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Figure 1.
FIGURE 1. Conformational changes associated with substrate
binding. a, superimposition of unliganded ABBB in the open form
(white) with AABB+UDP-Gal+DA in the closed form (yellow/red)
showing the internal and C-terminal loops (red), UDP-Gal and DA
(orange), and Mn^2+ (blue), and the location of Arg^176. b,
expanded view about the active site with an arrow indicating the
movement of the internal loop toward the donor in going from the
open state to both the semi-closed or closed states, and showing
the ordering of the C-terminal residues to form the closed
state. c, stereoview of electron density corresponding to the
internal loop in AABB+UDP showing two distinct conformations of
the enzyme (at 50% occupancy) corresponding to the open (yellow)
and semi-closed (green) forms of the enzyme. The disorder
converges at Met^189 (gray). d, the transformation of the
internal loop (residues 176-195) from the open (left) to the
semi-closed (right) conformation is accomplished by the merger
of two  -helices
(Arg^180-Met^186 and Arg^187-Asp^194) into a distorted helical
structure with alternating -3[10]- character. The pivot
point is indicated by a star.
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