The Crystal Structure of 5′-Deoxy-5′-methylthioadenosine Phosphorylase II from Sulfolobus solfataricus, a Thermophilic Enzyme Stabilized by Intramolecular Disulfide Bonds

The crystal structure of Sulfolobus solfataricus 5′-deoxy-5′-methylthioadenosine phosphorylase II (SsMTAPII) in complex with 5′-deoxy-5′-methylthioadenosine (MTA) and sulfate was determined to 1.45 Å resolution. The hexameric structure of SsMTAPII is a dimer-of-trimers with one active site per monomer. The oligomeric assembly of the trimer and the monomer topology of SsMTAPII are almost identical with trimeric human 5′-deoxy-5′-methylthioadenosine phosphorylase (hMTAP). SsMTAPII is the first reported hexameric member in the trimeric class of purine nucleoside phosphorylase (PNP) from Archaea. Unlike hMTAP, which is highly specific for MTA, SsMTAPII also accepts adenosine as a substrate. The residues at the active sites of SsMTAPII and hMTAP are almost identical. The broad substrate specificity of SsMTAPII may be due to the flexibility of the C-terminal loop. SsMTAPII is extremely thermoactive and thermostable. The three-dimensional structure of SsMTAPII suggests that the unique dimer-of-trimers quaternary structure, a CXC motif at the C terminus, and two pairs of intrasubunit disulfide bridges may play an important role in its thermal stability.