Pectic Acids and Pectins

Pectic Acids and Pectins.—Pectin has been converted into a j-triazinyl derivative having C-6 of the hexuronic acid residues incorporated into the heterocyclic ring. The actions of polygalacturonase and acid on the derivative were investigated. 

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Pectin is a long chain of pectic acid and pectinic acid molecules. Because these acids are sugars, pectin is categorized as a polysaccharide. It is prepared from citrus peels and the remains of apples after they are squeezed for juice. In the plant, pectin is the material that joins the plant cells together. When fungus enzymes break down the pectin in fruit, the fruit gets soft and mushy. 

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

Pectins are probably the most complex polysaccharides known, in terms of their chemistry and are certainly so in terms of their biosynthesis. Classically they were regarded as al,4-galacturonans, with various degrees of methyl esterification, and the terms pectic acid and pectinic acid referred to the non-esterified and partially esterified forms respectively. A third term protopectin , was used of insoluble pectin that could not be extracted from plant cell walls by hot solutions of chelating agents. It was considered that these three classes of pectin constituted a pectic triad . This view is now known to be erroneous, but it is still frequently put forward, especially in botanical texts. Consequently any discussion of the synthesis of pectins must be prefaced by a description of their chemistry, as it is now understood. 

Assays for PE. Because PE hydrolyzes pectin to pectic acid and methanol, the enzyme concentration can be assayed by measuring the rate at which free carboxyl groups or methanol is released from the substrate. Kertesz (18) titrated the free carboxyl groups as they were formed by the action of the enzyme on pectin. He used methyl red to indicate the pH (6.2) and added 0.1N NaOH at frequent intervals to maintain the pH relatively constant for 30 min. The pH meter replaced the use of indicators in subsequent modifications (17, 19). Current procedures use automatic pH titrators to titrate alkali at constant pH (25). A blank is used to correct for the consumption of alkali due to its reaction with atmospheric CO2, or the reaction solution is protected from C0 with a blanket of N2. 

Versteeg (8) speculated on the function of PE in vivo. He noted the high activity of PE in citrus fruit compared to the amount of available pectin. The fruit contain sufficient activity to deesterify the pectin to low methoxy pectin in 10 min at optimum pH. He suggested that the methyl transferase found by Kauss and Hassid (39) to esterify pectic acid to pectin in mung bean shoots and to be located in a lipid-membrane complex (31) functioned as pectinesterase after the lipid membranes were destroyed and the environment changed. However, no definitive experiments to establish the role of PE in fruits were reported. 

Pectin and pectic materials that contain relatively small amounts of other materials can be readily converted to insoluble calcium pectate. When these materials have been isolated from a hardwood or similar material, they consist of pectic acid mixed with hemicelluloses, lignin and coloring matter. To remove these impurities, the material is treated with liquid bromine. The pectic acid and hemicelluloses are precipitated by alcohol and washed with this solvent until free of brominated lignin. The pectic acid is then separated from the hemicellulose by conversion to insoluble calcium pectate. 

Glycuronans (Pectin, Pectic Acid, and Alginic Acid). 245... 

Oxalate-solrrble pectic substances (OXP)—include pectic acids and the colloidal pectinic adds of low methyl ester content... 

Leroy and Michaux found that sheep digested a high proportion of the pectic substances included in such foodstuffs as apple pulp, sugar beet pulp, hay, and straw. It was found that a sheep feeding on hay consumed from 75 to 102 g. daily of pectic substances (Michaux). The proportion of pectic acid to pectin is high in apple and sugar beet pulp, the ratio being... 

The second group can be further subdivided according to the substrate (pectin or pectic acid) and to the site of attack (endo-/exo-enzyme), as shown in Table 4.28. The endo-enzymes strongly depolymerize and rapidly reduce the viscosity of a pectin solution. 

Schneider and Ziervogel (Ref 2) obtained nitric esters of pectin as intermediate products which were then esterified by means of acet anhydr to the corresponding acetates. In 1949, Rogovin, Treyvas and Shorygina (Ref 5) prepd pectin nitric esters by nitrating pectic acid for 4 hrs at 20° in a mixt composed of 48% HN03,... 

The primary walls of growing plant cells are composed of 90% carbohydrate and 10% protein (51). Carbohydrate in the primary wall is present predominantly as cellulose, hemicellulose, and pectin. The pectic polysaccharides, are defined as a group of cell wall polymers containing a-l,4-linked D-galactosyluronic acid residues (62,76). Pectic polysaccharides are a major component of the primary cell waU of dicots (22-35%), arc abundant in gymnosperms and non-graminaceous monocots, and are present in reduced amounts (-10%) in the primary walls of the graminaceae (27,62). 

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