2.A.56.1.20
SiaP, sialic acid binding protein of 328 aas and 1 N-terminal TMS.  AaSiaP has been solved in both the open ligand-free and closed liganded conformations. An intermediate conformation, where AaSiaP is mostly closed and is bound to a non-cognate ligand, acetate, hints at how N-acetylneuraminate binding stabilises a fully closed state (King-Hudson et al. 2024). AaSiaP preferentially binds N-acetylneuraminate (K_D = 0.4 μM) compared to N-glycolylneuraminate (K_D = 4.4 μM), which is explained by the closed-N-acetylneuraminate bound structure. Small-angle X-ray scattering data alongside molecular dynamics simulations suggest the AaSiaP adopts a more open state in solution than in crystal. However, the open unliganded conformation can also sample closed conformations. Molecular dynamics simulations also demonstrate the importance of water molecules for stabilising the closed conformation. The results are consistent with an induced fit model of binding, but the open unliganded conformation may sample multiple states capable of binding substrate (King-Hudson et al. 2024).