4.2.2 Biology-Oriented Synthesis of Natural Product-Inspired libraries
Waldmann has developed a new concept for the design of combinatorial libraries based on natural products that he calls BIOS.(154-156) This concept is based on the recognition of fundamental and complementary properties of natural products and their protein targets. Nature, through evolution of natural products, has explored only a tiny fraction of the available small-molecule chemical space. The same holds true for the biological targets of natural products, which are mainly proteins. The number of three-dimensional protein folds have been shown to be even more conserved during evolution than the underlying sequences, since topologically similar shapes can be formed by different sequences. Estimates of the number of proteins in humans range between 100 000 and 450 000; the number of topologically different protein folds is actually much lower, with estimates of 600−8 000.(169) Since both the natural product space and the protein structure space explored by Nature are limited in size and highly conserved, these structure spaces have to be highly complementary. Thus, a natural product that is an inhibitor of a specific protein fold represents a biologically validated starting point for the development of closely related structures that may inhibit proteins with similar folds and even allow for the discovery of specificity. These concepts are fundamentally similar to the privileged structure concept,(129) but BIOS has the added dimension of using protein folding patterns as the basis for subsequent screens.