Galacturonans pectic acid

The specificity of pectinesterase is not so marked with respect to the alcohol moiety of the ester, as it hydrolyzes ethyl esters of D-galacturonans at a rate 6-16% that of de-esterification of the methyl esters.36 Citrus natsudaidai pectinesterase de-esterifies the ethyl esters of pectic acid at a rate l/5th to l/7th of that for the methyl esters the propyl and allyl esters are attacked at l/20th to l/80th of the rate.38 The tomato, citrus, alfalfa, and papaya pectinesterases do not hydrolyze the glycol and glycerol esters of pectin.39... 

Another method is based on the same principle,112 in which the [14C]labelled methyl ester of D-galacturonan is prepared by esterification of pectic acid with [,4C]diazomethane. In the course of the enzymic de-esterification, aliquots are removed, and the unreacted substrate is precipitated with acidified ethanol or 1-propanol. After centrifugation, the labelled methanol in the supernatant liquor is determined in a liquid scintillation counter. An advantage of this method lies in the possibility of using, as substrates, short-chain oligo-D-galactosiduronates partially esterified with [14C]methanol. These substrates, beginning with the trisaccharide, are not soluble in 1 4 80% phenol-diethyl ether, which is used for the extraction of enzymically released, labelled methanol. 

It was furthermore reported that the esterification of pectic acid by use of diazomethane is concomitant with the /3-eliminative degradation of the D-galacturonan chain. Oligosaccharides were obtained possessing the terminally linked, unsaturated deoxyhexopyranuronate residue, similar to the analogous degradation of heparin quaternary ammonium salts already described. 

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. 

The properties of pectic substances are altered by partial or complete demethylation, which increases the number of free carboxyl groups. Pectic acid is the completely demethylated galacturonan. It is a linear chain of 1— 4 linked a-D-galactopyranosyluronic acid units. 

ZiTKO, V., and C. T. Bishop Structure of a Galacturonan from Sunflower Pectic Acid. Canadian J. Chem. 44, 1275 (1966). 

Pectic polysaccharide, mol. wt. ca. 40000-45000. Contains galacturonan, apiogalacturonan and heteroglycanoga-lacturonan, interlinked by residues of galacturonic acid and rhamnose. Isol. from Zosteraceae spp. Shows antibacterial activity. [a]o +230 (c, 0.5 in H2O). 

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