RuBisCO-like proteins as the enolase enzyme in the methionine salvage pathway: functional and evolutionary relationships between RuBisCO-like proteins and photosynthetic RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the key enzyme in the fixation of CO2 in the Calvin cycle of plants. Many genome projects have revealed that bacteria, including Bacillus subtilis, possess genes for proteins that are similar to the large subunit of RuBisCO. These RuBisCO homologues are called RuBisCO-like proteins (RLPs) because they are not able to catalyse the carboxylase or the oxygenase reactions that are catalysed by photosynthetic RuBisCO. It has been demonstrated that B. subtilis RLP catalyses the 2,3-diketo-5-methylthiopentyl-1-phosphate (DK-MTP-1-P) enolase reaction in the methionine salvage pathway. The structure of DK-MTP-1-P is very similar to that of ribulose-1,5-bisphosphate (RuBP) and the enolase reaction is a part of the reaction catalysed by photosynthetic RuBisCO. In this review, functional and evolutionary relationships between B. subtilis RLP of the methionine salvage pathway, other RLPs, and photosynthetic RuBisCO are discussed. In addition, the fundamental question, 'How has RuBisCO evolved?' is also considered, and evidence is presented that RuBisCOs evolved from RLPs.