Details of the catalytic site. (a) A representation of RuBP in the active site of non-activated PsRubisco. (b) Comparison of non-activated RuBP-bound PsRubisco with Ca2+-activated inactive RuBP-bound spinach Rubisco. The substrate-binding channel in the PsRubisco structure is shown as a grey surface. Loop 6 of the PsRubisco L subunit (green loop) is folded over and pinned down by the overlapping Lys128 residue (grey surface), yielding a `closed' active-site conformation. This is in contrast to the `open' conformation observed for spinach Rubisco when activated in the presence of Ca2+ with RuBP bound (PDB entry 1rxo
), which shows loop 6 (yellow loop) folded back and away from the channel. In both panels, discrete atoms are shown with pink phosphate, red O, yellow C and blue N atoms.