The structure of human 5'-deoxy-5'-methylthioadenosine phosphorylase at 1.7 A resolution provides insights into substrate binding and catalysis.
BACKGROUND
5'-Deoxy-5'-methylthioadenosine phosphorylase (MTAP) catalyzes the reversible phosphorolysis of 5'-deoxy-5'-methylthioadenosine (MTA) to adenine and 5-methylthio-D-ribose-1-phosphate. MTA is a by-product of polyamine biosynthesis, which is essential for cell growth and proliferation. This salvage reaction is the principle source of free adenine in human cells. Because of its importance in coupling the purine salvage pathway to polyamine biosynthesis MTAP is a potential chemotherapeutic target.


RESULTS
We have determined the crystal structure of MTAP at 1.7 A resolution using multiwavelength anomalous diffraction phasing techniques. MTAP is a trimer comprised of three identical subunits. Each subunit consists of a single alpha/beta domain containing a central eight-stranded mixed beta sheet, a smaller five-stranded mixed beta sheet and six alpha helices. The native structure revealed the presence of an adenine molecule in the purine-binding site. The structure of MTAP with methylthioadenosine and sulfate ion soaked into the active site was also determined using diffraction data to 1.7 A resolution.


CONCLUSIONS
The overall quaternary structure and subunit topology of MTAP are similar to mammalian purine nucleoside phosphorylase (PNP). The structures of the MTAP-ligand complexes provide a map of the active site and suggest possible roles for specific residues in substrate binding and catalysis. Residues accounting for the differences in substrate specificity between MTAP and PNP are also identified. Detailed information about the structure and chemical nature of the MTAP active site will aid in the rational design of inhibitors of this potential chemotherapeutic target. The MTAP structure represents the first structure of a mammalian PNP that is specific for 6-aminopurines.
Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853, USA.
T C
TC
Appleby
M D
MD
Erion
S E
SE
Ealick
D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D015195:Drug Design; D004791:Enzyme Inhibitors; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011683:Purine-Nucleoside Phosphorylase / Q000737:chemistry*; D016415:Sequence Alignment; D013379:Substrate Specificity
D004791:Enzyme Inhibitors; D011683:Purine-Nucleoside Phosphorylase; C021784:5'-methylthioadenosine phosphorylase