Isolation and structural properties of the major protein fraction in Australian wattle seed (Acacia victoriae Bentham)

The major protein fraction of wattle (Acacia victoriae Bentham) seed was isolated by anion-exchange and gel permeation chromatography. The protein was then characterised by its amino acid composition, gel electrophoresis, fluorimetry and circular dichroism (CD) in order to elucidate its nature and structural properties. The major amino acids were found to be glutamate (14.4%), aspartate (11.1%) and lysine (9.13%) while the contents of sulphur-containing amino acids (cysteine and methionine) and tryptophan were very low. The native protein, with an isoelectric point of 6.85, was comprised of two subunits of molecular masses 62 and 125 kDa, the bigger unit being joined by at least one disulphide bond. Far-UV-CD spectra showed that the protein consisted mainly of equal amounts of β-sheets and random structures (39% each), about 19% β-turns and relatively little α-helix (3.6%). These structures were also found to be very stable to changes in pH (3–9), temperature and ionic strength. Based on the fluorescence emission and near-UV-CD data, however, the tertiary structure was more sensitive to pH, temperature and ionic strength.