D-Galactomannan

The disaccharide epimelibiose [0-a-D-galactopyranosyl-(l— 6)-D-mannopyranose] was first isolated by Whistler and Durso202 from a partial hydrolyzate of the D-galactomannan of guar by acid (see also, Ref. 203), and they also elucidated its structure. This disaccharide constitutes the branch points of the main D-galactomannan chain (see Section V,3). 

Two articles12,423 have been published that discuss the structure, properties, and biochemistry of D-galactomannans. In addition, several other reviews on this group of polysaccharides are available.1,424-428 In this Section, an overall description of D-galactomannans will be given, with inclusion of recent advances made in this field. 

In aqueous solution, D-galactomannans most probably exist in an extended-rod conformation43 "438, as was well demonstrated during gel-filtration experiments.437 McCleary and coworkers437 suggested that, in an aqueous solution of the polymer, the a-D-galactosyl stubs, when separated by no, or an even number of, D-mannosyl residues, lie on opposite sides of the main chain, and that those separated by an odd number of D-mannosyl residues lie on the same side of the chain, as illustrated. 

The main function of D-galactomannans in seeds is as reserve polysaccharides that are depleted during germination. The a-D-galactosyl stubs of the polysaccharide provide hydrophilic character to the molecule, thus endowing it with a water-retaining capacity this facilitates germination of the seed. 

The details of the metabolism of plant polysaccharides that contain a-D-galactopyranosyl residues are limited to D-galactomannans.12 However, published data on the exact mechanism of biosynthesis are still not available. 

The action of /3-D-mannosidase finally degrades the D-manno-oligo-saccharides, to afford free D-mannose molecules details of this process were given in an earlier article.12 An enzyme identified as oligo-/J-D-mannosyl-(l- 4)-phosphorylase719 may also take part in the mobilization of D-galactomannan in seeds. This enzyme, in the presence of orthophosphate, acts on D-manno-oligosaccharides (obtained from partial hydrolysis of D-mannan), and yields /3-D-mannosyl phosphate. 

Yeasts of the genus Trichosporon can also be sub-classified into three groups according to their polysaccharide components. Whereas a-D-linked D-galactomannans occur as cell components of Trichosporon fermentans, Trichosporon hellenicum, and Trichosporon penicillatum, pentosylmannans are formed by Trichosporon cutaneum, Trichosporon inkin, Trichosporon pullulans, Trichosporon undulatum, and Trichosporon sericeum. Trichosporon aculateum, on... 

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