Lysine metabolism in higher plants

The essential amino acid lysine is synthesised in higher plants via a pathw ay starting with aspartate, that also leads to the formation of threonine, methionine and isoleucine. Enzyme kinetic studies and the analysis of mutan ts and transgenic plants that overaccumulate lysine, have indicated that th e major site of the regulation of lysine synthesis is at the enzyme dihydro dipicolinate synthase. Despite this tight regulation, there is strong evide nce that lysine is also subject to catabolism in plants, specifically in th e seed. The two enzymes involved in lysine breakdown, lysine 2-oxoglutarate reductase (also known as lysine alpha -ketoglutarate reductase) and saccha ropine dehydrogenase exist as a single bifunctional protein, with the forme r activity being regulated by lysine availability, calcium and phosphorylat ion/dephosphorylation.