Food and healthcare applications: antimicrobial peptides

Antimicrobial peptides are found among all classes of life. They are always at low molecular mass (2-5 kDa) and sometimes called host-defense peptides. At usually between 12 and 50 amino acids, these antimicrobial peptides are categorized into four classes based on the structure properties:

1. Helical peptides:

An example is the alpha-helical peptide alamethicin, which can form voltage-gated ion channels in lipid membranes.

2. Cyclic peptides:

Some famous examples are amanitins, bacitracin, colistin, cyclotide, polymyxin, et al.

3. Peptides containing one to several disulfide bridges:

Hepcidin has a beta-sheet rich structure with 4 disulfide bonds. Another good example is a small cysteine-rich cationic protein called defensin, which consists of 18-45 amino acids including six (in vertebrates) to 8 conserved cysteine residues.

4. Peptides rich in certain amino acids:

Peptides rich in certain amino acids such as proline (Apidaecins for example), α-aminoisobutyric acid (Aib), cysteines and so on. Myticin is a novel cysteine-rich antimicrobial peptide.

Antimicrobial peptides are promising candidates for novel therapeutic agents and have biological significance. Many peptides are reported to have a wide range of biological activities, including being used as polypeptide antibiotics, anti-HIV, insecticidal, anti-tumour, antifouling, anti-microbial, trypsin inhibition, and uterotonic activities, to combat pathogenic microorganisms in human and veterinary medicine.

Some peptide synthesis techniques have been optimized to synthesize specific antimicrobial peptides. The physicochemical parameters of the peptides such as net charge, helicity and hydrophobic moment can be modified during the peptide synthesis process. Other strategies including using D-amino acids, Pro to Nlys substitution or fluorinated amino acids for regular amino acid substitution are found to be useful in breaking the secondary structure and thus to reduce hydrophobic interactions. Usually different analytical approaches are used for specific peptide synthesis. Mass spectrometry such as MALDI-TOF, LC-electrospray ionization (ESI), ion trap MS and direct infusion ESI Ion Trap MS are used to identify products of the solid phase synthesis and thereby optimize synthetic conditions.

Short synthetic peptides can be used as nutrition and healthcare foods as well. Compared with intact protein diets, free amino acid diets and parenteral nutrition support, peptides have advantage in stimulation of gut mass, reduction of bacterial translocation, maintenance of gut, improved visceral protein synthesis versus intact protein, maintenance of hepatic function versus TPN, and helping facilitate nitrogen absorption, utilization and tolerance. Recent studies show bioactive peptides can enhance the body’s antioxidant status, antisepsis capacity, immune function, anti-inflammatory capacity, mineral absorption, and appetite.