Enzyme glycosidases

Major enzymes catalyzing flavonoid reactions are oxidative enzymes (i.e., polyphenoloxi-dases and peroxidases) arising from grape but also from molds contaminating them. Various hydrolytic enzymes (glycosidases, esterases), excreted by the fermentation yeasts or fungi or present in preparations added for technological purposes (e.g., pectinases), are also encountered in wine. 

As well as complexing the substrate to the active site, many enzymes link covalently with the substrate, or a portion of it, to form an additional intermediate. Such intermediates occur in the action of enzymes as diverse as alkaline phosphatase (phosphoryl enzyme), serine and cysteine proteases (acyl enzymes), glycosidases (acylal enzymes) and aldolases. 

Digestion of lactose is facilitated by the p glycosidase lactase A deficiency of this enzyme makes it difficult to digest lactose and causes abdominal discomfort Lactose intolerance is a genetic trait it is treatable through over the counter formulations of lac tase and by limiting the amount of milk m the diet... 

One of the most important differences between cellulose and starch is that animals can digest starch Because the glycosidic linkages m starch are a an animal s a glycosidase enzymes can catalyze then hydrolysis to glucose When more glucose is... 

Fig. 2.—Possible Formation of an Enzyme-linked Acylal (—X = Carboxylate) from an Aldono-1,5-lactone at the Active Site of a / -Glycosidase (after Sinnot and Souchard ).

Inhibition of Glycosidases by AIdono-l,S-lactones and Aldohexoses Expressed by the Dissociation Constant K of the Enzyme-Inhibitor Complex... 

A case similar to the slow, practically irreversible inhibition of jack bean a-D-mannosidase by swainsonine is represented by the interaction of castanospermine with isomaltase and rat-intestinal sucrase. Whereas the association constants for the formation of the enzyme-inhibitor complex were similar to those of other slow-binding glycosidase inhibitors (6.5 10 and 0.3 10 M s for sucrase and isomaltase, respectively), the dissociation constant of the enzyme-inhibitor complex was extremely low (3.6 10 s for sucrase) or could not be measured at all (isomaltase), resulting in a virtually irreversible inhibition. Danzin and Ehrhard discussed the strong binding of castanospermine in terms of the similarity of the protonated inhibitor to a D-glucosyl oxocarbenium ion transition-state, but were unable to give an explanation for the extremely slow dissociation of the enzyme-inhibitor complex. 

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