The catalytic site of PsRubisco is located at the interface of interacting L subunits. An L2S2 unit is viewed, showing two active sites with bound RuBP substrate at the interface of the two twofold symmetry-related L subunits. The expanded image shows detail of the loops from both L-subunit symmetry mates comprising the catalytic site. Loops that undergo conformational variation in the `open' versus `closed' forms of the enzyme are labelled. The L-subunit symmetry mate is shown in grey, with the asymmetric unit refined L subunit shown in blue and both refined and symmetry-related S subunits shown in red. The substrate RuBP is shown with pink phosphate, red O and yellow C atoms.