Ribulose bisphosphate carboxylase like proteins, Rubisco-Form IV
Ribulose bisphosphate carboxylase (Rubisco) plays an important role in the Calvin reductive pentose phosphate pathway. It catalyzes the primary CO2 fixation step. Rubisco is activated by carbamylation of an active site lysine, stabilized by a divalent cation, which then catalyzes the proton abstraction from the substrate ribulose 1,5 bisphosphate (RuBP) and leads to the formation of two molecules of 3-phosphoglycerate. Members of the Rubisco family can be divided into 4 subgroups, Form I-IV, which differ in their taxonomic distribution and subunit composition. Form I-III have Rubisco activity, while Form IV, also called Rubisco-like proteins (RLP), are missing critical active site residues and therefore do not catalyze CO2 fixation. They are believed to utilize a related enzymatic mechanism, but have divergent functions, like for example 2,3-diketo-5-methylthiopentyl-1-phosphate enolase or 5-methylthio-d-ribulose 1-phosphate isomerase.