KINETIC CHARACTERISTICS, SUBSTRATE-SPECIFICITY AND CATALYTIC PROPERTIES OF PHOSPHOSERINE AMINOTRANSFERASE FROM THE GREEN-ALGA SCENEDESMUS-OBLIQUUS, MUTANT C-2A'

Phosphoserine aminotransferase (EC 2.6.1.52) has been purified from Sc enedesmus obliquus, mutant C-2A', as reported previously (Stolz and Do rnemann, 1994). The current studies on its catalytic properties, invol ving initial reaction velocities as a function of the phosphoserine co ncentration at various fixed concentrations of 2-oxoglutarate as amino acceptor, indicate a bi-bi ping pong mechanism. The application of a v ariety of substrate analogues of phosphoserine revealed no significant metabolisation of these compounds and thus a considerable specificity of the enzyme. 4,5-dioxovalerate with glutamate as aminodonor is effe ctive as competitive substrate to phosphohydroxypyruvate in the forwar d reaction and yields 5-aminolevulinate. 4,5-Dioxovalerate and glutama te-1-semialdehyde can both serve as competitive aminoacceptor in the r everse reaction with phosphoserine and as substrate with 2-oxoglutarat e as aminoacceptor. Comparison of the phosphoserine transamination wit h the transamination of 3,5-dioxovalerate revealed for both reactions a pH-optimum of 6.5-7.0 in Mes/Bis-Tris-buffer; However, the K-m-value s and the V-max for phosphoserine and 2-oxoglutarate on the one side, and 4,5-dioxovalerate and glutamate on the other were found to differ by orders of magnitude.