Sialoconjugate substrates modeled in the TS reaction center, in the context of the immunocomplex structure.

<p>(<b>A</b>) α(2,3)sialyllactose (SL, carbons in yellow) and MU-NANA (carbons in purple) are shown in stick representation, colored according to atom elements (oxygen in red, nitrogen in blue). The carbohydrates are grafted from PDB models 1S0I (for SL) and 1S0J (MU-NANA), after structural superposition of the TS molecules onto the immunocomplex, resulting in their specific positions within the TS catalytic pocket. TS and Fab molecules are shown in ribbons (TS in green, Fab light chain in magenta and heavy chain in blue), with their corresponding solvent-accessible surfaces on top. The surface has been cut to highlight the inner architecture of the TS catalytic pocket: this orientation does not allow appreciating that the site is open from above and beneath the plane of the paper. TS Y₁₁₉ (green sticks) is seen directly obstructing the sialic acid position, and its normal mobility is hindered by antibody's light chain S₃₀ (magenta sticks). (<b>B</b>) A similar representation as in panel (A), in a rotated orientation scene, to highlight the ‘roof’ formed by residues S₆₆–G₆₇ of the Fab light chain (in magenta sticks to the top left of the panel) in direct contact with TS residues R₃₁₁–W₃₁₂ (in pale green sticks, to the right of the figure). Note the expected clash of the glucosyl residue in sialyllactose against loop 66–67, and the better fit of the smaller MU-NANA substrate, still quite restricted in free torsional movements. Y₁₁₉ is again shown (strong green sticks), precluding entrance of the sialic acid moiety of both modeled sugar compounds.