Tyrosine micro-region of E. coli L-asparaginase.

The relationship between conformation change and activity of E. coli L-asparaginase has been studied with circular dichroism spectra and microcaloric methods. In many papers, it has been pointed out that the active site of L-asparaginase is closely related to tyrosyl residues. The present authors have studied the effects of L-cysteine on the activity and the conformation of L-asparaginase with UV difference spectra and kinetic methods. Moreover, we have studied the space arrangement of tyrosyl residues in the enzyme molecule. The results show that every enzyme molecule contains about 56 tyrosyl residues, 20 of which are in the hydrophobic core of the enzyme molecule, another 20 at the surface of the enzyme molecule, and the rest in the rifts and hollows of the enzyme molecule. Meanwhile, further study has also been made to determine the relationship between the changes of the enzyme activity and the ionization of tyrosyl residues as well as their chemical modification. By Zou Chenglu's graphical method we have proved that two tyrosyl residues at the surface of the enzyme molecule are the essential groups.