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netLibrary - eBook Summary Structure-based Drug Design by ...

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Figure 3<br />

C α trace of the ALR2 holoenzyme looking down the (β/α) 8 barrel. The NADPH<br />

cofactor is seen bound across the carboxy-terminal end of the β-barrel with the active<br />

nicotinamide moiety in the center. Figure produced using the MOLSCRIPT<br />

program [48].<br />

Page 233<br />

at the center of the barrel while the adenosine extends away to bind between α7 and α8. A belt<br />

composed of residues 213 to 227 folds over the pyrophosphate of the NADPH to sequester a large part<br />

of the cofactor from the solvent. It is fastened to the other side of the NADPH binding site via Asp216<br />

on the loop that forms bifurcated salt links with Lys21 and Lys262. The dominant interactions holding<br />

the coenzyme in place are directional hydrogen bonds and salt links from positively charged side chains<br />

to the phosphates. The interaction between the 2' phosphate on the NADPH and the side chains from<br />

Lys262 and Arg268 account for the enzyme's preference for NADPH over NADH. Earlier biochemical<br />

studies had shown that it is the 4-pro-R hydride that is transferred from the nicotinamide to the substrate<br />

[20]. This is ensured <strong>by</strong> a hydrogen-bonding network using side chains from Ser159 and Asn160 and the<br />

main chain of Gln183 to orient the amide. It is also determined <strong>by</strong> the stacking interactions with Tyr209,<br />

which is adjacent to the 4-pro-S side of the nicotinamide.<br />

B. Mechanism<br />

The catalytic site was unambiguously identified using the location of the nicotinamide moiety of the<br />

NADPH cofactor in the holoenzyme structure. The region surrounding the catalytic site is a 12-Å deep<br />

groove that measures<br />

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