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Page 234<br />

approximately 7 <strong>by</strong> 13 Å and is lined primarily with hydrophobic side chains. This is entirely consistent<br />

with earlier experiments showing that the enzyme has a marked preference for lipophilic substrates<br />

versus polar substrates such as sugars [21].<br />

The catalytic site also suggested a model for the enzyme's chemical mechanism, which is substantially<br />

similar to most other NAD(P)H-dependent oxido reductases. Upon binding of the reduced cofactor, the<br />

enzyme is able to form a ternary complex with the substrate. The pro-R hydride from the C-4 of the<br />

nicotinamide is transferred to the carbonyl carbon of the substrate, which in turn causes the carbonyl<br />

oxygen to abstract a proton from a general acid, which is presumably located on the protein, to form the<br />

alcoholic product. Three proton-donating side chains are located within 6 Å of the C-4 atom in the<br />

NADPH cofactor that could potentially fulfill this role: Tyr48, His110, and Cys298. Since it is not<br />

conserved in other members of the aldo-keto reductase family that exhibit enzymatic activity (Figure 8)<br />

Cys298 was unlikely as a candidate proton donor. The histidine is surrounded <strong>by</strong> several hydrophobic<br />

residues including Val47, Trp79, and Trp111, which would serve to lower the pK a of the side chain,<br />

making it less effective as a proton donor at physiological pHs. The tryrosine, which ordinarily has a pK a<br />

of approximately 11 engages in an interaction with the charged ammonium group of Lys77, which in<br />

turn charge-pairs with Asp43. This network serves to depress the pK a of the phenolic oxygen, increasing<br />

the exchangability of the proton.<br />

Subsequent activity studies involving site-directed mutants support this model [17,22]. The Tyr48<br />

rarrow.gif Phe mutation shows a complete lack of activity while the Asp43 rarrow.gif Asn, Lys77<br />

rarrow.gif Met, His110 rarrow.gif Asn, and Cys298 rarrow.gif Ser showed losses in catalytic<br />

efficiency of approximately 100-, 1000-, 10 6-, and 10-fold respectively when compared with the wildtype<br />

enzyme. These results correlate well with the functions predicted for each residue with the<br />

exception of the histidine.<br />

The structure of the ALR2 holoenzyme showed that the catalytic site was situated atop the nicotinamide<br />

moiety of the NADPH cofactor. The substrate binding site, which would determine the enzyme's<br />

specificity and also presumably bind inhibitors, appeared to be composed of a deep cleft (Figures 3 and<br />

4). It extended away from the catalytic site towards the loop composed of residues between β4 and α4<br />

and the last 20 residues of the carboxy-terminal meander. This hypothesis was supported <strong>by</strong> the<br />

appearance of poorly resolved density that occupied this region, which suggested the presence of an<br />

endogenously bound substrate or inhibitor in the structure of the holoenzyme [16]. Subsequent studies<br />

indicate that this electron density may be a citrate molecule, one of the components included in the<br />

crystallization mixture. Activity studies indicate that citrate is indeed one of the many inhibitors of the<br />

enzyme with a K i in the millimolar range [23].<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_234.html [4/5/2004 5:08:18 PM]

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