Quantitative comparison of human parainfluenza virus hemagglutinin-neuraminidase receptor binding and receptor cleavage

The human parainfluenza virus (hPIV) hemagglutinin-neuraminidase (HN) protein binds (H) oligosaccharide receptors that contain N-acetylneuraminic acid (Neu5Ac) and cleaves (N) Neu5Ac from these oligosaccharides. In order to determine if one of HN's two functions is predominant, we measured the affinity of H for its ligands by a solid-phase binding assay with two glycoprotein substrates and by surface plasmon resonance with three monovalent glycans. We compared the dissociation constant (K_d) values from these experiments with previously determined Michaelis-Menten constants (K_ms) for the enzyme activity. We found that glycoprotein substrates and monovalent glycans containing Neu5Ac MathematicalPi-Four.-1.H9251 2-3Gal MathematicalPi-Four.-1.H9252 1-4GlcNAc bind HN with K_d values in the 10 to 100 MathematicalPi-Four.-1.H9262 M range. K_m values for HN were previously determined to be on the order of 1 mM (M. M. Tappert, D. F. Smith, and G. M. Air, J. Virol. 85:12146-12159, 2011). A K_m value greater than the K_d value indicates that cleavage occurs faster than the dissociation of binding and will dominate under N-permissive conditions. We propose, therefore, that HN is a neuraminidase that can hold its substrate long enough to act as a binding protein. The N activity can therefore regulate binding by reducing virus-receptor interactions when the concentration of receptoris high.