D-Ribulose 1,5-Diphosphate Carboxylase from spinach

D-Ribulose 1,5-Diphosphate Carboxylase (RUBISCO) amounts to 50% of the total spinach leaves associated soluble protein.

Exists as a 557 kDa hexadecamer composed of eight heavy chains each with a molecular weight of approximately 56 kDa and eight light chains of molecular weight 14 kDa. Each molecule contains one magnesium ion.
pH optimum: ~7.9.
K_M for CO₂: ~0.45 mM.
Ribulose diphosphate becomes inhibitory at concentrations exceeding 0.7 mM. Orthophosphate and ammonium sulfate are competitive inhibitors. 3-Phosphoglycerate is a noncompetitive inhibitor.

D-Ribulose 1,5-Diphosphate Carboxylase from spinach has been used:

• as a test protein in pepsin digestion studies
• as an innocuous or non-hazardous protein sample to test its effect on human intestinal epithelial cell lines
• in isothermal titration calorimetry (ITC), and radiolabeled binding assays with abscisic acid

D-Ribulose 1,5-Diphosphate Carboxylase (RUBISCO) depends on Rubisco activase and chaperones for activation. It participates in plant photorespiration events by catalyzing the carboxylation and oxygenation of ribulose-1,5-bisphosphate. Abscisic acid inhibits the carboxylation activity of Rubisco.

One unit will convert 1.0 μmole of D-RuDP and CO₂ to 2.0 μmoles of D-3-phosphoglycerate per min at pH 7.8 at 25°C.