Structural and biochemical perspectives in plant isoprenoid biosynthesis

The isoprenoid family represents one of the most ancient and widespread classes of structurally and functionally rich biomolecules known to man. Although these natural products are synthesized in all organisms, the plant kingdom exhibits tremendous variation in their chemistry and roles, ranging from primary metabolism to secondary metabolism and specialized ecological interactions with the environment. Despite enormous diversity in structure and function, all isoprenoids are derived from the universal C-5 precursor isoprene. The isoprenoid biosynthetic pathway has three major stages, viz., (1) synthesis of the isoprene building blocks, followed by their (2) assembly into flexible linear and branched hydrocarbon substrates, which then undergo (3) multistep reaction cascades to generate the vast assortment of isoprenoid end products. One of the most interesting aspects of isoprenoid biosynthesis is its being finely tuned by a multilayered and complex regulatory network, which excellently controls the machinery producing one of the most heterogeneous groups of molecules in plants. Terpene synthases, enzymes of the final stage, are key players in the generation of isoprenoid diversity, catalyzing one of the most complex reactions known to chemistry and biology. Unraveling the mechanism by which a minimal pool of substrates is thus converted into tens of thousands of regiospecific and stereospecific products, is a promising research avenue: This knowledge may be practically used for rational design of novel compounds by metabolic engineering, in order to yield plants with improved nutritional efficacy, stress resistance, bio-pharmaceutical properties etc. This review is an attempt to summarize the biochemical, molecular, physiological, structural, genomic and evolutionary aspects of isoprenoid biosynthesis, providing new insights into how these enzymes utilize various innovative strategies for creation of the so-called final terpenome.