Streptococcus pneumoniae substrate binding protein SatA specifically recognises the α-anomer of N-acetylneuraminic acid

Streptococcus pneumoniae relies on sialic acid uptake for nutrition and human respiratory tract colonisation. The ATP binding cassette (ABC) transporter SatABC-MsmK facilitates this, with SatA being the substrate-binding protein (SBP). We show that SatA specifically recognises the α-anomer of N-acetylneuraminic acid (α-Neu5Ac). Crystallographic analysis, mutagenesis and binding affinity measurements identify conserved residues Phe87, Arg113, Gln216, Arg404 as critical for α-Neu5Ac coordination. Nuclear magnetic resonance spectroscopy confirms selective binding of α-Neu5Ac in buffered solution, despite its low equilibrium abundance. Isothermal titration calorimetry shows high affinity of SatA for the anomeric mixture of Neu5Ac ( K d ≈ 270 nM). The α-anomer preference of SatA, not previously observed in other SBPs, may confer selective advantage to S. pneumoniae by enabling uptake of α-Neu5Ac, the immediate product of sialidase-mediated glycan cleavage.