Pantothenate Biosynthesis in Higher Plants

Pantothenate (vitamin B-5) is an essential metabolite for all organisms because it is the precursor to the 4'-phosphopantetheine moiety of coenzyme A (CoA) and acyl carrier protein. Pantothenate must be obtained from the diet by animals, but plants, bacteria, and fungi can synthesise it de novo by the condensation of beta-alanine with pantoate, which is synthesised in two steps from alpha-ketoisovalerate, an intermediate in branched chain amino acid biosynthesis. In plants, the first and the last enzymes in this pathway, ketopantoate hydroxymethyltransferase and pantothenate synthetase, have been identified and characterised, but the enzyme responsible for the intermediate step of reduction of ketopantoate to pantoate has not been identified. Similarly, the source of beta-alanine for pantothenate biosynthesis in plants has not been established; we suggest that catabolism of the polyamine, spermidine, is the most likely source of this metabolite. In contrast, all five plant enzymes required to convert pantothenate into CoA have been identified and characterised. Strains of bacteria producing increased quantities of pantothenate have been generated by metabolic engineering, and knowledge of the bacterial pathway has been used in the first attempts to engineer plants with similarly increased pantothenate production.