Sponge derived bromotyrosines: structural diversity through natural combinatorial chemistry.

Sponge derived bromotyrosines are a multifaceted class of marine bioactive compounds that are important for the chemical defense of sponges but also for drug discovery programs as well as for technical applications in the field of antifouling constituents. These compounds, which are mainly accumulated by Verongid sponges, exhibit a diverse range of bioactivities including antibiotic, cytotoxic and antifouling effects. In spite of the simple biogenetic building blocks, which consist only of brominated tyrosine and tyramine units, an impressive diversity of different compounds is obtained through different linkages between these precursors and through structural modifications of the side chains and/or aromatic rings resembling strategies that are known from combinatorial chemistry. As examples for bioactive, structurally divergent bromotyrosines psammaplin A, Aplysina alkaloids featuring aerothionin, aeroplysinin-1 and the dienone, and the bastadins, including the synthetically derived hemibastadin congeners, have been selected for this review. Whereas all of these natural products are believed to be involved in the chemical defense of sponges, some of them may also be of particular relevance to drug discovery due to their interaction with specific molecular targets in eukaryotic cells. These targets involve important enzymes and receptors, such as histone deacetylases (HDAC) and DNA methyltransferases (DNMT), which are inhibited by psammaplin A, as well as ryanodine receptors that are targeted by bastadine type compounds. The hemibastadins such as the synthetically derived dibromohemibastadin are of particular interest due to their antifouling activity. For the latter, a phenoloxidase which catalyzes the bioglue formation needed for firm attachment of fouling organisms to a given substrate was identified as a molecular target. The Aplysina alkaloids finally provide a vivid example for dynamic wound induced bioconversions of natural products that generate highly efficient chemical weapons precisely when and where needed.