Terpene biosynthesis: modularity rules

Terpenes are the largest class of small-molecule natural products on earth, and the most abundant by mass. Here, we summarize recent developments in elucidating the structure and function of the proteins involved in their biosynthesis. There are six main building blocks or modules (α, β, γ, δ, ε, and ζ) that make up the structures of these enzymes: the αα and αδ head-to-tail trans-prenyl transferases that produce trans-isoprenoid diphosphates from C(5) precursors; the ε head-to-head prenyl transferases that convert these diphosphates into the tri- and tetraterpene precursors of sterols, hopanoids, and carotenoids; the βγ di- and triterpene synthases; the ζ head-to-tail cis-prenyl transferases that produce the cis-isoprenoid diphosphates involved in bacterial cell wall biosynthesis; and finally the α, αβ, and αβγ terpene synthases that produce plant terpenes, with many of these modular enzymes having originated from ancestral α and β domain proteins. We also review progress in determining the structure and function of the two 4Fe-4S reductases involved in formation of the C(5) diphosphates in many bacteria, where again, highly modular structures are found.