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Figure 4.
Fig. 4. Reciprocal orientation of ligands and flavin
plane. For clarity, the dioxygen species has been omitted.
Dashed lines represent H-bonds. (A) D-Ala is viewed along its N-
 C axis; the
electron density is shown at 2  . (B)
D-F[3]-Ala (Upper) and L-lactate (Lower). The green trace
represents the ideal line connecting the flavin N(5) and the
ligand C centers
(distance  3.2 Å).
Note that the C---H
function (grey) of L-lactate points away from the flavin. With
L-lactate, the position of the ---H
results from H-inclusion in the refinement. In the case of
D-F[3]-Ala (A), the number of observations (1.72 Å) does
not allow positioning. The strong H-bond interactions with the
-NH[2]/OH,
together with the electrostatic interaction of the substrate
carboxylate group with Arg-285, Tyr-238, and Tyr-223, provide
the rationale for substrate D-specificity in that they prevent
binding of the L-amino acid in a productive manner.
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