Investigation and Structural Studies of the Carbohydrates metabolized by the Green Algae Urospora and Mougeotia

Abstract

Part I: Investigation of the Carbohydrates synthesized by the marine green alga Urospora penicilliformis. The alga was sequentially extracted, and the following carbohydrates were isolated and characterised.

1) Low molecular weight carbohydrates: glyceric acid, myo-inositol,glucose, fructose, sucrose and a homologous series of malto-oligo-saccharides.

2) The aqueous extracts yielded a mixture of polysaccharides: a) Amylose was fractionated from the mixture as the starch-iodine complex, and characterised. b) An alpha-1,3-linked mannan was fractionated from the mixture by DE.52 cellulose, and characterised by classical techniques. c) The major polysaccharide, a sulphated glucuronoxylorhamnan, contains rhamnose, xylose and glucuronic acid in the approximatemolar proportions 5:7:2, together with 17--22% of sulphate. Each constituent sugar was separated and characterised.

The polysaccharide was examined by desulphation, methylation, periodate oxidation and autohydrolysis techniques. The major structural features are 1,3-linked rhamnose usually sulphated at C-2 and sometimes also at C-4; 1,4--linked xylose together with some units linked and, end group additionally through C-2 and end group; and 1,4-linked glucuronic acid. In addition small amounts of 1,4-linked rhamnose, and group xylose and 1,3-linked glucuronic acid and xylose, with lesser amounts of triply linked rhamnose, also occur.

3) Alkali extraction yielded a beta-1,4-linked glucan, thought to be a degraded cellulose. The residue also contains a cellulose-type glucan.

Part II: Mougeotia. This alga was also sequentially extracted, and the following carbohydrates isolated a) Low molecular weight carbohydrates showed sucrose, glucose, fructose, myo-inositol, galactose and a homologous series of malto-oligosaccharides.

b) The aqueous extract showed a complex mixture containing principally uronic acid, glucose, galactose, mannose, xylose, and arabinose, with lesser amounts of rhamnose and fucose. The glucose appeared to be derived from contaminating cell wall debris.

Fractionation on DE52-cellulose yielded five fractions with increasing proportions of uronic acid but of similar carbohydrate composition. None of these fractions contained glucose.

Periodate oxidation followed by hydrolysis gave erythritol, glycerol, erthythronic acid, and glyceric acid, together with uncleaved sugars, suggesting a highly branched polysaccharide mixture with 1,4-linked, 1,3-linked (or branched) and end group residues.

Hydrolysis led to the separation of a beta-D-glucuronosyl (1→3)-D-galactose, indicating that this is a structural feature of the macromolecule.

The presence of cellulose in the cell walls was inferred from a parallel study on a cell-debris rich extract.

c) Subsequent sequential extracts with acid and alkali showed a similar pattern of constituent sugar residues, with increasing proportions of glucose and xylose. The final residue again contains cellulose-type polysaccharide.

Appendix I: A preliminary study of the freshwater green alga Microspora sp. indicated that sucrose (major), glucose, fructose, galactose and myo-inosital constituted the low molecular weight carbohydrates. A complex mixture of polysaccharides containing galactose, glucose, mannose, xylose, rhamnose and fucose were also extracted. These defied fractionation on DE-cellulose.