apyrase and arabinogalactan

Mimosa pudica Linn leaves with pulvini contain unique isoforms (I and II) of Apyrase enzyme (EC 3.6.1.5). The activity of isoform I depends on divalent cation Mn2+. This isoform is associated noncovalently with the polysaccharide, containing mainly of galactose and arabinose sugars. The apparent molecular mass of these 2 isoforms are 36 and 34 Kd respectively. The association of the polysaccharide with the isoform I has been found to be Ca2+ dependent which is endogenously present in this isoform. Removal of Ca2+ and polysaccharide from the enzyme (isoform I) leads to an inactivation. The enzyme activity can be restored when both Ca2+ and endogenous polysaccharide fraction were added at an optimal molar ratio of Ca2+:protein of 7:1. The endogenous polysaccharide can be replaced by the standard arabinogalactan. No other sugar or polysaccharide except the arabinogalactan can restore the apyrase activity. Calcium mediates a conformational change in the protein which helps in association of polysaccharide as evidenced from fluorometric and far UV-CD studies to restore the enzymic activity. Neither any interaction of the polysaccharide with the protein is detected in absence of Ca2+ nor the enzyme activity could be recovered under such condition.

Topics: Amylases; Apoenzymes; Apyrase; Arabinose; Calcium; Calcium Channel Blockers; Calcium-Transporting ATPases; Chelating Agents; Circular Dichroism; Galactans; Galactose; Guanidine; Isoenzymes; Plant Leaves; Polysaccharides; Protein Structure, Secondary; Trifluoperazine; Tryptophan; Verapamil; Xylan Endo-1,3-beta-Xylosidase; Xylosidases