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 at high substrate concentrations its neuraminidase activ ity follows substrate-inhibition kinetics, in which the binding of a s econd molecule of substrate to the protein inhibits its enzymatic acti vity. Results show that the enzymatic activity is modulated by the sec ond substrate to a different extent when the protein is in different e nvironments. Taken together with results obtained by thermal denaturat ion studies of HN under varying conditions the data show that conforma tional changes leading to a loss of rigidity of HN are concomitant wit h the loss of catalytic activity. Also, electrophoretic and sucrose gr adient analyses show that the soluble domain of HN behaves as a monome r.