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

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) meas...

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Bibliographic Details
Published in:Journal of agricultural and food chemistry 2008-10, Vol.56 (19), p.9269-9276
Main Authors: Mahendran, T, Williams, P. A, Phillips, G. O, Al-Assaf, S, Baldwin, T. C
Format: Article
Language:English
Subjects:
Acacia senegal
alkaline hydrolysis
Amino Acids - analysis
arabinogalactan protein
arabinogalactans
Barium Compounds
Biological and medical sciences
chemical composition
chemical structure
deglycosylation
enzymatic treatment
Food additives
Food Chemistry/Biochemistry
Food industries
Fundamental and applied biological sciences. Psychology
gel permeation chromatography
General aspects
Glycosylation
gum arabic
Gum Arabic - chemistry
Gum arabic structure
Hydrogen-Ion Concentration
Hydrolysis
Molecular Weight
Mucoproteins - chemistry
multiangle light scattering
Peptide Hydrolases - metabolism
physicochemical properties
plant proteins
Plant Proteins - chemistry
wattle blossom model
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Summary: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 ∼2 × 106 Da to degrade, yielding material of molecular mass ∼4 × 105 Da, whereas the bulk of the material corresponding to the protein-deficient arabinogalactan component (AG) with molecular mass 4 × 105 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 ∼4.5 × 104 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 ∼4.5 × 104 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 ∼3 × 104 and ∼5 × 103 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.
ISSN:0021-8561
1520-5118
DOI:10.1021/jf800849a