Thapsigargin

Thapsigargin is non-competitive inhibitor of a class of enzymes known by the acronym SERCA, which stands for sarco / endoplasmic reticulum Ca2+ ATPase. Structurally, thapsigargin is classified as a sesquiterpene lactone, and is extracted from a plant, Thapsia garganica. It is a tumor promoter in mammalian cells. The anti-malarial drug artemisinin is also a sesquiterpene lactone, leading to a proposal that this class of drugs works by inhibiting the SERCA of malaria parasites such as Plasmodium falciparum;  this hypothesis awaits confirmation.

Thapsigargin raises cytosolic calcium concentration by blocking the ability of the cell to pump calcium into the sarcoplasmic and endoplasmic reticula which causes these stores to become depleted. Store-depletion can secondarily activate plasma membrane calcium channels, allowing an influx of calcium into the cytosol.

Thapsigargin is useful in experimentation examining the impacts of increasing cytosolic calcium concentrations.

Potential medical uses of thapsigargin

Thapsigargin is mentioned as a possible treatment for posterior capsular opacification.

Biosynthesis
The complete biosynthesis of thapsigargin has yet to be elucidated. A proposed biosynthetic pathway is shown below. The biosynthesis starts with the common terpene farnesyl pyrophosphate. The first step, I, is controlled by the enzyme germacrene B synthase. For step II, the C(8) position is easily activated for an allylic oxidation due to the position of the double bond. The next step is the addition of the acyloxy moiety by a P450 acetyltransferase; which is a well known reaction for the synthesis of the diterpene, taxol. In step III, the lactone ring is formed by a cytochrome P450 enzyme using NADP+. With the butyloxy group on the C(8), the formation will only generate the 6,12-lactone ring. Step IV is an epoxidation that initiates the last step of the base guaianolide formation. In step V, a P450 enzyme closes the 5 + 7 guaianolide structure. The ring closing is important, because it will proceed via 1,10 - epoxidation in order to retain the 4,5 - double bond needed in thapsigargin. It is not known whether the secondary modifications to the guaianolide occur before, or after the formation of thapsigargin, but will need to be considered when elucidating the true biosynthesis. It should also be noted, that several of these enzymes are P450's, therefore oxygen and NADPH are likely crucial to this biosynthesis as well as other cofactors such as Mg2+ and Mn2+ may be needed.