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Figure 4.
Figure 4. The water network at the active site of the
re-constituted Cu-Zn enzyme, monomer A. A sulphate ion sits on
the surface of the molecule at the substrate entry site. A
dual-occupancy water molecule (W1) is 2.6 Å from Cu(II)
and 3.7 Å from the position of the Cu(I) atom. W1 is
linked to the Thr137 O protein backbone via a H bond and to the
Gly141 O atom via H bonding to a second water molecule, W2,
which is also linked (2.8 Å) to water molecule W3, the
Gly141 residue and to a sulphate ion on the protein surface (2.6
Å). W3 forms a long (2.9 Å) H-bond to the Ny group
of Arg143. A full-occupancy water molecule, W5, is in direct
contact with the protein main chain, linked to the Lys136 O and
His63 N atoms. W5 and W3 are linked by W4, which is 4.4 Å
from Cu(II). The Gly141 O and Arg143 Ne atoms are bridged by W6.
Finally, W7, 5.3 Å from the (Zn site) Zn atom and a highly
conserved water molecule in all three structures (and in bovine
SOD), has H bonds with the secondary bridge residue Asp124 Od
(2.8 Å) and with Gly85 N (2.9 Å) and Gly72 O (2.8 Å) atoms.
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