1.3 Methodology for Stabilizing Peptide Secondary Structures
9
structures. This flexible structure makes the interface peptide easy to be degraded by
the enzymes, and at the same time, it is not conducive to the binding of the target
protein because of great entropy consumption during binding [47, 48].
Furthermore, most of the short linear peptides lack sufficient membrane penetration ability. A large number of researches suggested that helical conformation could
help peptide cross the cell membrane. To increase the cell penetration of peptides,
methods for enforcing peptides in helical conformation have been established. Introducing amino acids that easily form α helix, such as Aib [49], and covalently coupling
the side chains of amino acids at positions i, i + 4, and i + 7 in peptides, are commonly
used methods to enhance peptide helicity. The early reported cross-coupling methods
include disulfide bond formed by two cysteines [50], or amide bridge formed by lysine
and glutamic acid or aspartic acid [51]. Date back to the beginning of this century,
a new method named all-hydrocarbon stapled peptides that were yielded by olefin
metathesis reaction is reported by Professor Verdine et al. Compared to constrained
peptides generated by other cross-coupling methods, the all-hydrocarbon stapled
peptides exhibit higher structural rigidity. Moreover, the hydrophobic crosslinker
makes the peptides greater cell permeability [52].
In addition to ruthenium-catalyzed metathesis, other chemical bioconjugation
methods for constructing constrained peptides have also been reported. For example,
the bis(alkylation) of thiol-containing peptides [53–56]. Indeed, cysteine-based
cyclization has become the most important strategy for constructing cyclized
peptides. This method has many advantages. The peptide sequences containing designated cysteines can be synthesized via the recombinant method, which enables large
library screening of highly potent peptide inhibitors for protein targets. Moreover,
the cysteine-based cyclization can be occurred in ambient conditions to avoid the
use of any toxic heavy metal catalyst.
Recent papers indicate that new chemical techniques can construct side chain
‘tethered’ peptides on the surface of phages. In a recently published paper, Wang
et al. demonstrated that olefin-sulfhydryl coupling can be used to construct helical
peptides, which can selectively block the P53/MDM2 interactions and further induce
apoptosis of P53-wild type cancer cells [57]. Pentelute et al. used perfluorobenzene
as a linking group to synthesize a constrained peptide inhibitor of HIV-1 capsid
assembly polyprotein [58, 59]. The side chain tether constructed by perfluorinated
groups is lipophilic and can improve cellular uptake. Muppidi et al. used a series of
aryl and vinyl aryl groups with matching lengths to construct macrocyclic peptides
and studied the relationship between membrane penetration and the linker types [56].
Compared to the ‘inert’ crosslinker without active functional groups, a modifiable
tether possessing one or more modification sites are more attractive for synthesizing
multifunctional peptide inhibitors. Based on this, a series of modifiable linkers were
employed in stapled peptides. The use of modifiable crosslinkers provides possibilities to further optimize the peptides’ bioactivity via a second modification. For
example, Spring et al. developed an i, i + 7 double click stapling method, and based on
this method they synthesized a series of constrained peptides that bind to the MDM2
protein, and demonstrated the binding ability of these peptides to MDM2 protein is at
nanomolar [60]. However, the peptide with the highest binding affinity has a weaker
9
structures. This flexible structure makes the interface peptide easy to be degraded by
the enzymes, and at the same time, it is not conducive to the binding of the target
protein because of great entropy consumption during binding [47, 48].
Furthermore, most of the short linear peptides lack sufficient membrane penetration ability. A large number of researches suggested that helical conformation could
help peptide cross the cell membrane. To increase the cell penetration of peptides,
methods for enforcing peptides in helical conformation have been established. Introducing amino acids that easily form α helix, such as Aib [49], and covalently coupling
the side chains of amino acids at positions i, i + 4, and i + 7 in peptides, are commonly
used methods to enhance peptide helicity. The early reported cross-coupling methods
include disulfide bond formed by two cysteines [50], or amide bridge formed by lysine
and glutamic acid or aspartic acid [51]. Date back to the beginning of this century,
a new method named all-hydrocarbon stapled peptides that were yielded by olefin
metathesis reaction is reported by Professor Verdine et al. Compared to constrained
peptides generated by other cross-coupling methods, the all-hydrocarbon stapled
peptides exhibit higher structural rigidity. Moreover, the hydrophobic crosslinker
makes the peptides greater cell permeability [52].
In addition to ruthenium-catalyzed metathesis, other chemical bioconjugation
methods for constructing constrained peptides have also been reported. For example,
the bis(alkylation) of thiol-containing peptides [53–56]. Indeed, cysteine-based
cyclization has become the most important strategy for constructing cyclized
peptides. This method has many advantages. The peptide sequences containing designated cysteines can be synthesized via the recombinant method, which enables large
library screening of highly potent peptide inhibitors for protein targets. Moreover,
the cysteine-based cyclization can be occurred in ambient conditions to avoid the
use of any toxic heavy metal catalyst.
Recent papers indicate that new chemical techniques can construct side chain
‘tethered’ peptides on the surface of phages. In a recently published paper, Wang
et al. demonstrated that olefin-sulfhydryl coupling can be used to construct helical
peptides, which can selectively block the P53/MDM2 interactions and further induce
apoptosis of P53-wild type cancer cells [57]. Pentelute et al. used perfluorobenzene
as a linking group to synthesize a constrained peptide inhibitor of HIV-1 capsid
assembly polyprotein [58, 59]. The side chain tether constructed by perfluorinated
groups is lipophilic and can improve cellular uptake. Muppidi et al. used a series of
aryl and vinyl aryl groups with matching lengths to construct macrocyclic peptides
and studied the relationship between membrane penetration and the linker types [56].
Compared to the ‘inert’ crosslinker without active functional groups, a modifiable
tether possessing one or more modification sites are more attractive for synthesizing
multifunctional peptide inhibitors. Based on this, a series of modifiable linkers were
employed in stapled peptides. The use of modifiable crosslinkers provides possibilities to further optimize the peptides’ bioactivity via a second modification. For
example, Spring et al. developed an i, i + 7 double click stapling method, and based on
this method they synthesized a series of constrained peptides that bind to the MDM2
protein, and demonstrated the binding ability of these peptides to MDM2 protein is at
nanomolar [60]. However, the peptide with the highest binding affinity has a weaker