Crystal structure of rabbit cytosolic serine hydroxymethyltransferase at 2.8 angstrom resolution: Mechanistic implications

Serine hydroxymethyltransferase (SHMT) catalyzes the reversible cleavage of serine to form glycine and single carbon groups that are essential for man y biosynthetic pathways. SHMT requires both pyridoxal phosphate (PLP) and t etrahydropteroylpolyglutamate (H(4)PteGlu(n)) as cofactors, the latter as a carrier of the single carbon group, We describe here the crystal structure at 2.8 Angstrom resolution of rabbit cytosolic SHMT (rcSHMT) in two forms: one with the PLP covalently bound as an aldimine to the NE-amino group of the active site lysine and the other with the aldimine reduced to a seconda ry amine, The rcSHMT structure closely resembles the structure of human SHM T, confirming its similarity to the alpha-class of PLP enzymes. The structu res reported here further permit identification of changes in the PLP group that accompany formation of the geminal diamine complex, the first interme diate in the reaction pathway. On the basis of the current mechanism derive d from solution studies and the properties of site mutants, we are able to model the binding of both the serine substrate and the H(4)PteGlu(n) cofact or, This model explains the properties of several site mutants of SHMT and offers testable hypotheses for a more detailed mechanism of this enzyme.