Structural and metabolic studies of carbohydrates in algae

• Main Author: Smestad, Berit
• Published: Royal Holloway, University of London 1971
• Subjects: 579.8; Physiology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.704147

Investigation of the Carbohydrates synthesised by the marine green alga Ace tabulara a crenulata. By means of sequential extractions with different solvents the following carbohydrates were isolated and characterised. 1. From an alcoholic extract D-glucose, D-fructose, allulose, myo-inositol and an alcohol tentatively identified as allo-quercitol. This is only the second time allulose has been found in Nature and the first time in any alga. An homologous series of fructose-containing oligosaccharides were also separated and characterised as 2,1 linked units terminated by a molecule of sucrose at the potential reducing end. 2, Aqueous extraction gave a mixture of a fructan (major) and a sulphated heteropolysaccharide. a) These two polysaccharides were separated on a column of DEAE-cellulose. Using the classical techniques the fructan was characterised as an inulin type polysaccharide, b) (i) The sulphated polysaccharide contained D-glucuronic acid, B-galactose, L-rhamnose and small proportions of xylose and 4-0-methylgalactose. Each of the sugars were separated and characterised. The presence of the last sugar has not been reported as a constituent of any green algal polysaccharide before, (ii) By extraction of stalks and caps separately with cold and hot water and elution from the DEAE-cellulose with 0.5 and 1.0 M potassium chloride similar sulphated polysaccharideswith variable proportions of the different sugars and portions of sulphate were separated. (iii) Structural studies by partial desulphation, methylation, periodate oxidation and partial hydrolysis established the essential similarity of these fractions and that the main structural features are: highly branched molecules containing1,3-linked D-galactose 4-sulphate (major) and 6-sulphate, 1,2-linked Larhamnose and glucuronic acid, galactose and rhamnose all present as end groups. Glucuronic acid is linked to both rhamnose and galactose and galactose units are mutually linked in the macromoiecule. 3. linked mannan was extracted with alkali. Methylation,periodate oxidation and gel filtration studies proved this structure and indicated some degree of branching and a higher molecular weight than those of previously reported for mannans from green algae. PART II: Photosynthetic studies on 1. Acetabularia mediterranea, 2. Fucusvesiculosus and 3. Ulva lactuca1. Pulse labelling experiments with C0 on A.mediterraneafollowed by ethanolic extraction led to the separation of labelled sucrose, glucose, fructose and the first three oligosaccharides characterised in Part 1 (l). A possible biosynthetic inter-conversion of these carbohydrates is described from the results of these experiments. 2. a) Pulse labelling experiments with C0 on Fucus vesiculosas a marine bcrown alga, show that of the low molecular weight carbohydrates formed by photosynthesis, mannitol is formed first The possible conversion of mannitol into laminaran via mono-and di- glucosylmannitol is discussed*b) The polysaccharides, laminaran, xylogalactofucoglucuronan (A), xylogluouronogalactofucan (B), fucoidan (C) and alginic acid were extracted and separated by various fractionation techniques. The radioactivity in each was measured as was the radioactivity in the constituent sugars of the fucose-containing polysaccharides (all of which are sulphated). From the changes in the radioactivity of these polysaccharides in different samples it is postulated that (a) is synthesised first and transformed into (C) via (B).Low molecular weight carbohydrates present in the acid extract are suggested as precursors for the acid polysaccharides. The residual material after acid and alkali extraction was hydrolysed and the radioactivity of the sugars in the hydrolysate wasivmeasured. Glucose was the major radioactive sugar.3. Ulva lactuca, a marine green alga. Similar experiments on U.lactuca were carried out.a) Examination of the 80% ethanol extracts showed that sucrose is the first sugar to be synthesised and this is followed by glucose and fructose. Xylose, ribose(?) and myo-inositol also incorporated radioactivity.b) Starch and a sulphated glucuronoxylcrharrman were extractedand their radioactivities were measured. The former appears to be synthesised most rapidly and to be an active metabolite and the sulphated polysaccharide is laid down as a long term storage product or as part of the skeletal structure of the alga.c) The residual material after ethanolic and aqueous extractions was examined in the same way. Glucose is again the major sugar and the results indicate that the carbohydrate is laid down in the cell wall after other constituents.