Influenza virus sialidase: effect of calcium on steady-state kinetic parameters

Abstract

Ca 2+ increases the initial rate of activity of sialidase from influenza virus (A/Tokyo/3/67). Increasing ionic strength also activates influenza virus sialidase. When ionic strength is controlled, smaller but still significant Ca 2+ effects are observed, with V max/ K m increased from 0.8·10 5 to 1.4·10 5 M −1 s −1 and V max increased from 6.3 to 9.5 s −1 by saturating Ca 2+. The K i of the competitive inhibitor 2,3-dehydro-2-deoxy- N-acetylneuraminic acid was decreased from 2.7·10 −6 to 1.15·10 −6 M after the addition of saturating Ca 2+. The data show that Ca 2+ exerts a specific effect on V max/ K m, leading to an increased rate of interaction of substrate with the enzyme. The K d-app for the Ca 2+-sialidase complex is 2 mM. Except for Mg 2+ which behaves similarly to Ca 2+, other mono- and divalent cations have little specific effect on sialidase kinetics. Sequence analysis of a range of subtypes of sialidases from influenza virus supports the proposal that Ca 2+ binds at the subunit interface transmitting a conformational change to the enzyme active site. Ca 2+ activation may have a physiological role in switching on sialidase activity during the release of newly synthesised virions from the host cell surface.