Structural studies of an arabinogalactan-protein from the gum exudate of Acacia robusta

Abstract

The gum exudate from Acacia robusta (subspecies clavigera) has been found to contain protein (18% w/w), bound to an arabinogalactan having structural features typical of the gum polysaccharides from Acacias of Bentham's Series 4 (Gummiferae), of which A. robusta is a member. Three sequential Smith degradations yielded an oligosaccharide of molecular weight ∼700, indicating the presence in the galactan core of very small blocks of contiguous (1→3)-linked d-galactopyranosyl residues. Such an oligosaccharide was produced after only one Smith degradation, following removal of the l-arabinosyl units (constituting ∼50% of the total carbohydrate) by partial hydrolysis of the arabinogalactan with acid. This partial hydrolysis resulted in the detachment of a small proportion (∼25%) of the protein from the polysaccharide, whereas virtually all of it was detached by Smith degradation. These results have implications with regard to the nature of the linkages between the arabinogalactan and the 4-hydroxy- l-proline-rich, protein component of this gum.