Molecular and biochemical characterization of a serine racemase from Arabidopsis thaliana

A cDNA encoding a homolog of mammalian serine racemase, a unique enzyme in eukaryotes, was isolated from Arabidopsis thaliana and expressed in Escherichia coli cells. The gene product, of which the amino acid residues for binding pyridoxal 5′-phosphate (PLP) are conserved in this as well as mammalian serine racemases, catalyzes not only serine racemization but also dehydration of serine to pyruvate. The enzyme is a homodimer and requires PLP and divalent cations, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, or Ni²⁺, at alkaline pH for both activities. The racemization process is highly specific toward l-serine, whereas l-alanine, l-arginine, and l-glutamine were poor substrates. The V_max/K_m values for racemase activity of l- and d-serine are 2.0 and 1.4 nmol/mg/min/mM, respectively, and those values for l- and d-serine on dehydratase activity are 13 and 5.3 nmol/mg/min/mM, i.e. consistent with the theory of racemization reaction and the specificity of dehydration toward l-serine. Hybridization analysis showed that the serine racemase gene was expressed in various organs of A. thaliana.