We describe a model for the three-dimensional structure of E. coli ser
ine hydroxymethyltransferase based on its sequence homology with other
PLP enzymes of the alpha-family and whose tertiary structures are kno
wn. The model suggests that certain amino acid residues at the putativ
e active site of the enzyme can adopt specific roles in the catalytic
mechanism. These proposals were supported by analysis of the propertie
s of a number of site-directed mutants. New active site features are a
lso proposed for further experimental testing.