THE CRYSTAL-STRUCTURE OF HUMAN CYTOSOLIC SERINE HYDROXYMETHYLTRANSFERASE - A TARGET FOR CANCER-CHEMOTHERAPY

Background: Serine hydroxymethyltransferase (SHMT) is a ubiquitous enz yme found in all prokaryotes and eukaryotes, As an enzyme of the thymi dylate synthase metabolic cycle, SHMT catalyses the retro-aldol cleava ge of serine to glycine, with the resulting hydroxymethyl group being transferred to tetrahydrofolate to form 5,10-methylene-tetrahydrofolat e. The latter is the major source of one-carbon units in metabolism. E levated SHMT activity has been shown to be coupled to the increased de mand for DNA synthesis in rapidly proliferating cells, particularly tu mour cells. Consequently, the central role of SHMT in nucleotide biosy nthesis makes it an attractive target for cancer chemotherapy. Results : We have solved the crystal structure of human cytosolic SHMT by mult iple isomorphous replacement to 2.65 Angstrom resolution. The monomer has a fold typical for a class pyridoxal 5'-phosphate (PLP) dependent enzymes, the tetramer association is best described as a 'dimer of dim ers' where residues from both subunits of one 'tight' dimer contribute to the active site. Conclusions: The crystal structure shows the evol utionary relationship between SHMT and other alpha class PLP-dependent enzymes, as the fold is highly conserved, Many of the results of site -directed mutagenesis studies can easily be rationalised or re-interpr eted in light of the structure presented here. For example, His151 is not the catalytic base, contrary to the findings of others. A mechanis m for the cleavage of serine to glycine and formaldehyde is proposed.