New Insights into the Structural Characteristics of the Arabinogalactan−Protein (AGP) Fraction of Gum Arabic

Abstract

The structural characteristics of the gum exudate of Acacia senegal (gum arabic) have been investigated by monitoring the composition and physicochemical properties before and after treatment with proteolytic enzyme and various alkaline systems. Molecular mass ( M w) and radius of gyration ( R g) measurements were performed using gel permeation chromatography (GPC) coupled to refractive index, UV absorbance, and multiangle light scattering detectors and indicated that the macromolecules present have a compact structure. It was found that treatment with proteolytic enzyme caused the arabinogalactan-protein component (AGP) with average molecular mass approximately 2 x 10 (6) Da to degrade, yielding material of molecular mass approximately 4 x 10 (5) Da, whereas the bulk of the material corresponding to the protein-deficient arabinogalactan component (AG) with molecular mass 4 x 10 (5) remained unaffected. Barium hydroxide was found to hydrolyze the polysaccharide component (AG) itself in addition to the proteinaceous component as demonstrated in control experiments using dextran. However, sodium borohydride/sodium hydroxide treatments were unable to hydrolyze dextran and were assumed to hydrolyze only the proteinaceous component of gum arabic. The AGP component was completely degraded, yielding material of molecular mass approximately 4.5 x 10 (4) Da. It has been concluded, therefore, that the enzyme did not fully hydrolyze all of the protein present and that the AGP component of gum arabic consists of carbohydrate blocks of approximately 4.5 x 10 (4) Da linked to a polypeptide chain consistent with the wattle blossom structure. Because the AGP was degraded to differing extents using a mild and more severe sodium borohydride/sodium hydroxide treatment, it was concluded that the polysaccharide moieties were linked through both O-serine and O-hydroxyproline residues. The gum arabic sample was deglycosylated by treatment with anhydrous hydrogen fluoride and revealed the presence of two putative core proteins of approximately 3 x 10 (4) and approximately 5 x 10 (3) Da, respectively, which correspond to proteins of approximately 250 and 45 amino acids in length. A new model for the structure of the AGP component has been proposed.

Gum Arabic: More Than an Edible Emulsifier

“…Despite having different protein content, amino acid composition is similar in both gums. Recently, Mahendran et al, 2008, reported the GA amino acid composition in Acacia Senegal, being rich in hydroxyproline, serine, threonine, leucine, glycine, histidine, (Osman et al, 1993 andWilliams& Phillips, et al, 2000). Table 2.…”

“…Table 2. Aminoacid content in Gum Arabic taken from Acacia senegal (Mahendran et al, 2008). Table 3 shows some physicochemical properties used as international GA quality parameters, for example: moisture, total ash content, volatile matter and internal energy, with reference to gums taken from Acacia senegal species in Sudan (FAO, 1990, Larson & Bromley, 1991.…”

Green Synthesis: Nanoparticles and Nanofibres Based on Tree Gums for Environmental Applications

“…It is suggested that the high-molecular-weight fraction of the gum consists of large carbohydrate blocks with a molecular mass of approximately 2.5 · 10 5 Da attached individually to a polypeptide chain. GA is obtained from the stems and branches of Acacia Senegal and Acacia seyal, and being a branched polysaccharide, it exhibits unique structural, physical and chemical properties [84][85][86][87]. Consequently, it is widely used in food and pharmaceutical applications [88][89][90].…”

A novel analytical ultracentrifugation based approach to the low resolution structure of gum arabic

“…The polysaccharide component represents approximately 95% of the macromolecule and consists of a very complex, heavily branched β(1 3)D-galactopyranose backbone with a high proportion of arabinofuranose and rhamnopyranose residues (5) and terminal glucuronic acids. The protein component consists of a 250 amino acid chain, with regions of polysaccharide covalently O-linked to hydroxyproline and serine residues (6). This is often referred to as the wattle blossom model (3).…”

“…The overall hydrodynamic structure has previously been studied principally using light scattering and chromatographic based methods (6). From these, the "wattle blossom" model appears to have become favoured, with the overall conformation of the molecule being close to spherical.…”