Protopectin insolubility

The conversion of protopectin, the water-insoluble parent pectic substance, into soluble pectin and pectate and, further, into their cleavage products, is one of the mechanisms playing a role in the plant during its maturation, as well as in the process of infection of the plant. In the plant, protopectin has the function of an intercellular adhesive and, hence, its conversion into the soluble form results in a disruption of tissue rigidity, and in cell separation that is reflected in softening and subsequent liquefaction of the plant material. 

There is no major advance which can be reported in our knowledge of protopectinases. The possibility of the existence of a protopectinase different from PG seems now somewhat more remote than it was ten years ago. As stated above, the trend in thinking about protopectin is definitely in the direction that protopectin is not a specific cellulose-pectinic acid compound but pectinic acid which is insoluble either on account of large molecular size or due to the effect of polyvalent cations or both. In this sense there is really no need to assume the existence of a separate protopectinase. There is little doubt that this uncertainty is in a large degree attributable to the lack of precise knowledge of protopectin. 

The prevailing concept is that, in the softening that accompanies ripening, textural changes occur as insoluble, wall-bound protopectin of high molecular weight, after partial de-esterification by pectin methyl... 

Mention has already been made of the numerous effects attendant upon chemical substitutions on the polysaccharide linear chain. Natural branches impart a dispersion stability to amylopectin that is not afforded amylose. One only has to compare cellulose ethers, deesterified chitin, and the lysis product of protopectin with the underivatized parent compound to appreciate the impact of chemical substituents on functionality. The loosening of compact, parallel structures with alkyl, hydroxyalkyl, and alkoxyl groups facilitates hydration and transforms insoluble, refractory polysaccharides to soluble, reactive polysaccharides. Not only do these substituents obstruct the crystallization tendency, they almost always confer secondary functionalities like q enhancement and foam, suspension, and freeze-thaw stabilization. 

The pectin of commerce is the acid or enzyme hydrolyzate of protopectin—the insoluble parent polymer residing in the cell walls of higher plants, primarily apples and citrus fruits. The pectin is isolated by entrainment in polymeric aluminum hydroxide produced in situ by neutralization of an acidic aluminum salt in solution. The pectin isolate is freed of Al(III) by washing and dialysis in an acidified ethanol-water solvent. Advanced demethylation of extracted pectin with alkali or enzymes yields a series of low DE pectins. 

Protopectin is composed of water-insoluble pectic substances, which are fixed to the middle lamella and primary cell walls of plant cells. The neutral sugar side chain of the pectin is attached to the xyloglucan residues, which are bound to the cellulose fibers. The protopectin includes polyvalent such as calcium (Fig. 11.4-8). Protopectin is present in unripe fruits. During the maturation process of fruits or after harvesting, the protopectin is converted to soluble pectin11851. The insolubility of protopectin may be due to the polymerization of the molecule and to the cross-linking with divalent cations11861. 

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. 

Pectin analysis also needs more refining and emphasis. Pectic substances are a group of related substances (protopectin, pectin, and pectic acids), each with specific chemical characteristics (9). Both the protopectin and the pectic acid, pectinic acid, form insoluble complexes with metals, especially calcium. These too, like the protein, are concentrated in the persistent middle lamella-primary wall complex. 

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. 

Protopectin(s) a ground substance in plant cell walls. P. consists of insoluble Pectins (see) and are probably not pure homoglycans. Hiey are present in the cell wall as salts of calcium and magnesium. Hie constituent polygalacturonic acid chains of P. are linked to one another by salt linkages, phosphate bonds and esterification with arabinose. 

For several years now, dietary fibre has received considerable attention because of possible benefits to human health. Fibre is the collective name for the insoluble indigestible cell wall constituents of plant materials it consists of cellulose (a glucose polymer), hemicellulose (a heterogeneous group with pentosans predomi-nant-hexosans, and poly uronides), lignin (an aromatic compound), protopectin and some nitrogen-containing materials (Van Soest 1963 Worth 1967 Norman 1973 Hellendoorn et al. 1975). 

Protopectin the term protopectin is applied to the water insoluble parent pectic substances, which occurs in plants and which upon restricted hydrolysis, yields pectinic acids... 

Pectins are found mostly in large flowering plants and are located in both the primary cell wall and the intercellular space called the middle lamella [68], This results in the cell-cell adhesion effect and provides consistency and mechanical resistance to plant tissue. In immature fruits, the most abundant pectic substance is protopectin, very insoluble with a high molecular weight. Pectin together with cellulose and hemicelluloses forms the backbone of the cell wall [50]. 

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