Structural Determination, Total Synthesis, and Biological Activity of Iezoside, a Highly Potent Ca²⁺-ATPase Inhibitor from the Marine Cyanobacterium Leptochromothrix valpauliae

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a membrane protein on the endoplasmic reticulum (ER) that transports Ca²⁺ from the cytosol into the ER. As its function is associated with various biological phenomena, SERCA has been recognized as a promising druggable target. Here, we report the second-strongest SERCA-inhibitory compound known to date, which we isolated from the marine cyanobacterium Leptochromothrix valpauliae and named iezoside (1). The structure of iezoside (1) is fundamentally different from that of any other SERCA inhibitor, and its potency is the strongest among marine natural products (K_i 7.1 nM). In this article, we report our comprehensive analysis of iezoside (1), which covers its isolation, structural characterization supported by density functional theory (DFT) calculations and statistical analysis, total synthesis, and clarification of the mode of action of its potent antiproliferative activity (IC₅₀ 6.7 ± 0.4 nM against HeLa cells).