AN ESSENTIAL TYROSINE RESIDUE OF ASPERGILLUS POLYGALACTURONASE

Based on strict conservation of a tyrosine residue in 24 polygalacturo nases, tyrosine modification was assessed in two different forms of th e Aspergillus enzyme, The second subform was unknown in structure but submitted to sequence analysis and was found also to have the conserve d tyrosine residue, Results of chemical modifications are consistent i n showing inactivation of the proteins with all tyrosine-reactive agen ts tested, acetic anhydride, N-acetyl imidazole, and tetranitromethane , Furthermore, after acetylation, regeneration of enzyme activity was possible with hydroxylamine, Spectrophotometric pH titration showed th at one accessible tyrosine residue is ionized at pH 9.3-9.5, whereas t he remaining, masked residues are all ionized at pH 10.5, It is conclu ded that one tyrosine residue is catalytically important, in agreement with the inactivation and reactivation data, that this residue is acc essible, and that it is likely to correspond to the strictly conserved residue observed in all forms.