Papaya glutamine cyclase, a plant enzyme highly resistant to proteolysis, adopts an all-β conformation

Glutamine cyclases catalyse the conversion of L-glutaminyl-peptides into 5-oxoprolyl-peptides with the concomitant Liberation of ammonia. We report here biophysical characterisation of the glutamine cyclase present in the laticiferous cells of the plant Carica papaya. After purification to near homogeneity, this enzyme was subjected to limited proteolysis and found to exhibit a high resistance to degradation and nicking. The structural reasons for this property were examined using circular dichroism and infrared spectroscopies. By combining the analyses of the infrared and CD spectra of papaya glutamine cyclase, its susceptibility to proteolysis, and its hydrogen-deuterium exchange characteristics, we conclude that this protein contains extensive beta-sheet structure and is likely to have only short immobile loops connecting its beta-strands.