Peptides are short chains of amino acids linked by amide bonds, also known as peptide bonds. Typically, peptides consist of 2 to 50 amino acids, distinguishing them from proteins, which generally comprise 50 or more. Peptides play diverse and important roles in living systems, acting as hormones, neurotransmitters, and antimicrobial agents, influencing growth, immunity, and cellular communication.
Natural Biological Pathways
Living organisms produce peptides primarily through protein synthesis, followed by specific modifications. This begins with gene transcription, where DNA is copied into messenger RNA (mRNA). The mRNA then travels to ribosomes, the cellular machinery for translation, serving as a template for assembling amino acid chains.
During translation, transfer RNA (tRNA) brings amino acids to the ribosome, matching them to mRNA codons. These amino acids are joined by peptide bonds, forming a polypeptide chain. Often, this polypeptide is a precursor, known as a propeptide or a prepropeptide.
These precursor molecules undergo post-translational modification, a step where they are enzymatically cleaved into smaller, functional peptides. Enzymes, often proteases, precisely cut the propeptide at specific sites, releasing the active peptide. This processing ensures the peptide achieves its correct structure and biological activity.
Laboratory Chemical Synthesis
In laboratory settings, peptides are commonly synthesized using Solid-Phase Peptide Synthesis (SPPS). This method, pioneered by Robert Bruce Merrifield, builds the peptide chain by sequentially adding amino acids to a starting amino acid anchored to an insoluble resin. The resin provides a stable platform, allowing for easier purification as excess reagents and byproducts can be simply washed away by filtration.
The SPPS process involves a repetitive cycle. First, the amino group of the amino acid attached to the resin is deprotected to expose a reactive site. This is typically achieved using a base, such as piperidine, to remove a temporary protecting group like Fmoc. Next, a new, protected amino acid is coupled to the exposed amino group, forming a new peptide bond.
Protecting groups in SPPS prevent unwanted reactions at other reactive sites, such as side chains. Once the entire peptide sequence is assembled, the completed peptide is cleaved from the solid resin. This final cleavage often involves strong acids like trifluoroacetic acid (TFA), which also remove any remaining side-chain protecting groups, yielding the desired free peptide.
Biotechnological Production Methods
Peptides can also be produced using biotechnological methods, primarily genetic engineering and recombinant DNA technology. This approach manipulates an organism’s genetic material to direct it to produce a specific peptide. The process begins by identifying or synthesizing the DNA sequence encoding the desired peptide.
This DNA sequence is inserted into a suitable expression vector, often a plasmid. The expression vector acts as a carrier, introducing the peptide-encoding gene into host cells. Common host cells include bacteria (E. coli), yeast, or mammalian cells, chosen based on the peptide’s complexity and desired post-translational modifications.
Once inside the host cells, the gene is expressed, meaning the cellular machinery reads the inserted DNA and produces the peptide. These cells are cultured under controlled conditions to maximize peptide production. This biotechnological method offers advantages, particularly for producing larger quantities of peptides or those that are more complex and may require specific post-translational modifications for their activity, which can be challenging to achieve through chemical synthesis.