Jn. Scarsdale et al., Crystal structure at 2.4 angstrom resolution of E-coli serine hydroxymethyltransferase in complex with glycine substrate and 5-formyl tetrahydrofolate, J MOL BIOL, 296(1), 2000, pp. 155-168
Serine hydroxymethyltransferase (EC 2.1.2.1), a member of the a-class of py
ridoxal phosphate enzymes, catalyzes the reversible interconversion of seri
ne and glycine, changing the chemical bonding at the C-alpha-C-beta bond of
the serine side-chain mediated by the pyridoxal phosphate cofactor. Scissi
on of the C-alpha-C-beta bond of serine substrate produces a glycine produc
t and most likely formaldehyde, which reacts without dissociation with tetr
ahydropteroylglutamate cofactor. Crystal structures of the human and rabbit
cytosolic serine hydroxymethyltransferases (SHMT) confirmed their close si
milarity in tertiary and dimeric subunit structure to each other and to asp
artate aminotransferase, the archetypal alpha-class pyridoxal 5'-phosphate
enzyme. We describe here the structure at 2.4 Angstrom resolution of Escher
ichia coli serine hydroxymethyltransferase in ternary complex with glycine
and 5-formyl tetrahydropteroylglutamate, refined to an R-factor value of 17
.4 % and R-free value of 19.6 %. This structure reveals the interactions of
both cofactors and glycine substrate with the enzyme. Comparison with the
E. coil aspartate aminotransferase structure shows the distinctions in sequ
ence and structure which define the folate cofactor binding site in serine
hydroxymethyltransferase and the differences in orientation of the amino te
rminal arm, the evolution of which was necessary for elaboration of the fol
ate binding site. Comparison with the unliganded rabbit cytosolic serine hy
droxymethyltransferase structure identifies changes in the conformation of
the enzyme, similar to those observed in aspartate aminotransferase, that p
robably accompany the binding of substrate. The tetrameric quaternary struc
ture of Liganded E. coil serine hydroxymethyltransferase also differs in sy
mmetry and relative disposition of the functional tight dimers from that of
the unliganded eukaryotic enzymes. SHMT tetramers have surface charge dist
ributions which suggest distinctions in folate binding between eukaryotic a
nd E. coli enzymes. The structure of the E. coli ternary complex provides t
he basis for a thorough investigation of its mechanism through characteriza
tion and structure determination of site mutants. (C) 2000 Academic Press.