Pectic acid, enzymic hydrolysis

This enzyme [EC 3.2.1.82] catalyzes the hydrolysis of pectic acid from the nonreducing end, thereby releasing digalacturonate. 

The quality of extracted citrus juices depends on enzyme reactions that occur not only in the fruit during the development period, but also in the juice during processing. When juice is extracted from citrus fruit, enzymes are released from their normal restraint in the cell. Several of these enzymes catalyze reactions that adversely affect taste and appearance of the juice. Unless the reactions are controlled, the juice products will not meet the standards of quality set up by the USDA Food Safety and Quality Service. The two reactions of commercial importance are the hydrolysis of pectin to pectic acid, which clarifies juice, and the lactonization of limonoic acid A-ring lactone to the bitter compound, limonin. Research efforts to identify and characterize the reactions, to isolate and purify the enzymes, and to develop methods to control the reactions are described in this review. 

One group of enzymes which will tolerate solvents is the lipases. These catalyse the hydrolysis of fatty acid esters. Their substrates range from simple acetate esters of ethanol and glycerol to the esters of pectic acids and of steroid alcohols. Many of these esters are insoluble in water, but are soluble in organic solvents, and some of the lipases reflect this by showing their greatest activity in solvents containing only very small amounts (1-4% v/v) of water. 

Pectins represent complex molecules providing many opportunities for different types of enzyme action. Our understanding of the enz unic hydrolysis of protopectin is so unsatisfactory that a discussion of protopectinase seems unwarranted. The pol3uiionic skeleton of pectinic acids and pectic acids can be d raded through glycosidic hydrolysis by the various poly-Re/erences p. 244... 

This enzyme is also known as pectinase, and it hydrolyzes the glycosidic linkages in pectic substances according to the reaction pattern shown in Figure 10-7. The polygalacturonases can be divided into endoen-zymes that act within the molecule on a-1,4 linkages and exoenzymes that catalyze the stepwise hydrolysis of galacturonic acid... 

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