Arabinogalactan II

Arabinogalactan II (AG-II) (Figure 3.19b) has a more complex structure. It has a main chain of galactose units linked by /S-(l,3) oside bonds, with short lateral chains of galactose, bonded on /S-(l,6). In C3, these are replaced by individual... 

Fig. 3.18. Suggested structural model for acid pectic substances in grapes (Doco et al, 1995). A, arabinan HG, homogalacturonase AG-II, arabinogalactan II RG-I, rhamnogalacturonan I RG-II, rhamnogalacturonan II...

C6te W A Jr, Simson B W, Timell T E 1967 Studies on larch arabinogalactan. II. Degradation within the living tree. Holzforschung 21 85-88... 

Arabinogalactans (AGs) are widely spread throughout the plant kingdom. Many edible and inedible plants are rich sources of these polysaccharides. AGs occur in two structurally different forms described as type I and type II, associated with the pectin cell-wall component by physical bonds and some of them are covalently linked to the complex pectin molecule as neutral side chains. Commercial pectins always contain AG 10-15%). AG of type I has a linear (1 4)-y0-o-Galp backbone, bearing 20-40% of of-L-Ara/ residues (1 5)-linked in short chains, in general at position 3. It is commonly found in pectins from citrus, apple and potato [6]. Recently, this AG type has been isolated from the skin of Opuntia ficus indica pear fruits [372]. 

The arabinogalactans have more frequently been reported for activity in various biological systems. Arabinogalactans are often classified in three groups arabino-4-galactans (Type 1), arabino-3,6-galactans (Type II) and polysaccharides with arabinogalactan side chains (Type III) [14]. The latter type are also called the real pectins [10,11]. Only types I and II will be dealt with in this chapter, as Type III are equal to the pectins discussed below. 

Fig. 2 Average structure of the hairy or ramified region of an apectic substance, with a rhamnogalacturonan I backbone substituted at position 4 of the rhamnose units with arabinan and arabinogalactan type II side chains...

Fig. 7 Structure of the Arabinogalactan type II polymer from Larch...

Black pepper contains several polysaccharides of which one shows a strong effect as an immune enhancer based on the fact that the polymer is an anti-complementary polysaccharide. The polysaccharide has an Mw of approx. 40 kD. It is composed basically of rhamnose, arabinose, galactose and galac-turonic acid, and shows a high binding capacity for the Yariv reagent. This indicates that the side chain of the polymer is of the arabinogalactan type II, which is a common structure for several polysaccharides with an effect on the complement system [70]. 

The low amounts in 3- and 3,6-linked galactosyl, in 3- and 5-linked arabinosyl residues may be due to the presence of contaminating arabinogalactans and arabinans in apple and tomato juice RG-II preparations. 

A sensory study based on an incomplete factorial design allowed to demonstrate that astringency of procyanidins was reduced in the presence of rhamnogalaturonan II added at levels encountered in wine but was modified neither by anthocyanins nor by the other wine polysaccharides (mannoproteins and arabinogalactan proteins). Increase in ethanol level resulted in higher bitterness perception but had no effect on astringency. 

Multidrug-resistant M. tuberculosis (MDRTB) strains have become a problem. As pointed out previously, the mycobacterial cell wall presents a major barrier to the entry of chemotherapeutic drugs. Possible solutions are (i) to design drugs of increasing hydrophobicity that will pass through the cell wall, as with isoniazid derivatives [89, 90], (ii) to use a second antibiotic in combination with a known inhibitor of arabinogalactan or mycolic acid biosynthesis [238]. 

Pellerin, P, Vidal, S., Williams, R, Brillouet, J. M. (1995). Characterization of five type II arabinogalactan-protein fractions from red wine of increasing uronic acid content. Carbohydr. Res., 277, 135-143. 

Dufour and Bayonove (1999a) reported two criteria for polysaccharide discrimination acidity and protein content. Neutral peptic substances (type II arabinogalac-tans and arabinogalactans-proteins) represent 40% of the polysaccharides in wine and acidic pectic polysaccharides, (e.g. homogalacturonans and rhamnogalacturo-nans) account for 20% of them. Because of the difficulty in purifying wine polysaccharides, most of the studies on interactions between wine polysaccharides and aroma compounds have been carried out with exocellular and cell wall mannoproteins (thus mainly glycoproteins) of Saccharomyces (see effect of yeast and derivatives in the next section). 

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