ABSTRACT
In plants, the plastids appear to be the major site of biosynthesis of the aromatic amino acids in which all of the enzymes of the pathway can be detected (Mousdale and Coggins 1985). The entry point for the shikimate pathway is 3-deoxy-D-arabino-heptulosonate 7 phosphate synthase (DAHPS; refer to Scheme 1), and it has been under study as a possible herbicide target. This enzyme catalyzes the condensation of o-erythrose-4-phosphate (E4P) with phosphoenolpyruvate (PEP) to form DAHP. The DAHPS-catalyzed reaction is uncommon for PEP-using enzymes, in that it results in C-o, rather than P-o, bond cleavage of PEP, with resulting C-e bond formation. EPSPS catalyzes C-O bond cleavage as well, but results in C-O-C enol ether linkage by a very different chemical mechanism (discussed later). In the DAHPS reaction the C-3 carbon of PEP is the nucleophile that attacks the carbonyl carbon of E4P, whereas the C-2 carbon of PEP undergoes nucleophilic addition of water.
