Pectic acid function

Activation-Inhibition and Function In Vivo. When 0.15M NaCl was added to an orange PE-pectin reaction mixture at pH 7.5, activity was increased 5-fold, and at pH 5 it was increased 100-fold (17). As explained by Lineweaver and Ballou (19), NaCl caused the apparent activation by freeing the enzyme from the inactive ionic complex (pectin-carboxyl). They showed that at pH 5.7 pectic acid inhibited alfalfa PE activity 55% in 0.015M NaCl but only 17% in 0.2M NaCl. At pH 8.5 pectic acid inhibited PE activity only 9% in 0.015 NaCl. They concluded that the stimulation of activity by cations at low pH (17) did not show that cations were essential for activity, but, rather, that cations function by preventing product inhibition, which is greater at low pH. 

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. 

The biochemical role of minerals in metabolic processes of various organisms has been covered in several reviews by Malstrom and Neilands (20A60), McElroy and Nason (20A65), Nason and McElroy (20A81), and Nicholas (20A84). The function of C, H, and O are obvious since they are constituents of fats, carbohydrates, and proteins. N and S are constituents of amino acids, proteins, coenzymes, and other compounds. Ca forms a complex with pectic acid and functions as a constituent of the middle lamella of cell walls. Dixon and Webb (20A13) showed that Ca also plays a role as a cofactor for certain adenosine triphosphate hydrolyzing enzymes, for phospholipases, and as a cofactor for the amylases from a variety of plants. Many elements, especially the cations, play essential roles as cofactors for the enzymes of various metabolic sequences. 

Physical Methods. Infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopic methods have been applied to structural analysis of polysaccharides such as pectin. These applications have been reviewed ( ), and reference IR spectra of pectic substances have been published ( ). Quantitative IR has been used to estimate acid dissociation constants of polyuronides from the ratio of -CO2H to -CO2- as a function of pH ( ). Also, by... 

A better understanding of this subject is possible when the location and function of the pectic substances in plants are reviewed. The movement of water and plant fluids to the rapidly growing fruits and the retention of form and firmness of fruits are functions of pectin. This intercellular substance in plants is similar in action to the intercellular substance of the vertebrates—collagen (the precursor of gelatin). Protopectin, the water-insoluble precursor of pectin, is abundant in immature fruit tissues. Ripening processes involve hydrolytic changes of protopectin to form pectin and later, as maturity is passed, enzymic demethylation and depolymerization of pectin to form pectates and eventually soluble sugars and acids. 

After 70 years of effort, a biochemical function for boron in plants was identified. In primary cell walls, boric acid cross-links two chains of pectic polysaccharide at the rhamnogalacturonan-II region through borate-diester bonding between two side-chain apiosyl residues, thus forming a network of pectic polysaccharides (O Neill et al. 1996, Matoh 1997). Boron is suspected to have additional essential functions... 

A number of reports are in record on the chemistry, characterization, functional physiological properties, and applications of the pectic polysaccharides, a natural polymer. The foregoing discussion divulge that the long chain polymer of a-D-galacturonic acids, which find their existence outside the protoplasm of a plant cell have a number... 

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