Ribulose bisulphate carboxylase: occurance, function and significance.

Ribulose bisulphate carboxylase 

Ribulose bisulphate carboxylase is world’s most abundant enzyme. The enzyme is found in plant containing eight copies of two types of subunits. The larger subunit, which has a molecular weight of 56000 and contains the catalytic site, is synthesized within the chloroplast under the direction of chloroplast DNA and ribosomes. The smaller subunit, with a molecular weight of 14000 is synthesized on cytoplasmic ribosome under the direction of nuclear DNA.

CO2 acts as a substrate as well as activates ribulose bisulphate caboxylase by binding to the amino group of a lysyl residue in the large subunit to form a carbamate (lysine-NH-CO2⁻). This process is catalyzed by another enzyme and requires Mg⁺⁺, which binds to the carboxyl group of the carbamate. The Mg⁺⁺ in turn forms part of the binding site for a second molecule of CO2, which acts as the substrate in the carboxylase reaction. The reactive molecule of CO2 adds to the enolate of ribulose-1, 5-bisulphate to form 2-carboxy-3-ketoarabinitol-1, 5-bisulphate as an intermediate.

Studies showed that Ribulose bisulphate carboxylase has the same active site that catalyses a competing reaction in which O2 replaces CO2 as a substrate. It means enzyme acts as oxygenase.

REACTION MECHANISM

REACTION MECHANISM