Shikimate kinase from spinach chloroplasts : purification, characterization, and regulatory function in aromatic amino Acid biosynthesis

Shikimate kinase was purified to near homogenity from spinach Spinacia oleracea L. chloroplasts and found to consist of a single 31 kilodalton polypeptide. The purified enzyme was unstable, but could be stabilized by a variety of added proteins, including oxidized and reduced thioredoxins. Whereas the isolated enzyme was stimulated by mono- and dithiol reagents, the enzyme in intact chloroplasts was unaffected by added thiols and showed only minor response to dark/light transitions. These results indicate that the previously reported stimulation of shikimate kinase activity by reduced thioredoxins is due to enzyme stabilization rather than to activation. In the current study, the purified enzyme was inhibited by added ADP and showed a strong response to energy charge. When intact chloroplasts were incubated in the dark in presence of shikimate, phosphoenolpyruvate and a source of ATP (dihydroxyacetone phosphate or ATP itself under appropriate conditions), aromatic amino acids were formed: phenylalanine and tyrosine. The data indicate that energy charge plays a role in regulating shikimate kinase, thereby controlling the shikimate pathway. An unidentified enzyme of the latter part of the pathway, leading from shikimate-3-phosphate to phenylalanine, appears to be activated by light.