and isoprenoids. Various studies have located FPPS in chloroplasts, mitochondria, cytosol, and peroxisomes. In cholesterol synthesis, the product, farnesyl pyrophosphate, is consumed in a reductive tail-to-tail condensation with another farnesyl pyrophosphate to form a 30-carbon compound called squalene by squalene synthase. Through several more biosynthetic steps, squalene is transformed into lanosterol, a direct precursor for cholesterol. Notably, sterols control FPPS expression through two cis regulatory factors (an inverted CAAT box and SRE-3) in the proximal FPPS promoter. In plants, porphyrin and carotenoids constitute accessory pigments that help capture light in the photosystems. Ubiquinone is a key electron carrier in the electron transport chain of cellular respiration. Isoprenoids are a large group of compounds that serve as biosynthetic precursors for lipids and hormones. Farnesyl and geranyl pyrophosphate also serve as precursors for prenylated proteins. Prenylation is a common type of covalent post-translational modification at C-terminal CaaX motifs that allows proteins to localize to membranes or bind to one another. A notable example of the former is the farnesylation of small G-proteins including Ras, CDC42, Rho, and Rac. The attachment of a hydrophobic aliphatic chain as those present in farnesyl or geranylgeranyl groups allows small G-proteins to tether from membranes and carry out effector functions. Drug targeting FPPS is the target of bisphosphonate drugs such as Fosamax (alendronate) and Actonel (risedronate). Bisphosphonate drugs are commonly prescribed for bone diseases including Paget’s disease, osteolytic metastases, and post-menopausal osteoporosis. Bisphosphonate drugs help maintain bone tissue in osteoporotic patients and reduce blood calcium levels in hypercalcemic patients by inhibiting FPPS in bone-reabsorbing osteoclasts. An FPPS-IPP-risendronate ternary complex demonstrated that risendronate binds to the trinuclear Mg2+ complex and interacts with the hydrophobic pocket in a manner similar to DMAPP. References EC 2.5.1 Enzymes of known structure