Dihydrodipicolinate synthase and Dihydrodipicolinate reductase (DHDPS and DHDPR) are two important enzymes in the diaminopimelate (DAP) pathway; a lysine biosynthesis pathway. Lysine is the limiting amino acid in staple grains such as rice and wheat. DHDPS catalyses the first committed step of the pathway and has been a subject of much investigation in bacteria and plants which have different dimeric arrangements. Both bacterial and plant DHDPS typically exist in a tetrameric “dimer of dimers” formation. However the arrangement of the dimers is structurally different. A structural difference is also observed in DHDPR, the enzyme that catalyses the 2nd committed step of the pathway. In bacteria, it exists as a tetrameric enzyme. However, in plants it exists as a dimeric enzyme. Both of these differences are the result of an evolutionary divergence that occurred at some point in the evolutionary lineage; possibly in algal organisms. This makes these enzymes interesting targets in the evolution of plant and bacterial enzymes.
Chlamydomonas reinhardtii; a green algal DHDPR has been shown to exist in equilibrium between tetramer and dimer in analytical ultracentrifugation (AUC). As this protein lies in the evolutionary lineage between dimeric plant forms and tetrameric bacteria forms, this protein could be a missing link in the evolutionary lineage between the bacterial and plant DHDPR's. Selaginella moellendorffii DHDPS is another subject of interest. This lycophyte DHDPS exhibits dynamic quaternary structure as it exists as a dimer with the allosteric regulator lysine bound but as a tetramer with substrate pyruvate bound. Due to its position on the evolutionary lineage between bacterial and plant type DHDPS', this could prove to be an interesting target in the evolution of plant and bacterial enzymes.