Details

Autor(en) / Beteiligte

• Prado F.E
• Lazaro J.J
• Gorge J.L

Titel

Regulation by Ca2+ of a cytosolic fructose-1,6-bisphospatase from spinach leaves

Ist Teil von

• Plant physiology (Bethesda), 1991, Vol.96

Erscheinungsjahr

1991

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• Cytosolic fructose-1,6-bisphosphatase from spinach (Spinacia oleracea L.) leaves was purified over 1700-fold. The final preparation was specific for fructose-1,6-bisphosphate in the presence of either Mg2+ or Mn2+ and was free of interfering enzyme activities. Ca2+ was an effector of fructose-1,6-bisphosphatase activity, and showed different kinetics, depending on whether Mg2+ or Mn2+ was used as cofactor. In the presence of 5 millimolar Mg2+, Ca2+ appeared as activator or as inhibitor of the enzyme at low or high levels of substrate, respectively. In both cases, a rise in affinity for fructose-1,6-bisphosphate was observed. A model is proposed to describe the complex interaction of fructose-1,6-bisphosphatase with its substrate and Ca2+. However, with Mn2+ (60 micromolar) as cofactor, Ca2+ exhibited the Michaelis-Menten kinetics of a noncompetitive inhibitor. When assayed at constant substrate concentration, Ca2+ behaves as a competitive or non-competitive inhibitor, depending on the use of Mg2+ or Mn2+ as cofactor, respectively, with a positive cooperativity in both cases. Fructose-2,6-bisphosphate showed a classic competitive allosteric inhibition in the presence of Mg2+ as cofactor, but this effect was low with Mn2+. From these results we suggest that Ca2+ plays a role in the in vivo regulation of cytosolic fructose-1,6-bisphosphatase.