By Hudson Roth Spotlight #1
Matsumori, N. et al. Biochemistry 2013, 52, 2410-2418.
Natural products isolated from plants, bacteria, and marine organisms serve as an almost limitless source of new fragrances, flavors, and therapeutics. The marine sponge Theonella sp. serves as the source of the family of bicyclic dodecapeptides known as theonellamides (TNMs). These natural products were first isolated in 1989 and have been shown to exhibit antifungal and anticancer properties. TNMs have also been shown to recognize lipid bilayers containing cholesterol and other 3β-hydroxysterols and induce 1,3-β-D-glucan biosynthesis – both of which likely play a role in its antifungal and cytotoxic effects. However, prior to this study little was known about the chemical basis for this interaction. The authors of this paper sought to understand the means by which TNMs recognize and bind to lipid bilayers containing such 3β-hydroxysterols, information that is important for understanding their biological activity.
The authors isolated theonellamide-A (TNM-A) from Theonella sp. and prepared liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and varying amounts of the following sterols: cholesterol, ergosterol, and epicholesterol (a 3α-hydroxysterol). Kinetic data collected with surface plasmon resonance (SPR) studies reveals a two-step binding process in which the first step is greatly accelerated by the presence of 3β-hydroxysterols, while the second remains unaffected. This suggests an initial interaction between the TNM and the sterols followed by another biochemical change that does not directly involve said sterols. In order to confirm that TNM-A interacts directly with the 3β-hydroxysterols, solid-state 2H NMR was performed using 3-d-sterols. Initially, the sterols exhibit a Pake doublet, indicating rapid rotation within the lipid bilayer; upon addition of TNM-A, the NMR signal for cholesterol and ergosterol, but not epicholesterol, decreases drastically, revealing a decrease in rotation and most likely a direct intermolecular interaction between the 3β-hydroxysterols and TNM.
This study began to reveal the mechanism by which TNMs interact with phospholipid bilayers. Though the exact chemistry was not determined, this study showed TNM-A undergoes a two-step binding process with lipid bilayers that involves a direct interaction with cholesterol and other 3β-hydroxysterols present. Further investigation into the details of how both binding steps occur and TNMs influence membrane structure and function will be necessary to better understand how these molecules exhibit antifungal and cytotoxic effects.