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Table 3 Residues Involved in ADP Binding in Glycosomal and Human PGK<br />

T.brucei Human_1 a Human_2 b In contact with ADP moiety<br />

Ala 242 Gly 238 Gly 238 Adenine<br />

Tyr 245 Phe 241 Tyr 241 Adenine<br />

Lys 259 Leu 256 Leu 256 Adenine<br />

Ala 314 Gly 309 Gly 309 Adenine<br />

Ser 378 Thr 375 Thr 375 β-phosphate<br />

a Somatic PGK [67].<br />

b PGK in spermatogenic cells [68].<br />

Page 377<br />

commercially available. However, its sequence has been determined [67] and appears to be 97%<br />

identical to horse and pig PGK. For completeness it should be mentioned that there is a second human<br />

PGK in testis tissue that is 87% identical to the somatic enzyme [68].<br />

The crystal structures of the apo-enzyme from horse [31] and of the binary complex between pig PGK<br />

and its substrate [32] (Figure 8) are available from the Protein Databank. The substrate was found to<br />

bind to the N-terminal domain of the enzyme. The binding site for ADP is known from the structure of<br />

its binary complex with PGK from B. stearothermophilus [69]. It resides in the Cterminal domain. Since<br />

the substrate and ADP binding sites are 10 Å apart, a hinge-bending motion between the two domains<br />

has been postulated to occur during catalysis [70].<br />

Kinetically glycosomal PGK from T. brucei and mammalian PGK are very similar: the K m values for<br />

ATP are 0.29 and 0.46 and mM, respectively; the K m values for 3-phosphoglycerate are 1.62 and 0.62<br />

mM, respectively (due to the unavailability of the human enzyme the rabbit muscle enzyme was used as<br />

a substitute) [71]. The residues responsible for binding the substrate [32] are identical between human<br />

and glycosomal PGK [72]. However, five of the residues involved in the binding of ADP differ between<br />

the two enzymes (Table 3). Apparently, the biggest difference between human PGK and the parasite<br />

enzyme is the charged residue Lys259, which has the apolar Leu256 as a human counterpart. Molecules<br />

that bind at the ADP binding site and specifically recognize Lys259 might therefore be good starting<br />

points for drug design.<br />

III. Search For New Leads<br />

A. Triosephosphate Isomerase: The Crystallographic Cocktail Soak Approach<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_377.html (1 of 2) [4/5/2004 5:39:43 PM]

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