Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: the self-similarity hypothesis of assembly from a common building block.

Arabinogalactan (AG) and arabinogalactan-protein (AGP) fractions were treated enzymatically using several proteases in acidic (pH 4) and alkaline (pH 7) conditions in order to go deeper insight into the structure and conformations of the two main fractions of Acacia senegal gum. Endoproteinase Glu-C, pepsin and phosphatase acid were thus used in acidic conditions while subtilisin A, pronase, trypsin, papain and proteinase K were used in alkaline conditions to cleave protein moieties of the two fractions. Structures of AG and AGP were probed using HPSEC-MALLS, small angle neutron scattering and far-UV circular dichroism. Enzymes did not affect AG fraction structure whatever the pH conditions used, highlighting the inaccessibility of the peptide backbone and the remarkable stability of this fraction in acidic and alkaline conditions. This result was in agreement with the thin oblate ellipsoid model we previously identified for the AG fraction where the 43 amino-acid residues peptide sequence was supposed, based on spectroscopic methods, to be totally buried. Contrary to AG fraction, AGP protein component was therefore cleaved using enzymes in alkaline conditions, the absence of enzymatic efficiency in acidic conditions being probably ascribed to long range electrostatic repulsions occurring between negatively charged AGP and enzymes at pH 4. The decrease of AGP molecular weight after hydrolysis in alkaline conditions went from 1.79 × 10(6) g mol(-1) for control AGP to as low as 1.68 × 10(5) g mol(-1) for papain-treated AGP. The overall structure of the enzyme-treated AGPs was found to be surprisingly quite similar whatever the enzyme used and close, with however some subtle differences, to AG unit. A tri-axial ellipsoid conformation was found in enzyme-treated AGPs and the two main preferential distances identified in the pair distance distribution function would claim in favor of rod-like or elongated particles or alternatively would indicate the presence of two particles differing in dimensions. The secondary structures content of control and enzyme-treated AGPs were similar, highlighting both the high rigidity of the protein backbone and the overall symmetry of AGP. This conclusion was reinforced by the more compact structures found when AGP was intact compare to the more elongated structures found when AGP was enzymatically cleaved. Finally, the structural similarities found in enzyme-treated AGP together with the theoretical calculations to analytically probe the type of branching would suggest that AGP would be made of a self-similar assembly of two types of building blocks, the second being a five-fold repetition of the first one, for which palindromic amino acid sequence would ensure a self-ordering of carbohydrate moieties along the polypeptide chains. The cleavage would therefore lead to hydrolysed building blocks with similar secondary structures and conformations whatever the enzyme used.