Conspectus
Although macrocyclic peptides bearing exotic
building blocks have
proven their utility as pharmaceuticals, the sources of macrocyclic
peptide drugs have been largely limited to mimetics of native peptides
or natural product peptides. However, the recent emergence of technologies
for discovering de novo bioactive peptides has led
to their reconceptualization as a promising therapeutic modality.
For the construction and screening of libraries of such macrocyclic
peptides, our group has devised a platform to conduct affinity-based
selection of massive libraries (>1012 unique sequences)
of in vitro expressed macrocyclic peptides, which
is referred to as the random nonstandard peptides integrated discovery
(RaPID) system. The RaPID system integrates genetic code reprogramming
using the FIT (flexible in vitro translation) system,
which is largely facilitated by flexizymes (flexible tRNA-aminoacylating
ribozymes), with mRNA display technology.
We have demonstrated
that the RaPID system enables rapid discovery
of various de novo pseudo-natural peptide ligands
for protein targets of interest. Many examples discussed in this Account
prove that thioether-closed macrocyclic peptides (teMPs) obtained
by the RaPID system generally exhibit remarkably high affinity and
specificity, thereby potently inhibiting or activating a specific
function(s) of the target. Moreover, such teMPs are used for a wide
range of biochemical applications, for example, as crystallization
chaperones for intractable transmembrane proteins and for in vivo recognition of specific cell types. Furthermore,
recent studies demonstrate that some teMPs exhibit pharmacological
activities in animal models and that even intracellular proteins can
be inhibited by teMPs, illustrating the potential of this class of
peptides as drug leads.
Besides the ring-closing thioether linkage
in the teMPs, genetic
code reprogramming by the FIT system allows for incorporation of a
variety of other exotic building blocks. For instance, diverse nonproteinogenic
amino acids, hydroxy acids (ester linkage), amino carbothioic acid
(thioamide linkage), and abiotic foldamer units have been successfully
incorporated into ribosomally synthesized peptides. Despite such enormous
successes in the conventional FIT system, multiple or consecutive
incorporation of highly exotic amino acids, such as d- and
β-amino acids, is yet challenging, and particularly the synthesis
of peptides bearing non-carbonyl backbone structures remains a demanding
task. To upgrade the RaPID system to the next generation, we have
engaged in intensive manipulation of the FIT system to expand the
structural diversity of peptides accessible by our in vitro biosynthesis strategy. Semilogical engineering of tRNA body sequences
led to a new suppressor tRNA (tRNAPro1E2) capable of effectively
recruiting translation factors, particularly EF-Tu and EF-P. The use
of tRNAPro1E2 in the FIT system allows for not only single
but also consecutive and multiple elongation of exotic amino acids,
such as d-, β-, and γ-amino a...