Table 1.

Values of the model parameters for influenza virus-host cell adhesion

NHA	NSA	K0 (mM)	Veff (mM−1)
3 × 50	2000	1.0 ± 0.5 × 10−5	2.7 ± 0.3 × 103

The parameters K₀ and V_eff are fitted to three experimentally measured isotherms. N_HA is the number of HA monomers, and N_SA is the number of SA receptors at maximum sugar loading. The number of ligands in our model is then N_L = N_HA, and the number of receptors N_R = N_SA × relative sugar loading. In principle, N_HA and N_SA can be treated as fitting parameters as well. The fitting problem then becomes underrestrained, however, given the small amount of isotherm data, and there are large statistical uncertainties in the fitted N_HA, N_SA, and K₀ parameters. Instead, we used rough estimates of N_HA and N_SA from previous publications. It has been estimated (19) that 50 HA trimers per virus participate in forming adhesive connections, and thus N_HA ≈ 3 × 50. The estimate for N_SA ranges from 600 to 4500 (19, 32, 39). We used an intermediate value. Different values of N_SA used in fitting result in compensatory values of V_eff, but the consequent isotherms are little changed. More accurate measurements of N_HA and N_SA can help determine more reliably the parameters K₀ and V_eff. We have also fit the parameters K₀ and V_eff using the exact solution (Eqs. 3 and 4), obtaining the same values.