Uronic acid, occurrence

As investigations with polysaccharides of microbiological, plant, and animal origin revealed the presence in these substances of uronic acids, it became necessary to know more about the acids. Moreover, the occurrence of uronic acids in plant gums and pectic substances, and the structures of urinary... 

The parent that includes the functional group most preferred by general principles of organic nomenclature [13,14], If there is a choice, it is made on the basis of the greatest number of occurrences of the most preferred functional group. Thus aldaric acid > uronic acid/ketoaldonic acid/aldonic acid > dialdose > ketoal-dose/aldose > diketose > ketose. 

The occurrence of uronic acid provides another means of easy access to bicyclic lactones, which can be used for the synthesis of various targets, such as surfactants or pseudo glycopeptides. 

The occurrence in many polyglycosiduronic acids of relatively resistant linkages, usually those between uronic acid residues and adjacent residues, has resulted in the isolation of several aldobiouronic acids on graded hydrolysis of acidic polysaccharides. The isolation of such aldobiouronic acids, and subsequent conversion to their fully methylated derivatives, has become a standard procedure in structural studies on polyglycosiduronic acids, especially those of plant gums and mucilages.1 In other cases, partially methylated aldobiouronic acids have been isolated from the hydrolyzates of methylated polysaccharides. The sources and methods of isolation of the methyl ethers of aldobiouronic acids so far examined are given in Table I. Some properties of derivatives are recorded in Table Y. 

Almost all the polysaccharide fractions (glycogen excepted) isolated from M. tuberculosis contained small amounts of nitrogen and phosphorus. Amino sugars have been detected in some polysaccharides, but in the main, the mode of occurrence of the less well-defined constituents remains undetermined. Similarly, some of the polysaccharide fractions have been stated to possess acid properties. Uronic acids have not, however, been identified in the hydrolysis products, nor have any other acidic components been conclusively identified. 

Polyuronides may be defined as polysaccharides that contain one or more uronic acid units in their molecular structures. They have a frequent and wide occurrence in nature. Much of the carbohydrate material in plants belongs to the group. It includes all pectic materials and plant gums and many plant mucilages, hemicelluloses and gel-forming substances and some microbial polysaccharides. These substances are to be looked for in water-soluble plant exudates and mucilages, as well as in water and alkaline extracts of most plant materials. This review will be limited to the polyuronides occurring in plants. 

Properties and Occurrence of Glycosaminoglycans Composed of Amino Sugars and Uronic Acids... 

D-Glucosiduronic acids are of rare occurrence in plants, although in those species where they do occur they sometimes accumulate in high concentration. The mechanism of their formation in plants is undoubtedly identical with that in animals. An enzyme preparation from leaves of the French bean has been shown to catalyze the formation of quercetin-3-yl /3-D-glucopyranosiduronic acid from uridine 5-(a-D-glucopyranosyl-uronic acid pyrophosphate) and quercetin. In contrast to the liver enzyme, the transferase from beans was found to be specific for the acceptor. 

Although uronic acids are produced by marine biota and the residues may be found in particulate matter and sediments, with the exception of a limited number of analyses (Mopper, 1977 Mopper and Larsson, 1978), data on the occurrence of uronic acids, in either the monomeric or polymeric dissolved fractions of seawater, are non-existent. The values reported for five identified uronic acids by the above authors are in the range between 10— 100 nmole 1" for individual acids in a single sample of hydrolysed North Sea surface water. 

Sometimes, however, the distribution of B in A is neither regular nor random. An example is the occurrence of rhamnosyl residues in the a 1,4 galacturonan chains of pectinic acids. In this instance rhamnose and galact-uronic acid are too unlike each other to make it at all probable that a single transferase can add them both and it is very hard to see how the rhamnosyl insertion could take place other than by the action of specific transferases, which are in some way externally triggered. 

C. Uronic Acids 64a, b, c) a. Preparation and Occurrence See also Chapter XII)... 

Aldobiouronic acids are readily isolated because of the strong resistance of the biouronic linkage to acid hydrolysis. Whereas 4 % acid at 100 to 120 is often used for the isolation of uronic acids, O Dwyer (7 ) isolated an aldobiouronic acid from oakwood hemicellulose by the action of 1% sulfuric acid at 100 . This resistance to hydrolysis may explain the occurrence of uronides in soil. Some 10 to 15 % of the organic carbon in surface soil appears to be combined uronic acids, and the amount increases with the depth of the soil (73) (see also p. 669, 717, 719). 

Modified aldoses, of wide natural occurrence, are the uronic acids. In these molecules the terminal CH2OH group of a hexose has been oxidised to carboxyl. Polysaccharides consisting almost entirely of uronic acid units are pectic acid (D-galacturonic acid) (LXXVIII), the main component of pectin found in plant tissues, and alginic acid (D-mannuronic (LXXIX) and L-guluronic acid (LXXX)) synthesised by the brown algae. For a full description of these acids see Chapter II, p. 73. 

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