HIGH RESOLUTION STRUCTURE OF CADMIUM CARBOXYPEPTIDASE A.
Anette Frost Jensen1, Jens Bukrinsky1,2, Rogert Bauer3, Morten J. Bjerrum2 and Sine Larsen1
1Centre for Crystallographic Studies,
University of Copenhagen, Universitetsparken 5, DK-2100
Copenhagen, Denmark.
2 Chemistry Department, and
3Physics and Mathematics Department,
Royal Veterinary and Agri\-cultural University, Thorvaldsensvej
40, DK-1871 Frederiksberg, Denmark. E-mail: frost@xray.ki.ku.dk
Keywords: Protein crystallography, metalloproteins, coordination chemistry.
Carboxypeptidase A is a 307 amino acid enzyme which hydrolyses peptide bonds from the C-terminal. When the Zn2+ ion has been exchanged with Cd2+ by dialysis the enzyme retains 30 % of its original activity. The structure of the cadmium enzyme in P21 has been determined by molecular replacement using the native zinc enzyme as a search model.[1] Two experimental conditions for dialysis, one at low ionic strength and one at high ionic strength (0.25 M LiCl) and both at the native pH of 7.5, have been investigated. As in the native zinc enzyme the Glu72 oxygen atoms and one nitrogen atom from His69 and one from His196 coordinate the cadmium. However, for the low ionic strength condition one water molecule is bound to cadmium, whereas for high ionic strength two water molecules are bound. Present figure of merits are: for A) R = 18.2 % and Rfree= 23.4 % at 1.57 A resolution using 25842 data as working set and for B) R = 19.9 % and Rfree= 23.4 % at 1.70 A resolution using 26776 data as working set. Differences in coordination chemistry of the two forms will be discussed.