Modulation of the neuraminidase activity of HN protein from Newcastle disease virus by substrate binding and conformational change: kinetic and thermal denaturation studies.

A statistical study of the enzyme kinetics of the soluble fraction of Haemagglutinin-Neuraminidase (HN) protein from Newcastle Disease virus reveals that a high substrate concentrations its neuraminidase activity follows substrate-inhibition kinetics, in which the binding of a second molecule of substrate to the protein inhibits its enzymatic activity. Results show that the enzymatic activity is modulated by the second substrate to a different extent when the protein is in different environments. Taken together with results obtained by thermal denaturation studies of HN under varying conditions the data show that conformational changes leading to a loss of rigidity of HN are concomitant with the loss of catalytic activity. Also, electrophoretic and sucrose gradient analyses show that the soluble domain of HN behaves as a monomer.