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Figure 4.
Figure 4. Proposed Mechanism of Acyl Transfer(A) The
stereodiagram of acyl-phosphopantetheine modeled into the EsaI
active-site cavity viewed as in Figure 2A. The electrostatic
surface, generated using GRASP (Nicholls et al., 1993) and
Photoshop (Adobe), is colored red, white, and blue to indicate
negatively charged, neutral, or positively charged regions of
the surface, respectively. The individual atoms in the modeled
phosphopantetheine are colored according to atom type.(B) The
acylation cleft of EsaI and relevant residues are shown in
gray, the modeled phosphopanteteine is shown in cyan, and the
well-ordered water molecules observed in the native structure
that lie along β4 are shown as red spheres.(C) The proposed
N-acylation reaction is catalyzed via nucleophilic attack on the
1-carbonyl of acyl-ACP by the free amine electrons of SAM after
proton abstraction by a water molecule stabilized by Glu97 or
Ser99.
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