Structural and Functional Characterization of Periplasmic Sialic Acid Binding Proteins from Pathogenic Bacteria

Abstract

Pathogenic bacteria have evolved different strategies to escape the host immune system. One of the strategies is to scavenge and coat themselves with host sialic acids, which allows the bacteria to deceive the host immune system. This is also called ‘molecular mimicry’. N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) are the two common sialic acids and many pathogenic bacteria scavenge these sugars and they are added to the terminus of lipooligosaccharide (LOS) or lipopolysaccharides (LPS) as non-reducing sugars. In addition, part of the scavenged sialic acid is also utilized by the bacteria as carbon and nitrogen source. Detailed structural and functional studies of sialic acid binding proteins will help in rational design of drugs against different pathogenic bacteria. Pathogenic bacteria like Haemophilus influenzae (Hi), Fusobacterium nucleatum (Fn), Pasteruella multocida (Pm) and Vibrio cholerae(Vc) use Sialic acid binding protein (SiaP), a component of the TRAP (Tripartite ATP-independent) transport system, to scavenge sialic acid present in the periplasm. Further, SiaP delivers the bound sugar to a membrane protein that transports the sugar from the periplasm to the cytoplasm. SiaP's are two domain proteins, with a very well conserved ligand-binding pocket. Our thermodynamic studies illustrate that the binding of Nue5Ac and Neu5Gc to SiaP is an enthalpically driven process. The structure of the sialic acid transport protein (SatA), a component of ABC (ATP binding cassette) transport system, from Haemophilus ducreyi(Hd) displays a different sialic acid binding pocket when compared to SiaP. The structural differences reflect in our thermodynamic analysis with SatA binding to Neu5Ac and Neu5Gc being an entropically driven process. Altogether, our structural and functional analyses show that, sialic acid binding proteins bind to same ligand with similar binding affinities, but by using different binding topologies.