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Figure 1.
Figure 1. Structural Aspects of CAD(A) The monomeric
structure. The secondary structures in Domains C2 and C3 are
numbered in the order of appearance in the primary sequence. The
invisible Domain C1 is represented as a sphere. The
catalytically important histidine and lysine residues are shown
in ball-and-sticks. The Zn^2+ binding site is shown with the
Zn^2+ in orange and the cysteinyl sulfur in yellow. The inset
shows the detailed interactions of the catalytic residues
together with the final 2F[o] − F[c] electron density map
(2.6 Å, 1.0 σ). The putative Mg^2+ is represented as a
yellow sphere. Asp262, His308, and two loosely bound water
molecules provide the metal coordination arms. Asn260 and a
water molecule are on the hydrogen-bonded network with a
metal-coordinating water.(B) The dimeric structure. In the side
view (top), one subunit in blue is oriented similar to that in
(A). The catalytic residues and the Zn^2+ binding site are
shown. A DNA strand shows CAD on the same scale. The top view
(bottom) is looking down along the molecular 2-fold axis. The
crevice between the C3 domains spans 14 base pairs of the
modeled DNA. Of note, the longest helix α4 fits into the major
groove of the DNA.
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