J. Biol. Chem. doi:10.1074/jbc.M114.578625

Credit: SUM CHAN

The inefficient enzyme RuBisCO fixes CO2, enabling life, but improvements of its catalytic activity would facilitate applications in carbon capture and metabolic engineering. Structures are available for the open (apo) and closed (ligand-bound) states of two of the three known forms (I and III), but only an open structure exists for form II of the homolog from Rhodospirillum rubrum. Satagopan et al. now report the closed structure of a form II RuBisCO from Rhodopseudomonas palustris with a transition state analog. This RuBisCO, which forms a hexamer, unlike the dimeric R. rubrum homolog, cants at a different angle compared to the form I and III oligomers, creating a distinct subunit interface. Mutations to nonconserved amino acids neighboring the active site such as I165A were generally deleterious, indicating roles in enzyme function even if not directly in catalysis. The C terminus, representing a second point of divergence between the three forms, could not be exchanged between forms, though adding a C-terminal extension derived from the R. rubrum enzyme enhanced the activity of the R. palustris enzyme substantially. Finally, the slow reactivation of the I165A mutant after inhibition suggested that accessory proteins needed for regulation of the eukaryotic and cyanobacterial enzymes may not be required for R. palustris. These results highlight similarities and differences in enzyme structure and function that may lead to new mechanistic understanding of this important family of enzymes.