PAPAYA GLUTAMINE CYCLASE, A PLANT ENZYME HIGHLY RESISTANT TO PROTEOLYSIS, ADOPTS AN ALL-BETA CONFORMATION

Glutamine cyclases catalyse the conversion of L-glutaminyl-peptides in to 5-oxoprolyl-peptides with the concomitant Liberation of ammonia. We report here biophysical characterisation of the glutamine cyclase pre sent in the laticiferous cells of the plant Carica papaya. After purif ication to near homogeneity, this enzyme was subjected to limited prot eolysis and found to exhibit a high resistance to degradation and nick ing. The structural reasons for this property were examined using circ ular dichroism and infrared spectroscopies. By combining the analyses of the infrared and CD spectra of papaya glutamine cyclase, its suscep tibility to proteolysis, and its hydrogen-deuterium exchange character istics, we conclude that this protein contains extensive beta-sheet st ructure and is likely to have only short immobile loops connecting its beta-strands.