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M105694200v1
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Papers In Press, published online ahead of print August 6, 2001
J. Biol. Chem, 10.1074/jbc.M105694200
Submitted on June 20, 2001
Revised on August 3, 2001
Accepted on August 6, 2001

Three-dimensional structure of a hyperthermophilic 5'-deoxy-5'-methylthioadenosine phosphorylase from Sulfolobus solfataricus

Todd C. Appleby, Irimpan I. Mathews, Marina Porcelli, Giovanna Cacciapuoti, and Steven E. Ealick

Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853

Corresponding Author: see3{at}cornell.edu

The structure of 5'-deoxy-5'-methylthioadenosine phosphorylase from Sulfolobus solfataricus (SsMTAP) has been determined alone, as ternary complexes with sulfate plus substrates 5'-deoxy-5'-methylthioadenosine, adenosine or guanosine or with the noncleavable substrate analog Formycin B and as binary complexes with phosphate or sulfate alone. The structure of unliganded SsMTAP was refined at 2.5 Å resolution and the structures of the complexes were refined at resolutions ranging from 1.6 to 2.0 Å. SsMTAP is unusual both for its broad substrate specificity and for its extreme thermal stability. The hexameric structure of SsMTAP is similar to that of purine nucleoside phosphorylase (PNP) from Escherichia coli, however, only SsMTAP accepts 5'-deoxy-5'-methylthioadenosine as a substrate. The active site of SsMTAP is similar to that of E. coli PNP with 13 of 18 nearest residues being identical. The main differences are at Thr89, which corresponds to serine in E. coli PNP, and Glu163, which corresponds to proline in E. coli PNP. In addition, a water molecule is found near the purine N7 position in the guanosine complex of SsMTAP. Thr89 is near the 5'-position of the nucleoside and may account for the ability of SsMTAP to accept either hydrophobic or hydrophilic substituents in that position. Unlike E. coli PNP, the structures of SsMTAP reveal a substrate induced conformational change involving Glu163. This residue is located at the interface between subunits and swings in towards the active site upon nucleoside binding. The high-resolution structures of SsMTAP suggest that the transition state is stabilized in different ways for 6-amino versus 6-oxo substrates. SsMTAP has optimal activity at 120º C and retains full activity after 2 hours at 100º C. Examination of the three-dimensional structure of SsMTAP suggests that unlike most thermophilic enzymes, disulfide linkages play a key in role in its thermal stability.


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