A hypothetical model for oxygen diffusion into the RubisCO active site via the hydrophobic pocket of residues. The figure shows the cross-sectional view of a surface rendering of the α/β-barrel motif of a single large subunit from the Synechococcus 6301 RubisCO (1RBL) (37). The active site, marked by the presence of CABP (black), is indicated. The active-site residues, loop-6 Lys-331 and Ser-376 (blue), are on either side of the carbon that is attacked by CO2 or O2 in the rate-limiting step of the reaction mechanism. Ser-376 is adjacent to the hydrophobic pocket of residues that have been proposed to play a role in oxygen sensitivity. Residue Phe-391 (red), one of the hydrophobic-pocket residues, appears to be positioned at the surface opposite to the active site.