Role of Pro-297 in the catalytic mechanism of sheep liver serine hydroxymethyltransferase

Serine hydroxymethyltransferase belongs to the a class of pyri-doxal-5«-phosphate enzymes along with aspartate amino-transferase. Recent reports on the three-dimensional structure of human liver cytosolic serine hydroxymethyltransferase had sug-gested a high degree of similarity between the active-site geome-tries of the two enzymes. A comparison of the sequences of serine hydroxymethyltransferases revealed the presence of several highly conserved residues, including Pro-297. This residue is equivalent to residue Arg-292 of aspartate aminotransferase, which binds the c-carboxy group of aspartate. In an attempt to change the reaction specificity of the hydroxymethyltransferase to that of an aminotransferase and to assign a possible reason for the conserved nature of Pro-297, it was mutated to Arg. The mutation decreased the hydroxymethyltransferase activity signifi