In Vivo Activity, Purification, and Characterization of Shikimate Kinase from Sorghum

Sorghum bicolor stems and shoots were observed to phosphorylate exogenously supplied shikimate, forming a product which could not be distinguished from shikimate 3-phosphate by anion exchange and thin layer chromatography. Upon treatment with phosphatase, this product gave rise to a compound which co-chromatographed with shikimate.Shikimate kinase from stems was separated from phosphatase and ATPase and partially characterized. The stoichiometry of the reaction required equimolar quantities of ATP and shikimate to produce ADP and shikimate 3-phosphate. Maximal enzymic activity was observed near pH 9 in the presence of 11 millimolar MgCl(2). Graphs of reaction velocity as a function of substrate concentration were hyperbolic for both substrates and K(m) values of 0.2 and 0.11 millimolar were calculated for shikimate and ATP, respectively. Shikimate kinase was not inhibited by phenylalanine, tyrosine, or tryptophan, either alone or in combination. Slight inhibition was caused by p-coumarate and greater inhibition by caffeate. Inhibition was also observed in the presence of ADP and AMP, but the reaction velocity was not highly responsive to adenylate "energy charge" in experiments with ADP-ATP mixtures.