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.