been identified from X-ray crystallography. The aspartate residue has been shown to form a covalent bond with the Cl position of the substrate in X-ray structure of a complex formed by a structurally related glucosyltransferase. " The glutamate residue is located in vicinity to the chloride ion and acts as the acidic catalyst in the reaction. The catalytic site of a-amylases is located in a V-shaped depression on the surface of the enzyme. 

The treatment of free sugars with alcohols in the presence of acid catalysts is the procedure most commonly used to prepare simple alkyl glycosides of the type frequently used in synthetic work. Although this is one of the oldest and most widely used reactions of carbohydrates it has not, until recently, been investigated in any detail on account of the variety of competing processes involved and the complexities of the products. Alternatively, the reverse reaction, the acid catalysed hydrolysis of glycosides, which is appreciably easier to study because of the single products obtained, has been the subject of many detailed studies and is now well documented and understood. 

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