Mimosa pudica apyrase requires polysaccharide and Ca2+ for the activity

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.