Mevalonate biosynthesis in plants

Abstract

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR, R-mevalonate: NADP+ oxidoreductase, CoA-acylating, EC 1.1.1.34) catalyzes the reductive conversion of HMG-CoA to mevalonic acid (MVA). This reaction is generally considered as a key controlling step in plant isoprenoid biosynthesis.1–4 However, the role of HMGR in the overall control of plant isoprenoid biosynthesis has not yet been unequivocally established. In spite of the interest in plant HMGR, the molecular characterization of this enzyme was hampered by the fact that it is membrane bound and difficult to purify (see Bach et al.3,5 for literature). That is why many of the molecular properties of plant HMGR did not emerge until the genes encoding the enzyme had been cloned. After characterization of a considerable number of HMGR genes (for literature see Stermer et al.4), it now seems clear that plant HMGR is encoded by multigene families. The number of genes comprising each multigene family varies, depending on the species, ranging from the two genes found in Arabidopsis thaliana6,7 to the at least seven genes reported to occur in potato.4 This is in sharp contrast with animal systems in which the enzyme is encoded by a single gene. The presence of multiple plant HMGR isozymes is consistent with the proposed key role of this enzyme in plant isoprenoid biosynthesis. Up to now, Arabidopsis is the only plant species from which all the HMGR genes have been cloned and characterized.6,7 The simplicity of the Arabidopsis HMGR gene family, together with the well recognized advantages of this plant for molecular and genetic studies, make Arabidopsis an attractive model system to elucidate the regulatory role of HMGR in plant isoprenoid biosynthesis.