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
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