Crystal Structure of Rabbit Cytosolic Serine Hydroxymethyltransferase at 2.8 Å Resolution:  Mechanistic Implications

• Published in: Biochemistry (Easton) 1999-06, Vol.38 (26), p.8347-8358
• Main Authors: Scarsdale, J. N, Kazanina, G, Radaev, S, Schirch, V, Wright, H. T
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino Sugars - chemistry; Animals; Binding Sites; Crystallization; Crystallography, X-Ray; Cytosol - enzymology; Dimerization; Glycine Hydroxymethyltransferase - chemistry; Humans; Hydrogen Bonding; Models, Molecular; Molecular Sequence Data; Pyridoxal Phosphate - chemistry; Rabbits; Sequence Homology, Amino Acid; Sheep; Structure-Activity Relationship; Substrate Specificity
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi9904151

Serine hydroxymethyltransferase (SHMT) catalyzes the reversible cleavage of serine to form glycine and single carbon groups that are essential for many biosynthetic pathways. SHMT requires both pyridoxal phosphate (PLP) and tetrahydropteroylpolyglutamate (H4PteGlu n ) as cofactors, the latter as a carrier of the single carbon group. We describe here the crystal structure at 2.8 Å resolution of rabbit cytosolic SHMT (rcSHMT) in two forms:  one with the PLP covalently bound as an aldimine to the Nε-amino group of the active site lysine and the other with the aldimine reduced to a secondary amine. The rcSHMT structure closely resembles the structure of human SHMT, confirming its similarity to the α-class of PLP enzymes. The structures reported here further permit identification of changes in the PLP group that accompany formation of the geminal diamine complex, the first intermediate in the reaction pathway. On the basis of the current mechanism derived from solution studies and the properties of site mutants, we are able to model the binding of both the serine substrate and the H4PteGlu n cofactor. This model explains the properties of several site mutants of SHMT and offers testable hypotheses for a more detailed mechanism of this enzyme.