(
a) Asymmetric unit of trimer hPNP with the catalytic site of subunit-A located near the trimer interface and covered with the loop including Phe-159
C of subunit C. Mutated residues His
104Arg and Lys
22Glu are located 25 Å and 45 Å away from the catalytic site of the adjacent subunit and 26 Å and 32 Å from the catalytic site of their own subunit. (
b) The active site residues near ImmG and the phosphate nucleophile. (
c) Structure of ImmG at the catalytic site of hPNP. Active site residues in contact with ImmG and the nucleophile phosphate are shown. The ImmG:O5′, ImmG:N4′, H257:N
δ, and

:Op atoms are highlighted in red.