11.3 Functional Peptides 223apoptosis in mammalian cells [76], or to establish gene networks for synthetic
biology [5, 17]. Further well‐characterized potyvirus proteases are the plum pox
virus (PPV) protease, which recognizes the 7 aa sequence NVVVHQ/A [80] and
the tobacco vein mottling virus (TVMV) protease being specific for ETVRFQG/S
[81]. While PPV and TVMV proteases have been used for processing of fusion
proteins in vivo and in vitro [82, 83], they have not yet been extensively explored
as tools for systemic functionality studies of target proteins in vivo. However,
potyvirus proteases are orthogonal to each other, meaning they cannot recognize
the cleavage sites of each other efficiently [74, 84], which might facilitate com-
bined employment in vivo, for example, for synthetic posttranslational modifica-
tion networks.
11.3.4 Reactive Peptides
All before mentioned peptides offer “passive” functions like “binding” or “being
recognized” and are acted upon by separately encoded enzymes. Peptides, which
encode an “active” function, for example, catalytic activity, constitute an interest-
ing expansion to the functional portfolio of peptides, especially when these activ-
ities could be added in an orthogonal manner to the activity of an already
functional scaffold protein.
One outstanding example of a reactive peptide is the 13 aa SpyTag peptide that
rapidly forms an isopeptide bond between a peptide‐internal lysine and an aspar-
tate residue in its target protein SpyCatcher (138 aa, 15 kDa) [85, 86]. In the dem-
onstrated setting, SpyTag was reactive irrespective of the location in the scaffold
protein (terminal or internal). Recently, a second orthogonal isopeptide bond
forming peptide tag/target protein pair (SnoopTag/SnoopCatcher) was intro-
duced that is completely orthogonal to the SpyTag/SpyCatcher system [87]. Both
pairs have been used together to build synthetic polyproteins [87], to design opti-
mized vaccines [88], and to assemble bioactive protein hydrogels [89]. The
hydrogel assembly was achieved by combining internal and terminal SpyTag
insertions.
11.3.5 Pharmaceutically Relevant Peptides: Peptide Epitopes,
Sugar Epitope Mimics, and Antimicrobial Peptides
The structural flexibility of proteins to accept additional residues makes them
suitable scaffolds to structurally stabilize or protect peptides from degradation.
While the functional peptides that were discussed before can be seen as tools to
facilitate the study of properties or the (modulation of the) in vivo behavior of a
certain protein of interest, the functional peptides of the following section will
themselves be the actual targets of interest, and the protein in which it is
inserted is a tool to facilitate its production or application. This section is only
meant to give a notion on the diversity of pharmaceutically relevant functions
that can be adopted by peptides and to discuss the potential impact that an
extended knowledge about permissive sites could make to the field of therapeu-
tic peptides. For a broader treatment, the reader is referred to an excellent
recent review [90].