Cystathionine γ-synthase catalyses the first reaction specific for methionine biosynthesis in plants, the γ-replacement of the phosphoryl substituent of O-phosphohomoserine by cysteine. A cDNA encoding cystathionine γ-synthase from Arabidopsis thalianahas been cloned and used to overexpress the enzyme in Escherichia coli.The native recombinant enzyme is a homotetramer composed of 53 kDa subunits, each being tightly associated with one molecule of pyridoxal 5´-phosphate that binds at lysine-379 of the protein precursor. The replacement reaction follows a Ping Pong mechanism with a Vmax of 33.6 units/mg and Km values of 2.5 mM and 460 µM for O-phosphohomoserine and cysteine respectively. The protective effect of O-phosphohomoserine against enzyme inactivation by propargylglycine indicated that the Kd for the substrate is approx. 1/2500 of its Km value. Thus most of these biochemical properties are similar to those previously reported for plant and bacterial cystathionine γ-synthases. However, the plant enzyme differs markedly from its enterobacterial counterparts because it catalyses a very faint γ-elimination of O-phosphohomoserine in the absence of cysteine, this process being about 1/2700 as fast as the γ-replacement reaction and approx. 1/1500 as fast as the γ-elimination catalysed by the E. colienzyme. This huge difference could be attributed to the inability of the A. thalianacystathionine γ-synthase to accumulate a long-wavelength-absorbing species that is characteristic for the efficient γ-elimination reaction catalysed by the enterobacterial enzyme.

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