Synthesis and Structural Characterization of Ricin Inhibitors Targeting Ribosome Binding Using Fragment-Based Methods and Structure-Based Design

Ricin toxin A subunit (RTA) is the catalytic subunit of ricin, which depurinates an adenine from the sarcin/ricin loop in eukaryotic ribosomes. There are no approved inhibitors against ricin. We used a new strategy to disrupt RTA-ribosome interactions by fragment screening using surface plasmon resonance. Here, using a structure-guided approach, we improved the affinity and inhibitory activity of small-molecular-weight lead compounds and obtained improved compounds with over an order of magnitude higher efficiency. Four advanced compounds were characterized by X-ray crystallography. They bind at the RTA-ribosome binding site as the original compound but in a distinctive manner. These inhibitors bind remotely from the catalytic site and cause local conformational changes with no alteration of the catalytic site geometry. Yet they inhibit depurination by ricin holotoxin and inhibit the cytotoxicity of ricin in mammalian cells. They are the first agents that protect against ricin holotoxin by acting directly on RTA.

Automated summary

In this study, a new strategy was used to disrupt RTA-ribosome interactions and improve the affinity and inhibitory activity of small-molecular-weight lead compounds by fragment screening and structure-guided approaches. The lead compounds were further optimized to obtain improved compounds with over an order of magnitude higher efficiency, which bind at the RTA-ribosome binding site in a distinctive manner and inhibit ricin activity by causing local conformational changes without altering the catalytic site geometry. These compounds represent the first agents that can protect against ricin holotoxin by acting directly on RTA.