Chemical Insight into the Emergence of Influenza Virus Strains That Are Resistant to Relenza

A reagent panel containing ten 4-substituted 4-nitrophenyl alpha-D-sialosides and a second panel of the corresponding sialic acid glycals were synthesized and used to probe the inhibition mechanism for two neuraminidases, the N2 enzyme from influenza type A virus and the enzyme from Micromonospora viridifaciens. For the viral enzyme the logarithm of the inhibition constant (K-i) correlated with neither the logarithm of the catalytic efficiency (k(cat)/K-m) nor catalytic proficiency (k(cat)/K(m)k(un)). These linear free energy relationship data support the notion that these inhibitors, which include the therapeutic agent Relenza, are not transition state mimics for the enzyme-catalyzed hydrolysis reaction. Moreover, for the influenza enzyme, a correlation (slope, 0.80 +/- 0.08) is observed between the logarithms of the inhibition (K-i) and Michaelis (K-m) constants. We conclude that the free energy for Relenza binding to the influenza enzyme mimics the enzyme-substrate interactions at the Michaelis complex. Thus, an influenza mutational response to a 4-substituted sialic acid glycal inhibitor can weaken the interactions between the inhibitor and the viral neuraminidase without a concomitant decrease in free energy of binding for the substrate at the enzyme-catalyzed hydrolysis transition state. The current findings make it clear that new structural motifs and/or substitution patterns need to be developed in the search for a bona fide influenza viral neuraminidase transition state analogue inhibitor.