2.1. Direct peptide synthesis versus recombinant protein synthesis
2.2. Protein engineering strategies for designing eECM
As protein technologies continue to evolve, there are now several
toolkits available to achieve reproducible and tunable eECM
with virtually limitless possibilities. To design a multifunctional
eECM, the first step is the selection of primary amino acid sequences
with desirable structural, gelation, degradation or bioactive
roles. Once the desired amino acid sequence has been
designed, two common techniques can be used to synthesize the
eECM: direct chemical synthesis of small peptide building blocks
or recombinant biochemical synthesis of large proteins (Fig. 1). Direct
peptide synthesis is usually realized through solid-phase peptide
synthesis (SPPS) using a peptide synthesizer. The advent of
peptide synthesizers allows for automatic and efficient production
of up to 30–50 amino acid residues in sub-gram quantities at reasonable
cost [49]. These small peptides can be ligated together to
create much longer sequences of up to 150 amino acids [50]. SPPS
ensures precise chemical structures and enables the addition of
modifications to the functional groups of individual amino acids
during synthesis. Liquid-phase peptide synthesis (LPPS) is a classical
method that requires sequential incorporation of amino acids
followed by removal of protecting groups [51]. LPPS is still commonly
used for large-scale production of gram-scale quantities of
a given peptide, although this synthetic route is much slower
and more labor intensive than SPPS.
Recombinant protein synthesis can produce much larger proteins
with more complex structural features, resulting in more
complex functionalities. This biochemical synthesis strategy requires
several genetic engineering steps, each of which may require
optimization to achieve efficient yields of functional
protein. First, a DNA template that encodes the target amino acid
sequence is designed and chemically synthesized. This engineered
gene is then inserted into a plasmid vector that enables gene replication
and transcription. The vector is transformed into a host
organism that expresses the target protein. The target protein is
then harvested and purified from other endogenous proteins and
contaminants. This templated synthesis offers precise control of
long protein sequences with multiple functional modules each presented
at a specific location, facilitating independent tuning of
mechanical properties and bioactivity within the final biomaterial.