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 are sometimes called host-defense peptides. They are usually between 12 and 50 amino acids in size and are categorized into four classes based on their structural properties.

1. Helical peptides:

The alpha-helical peptide alamethicin, which can form voltage-gated ion channels in lipid membranes, is a helical peptide.

2. Cyclic peptides:

Some famous examples are amanitins, bacitracin, colistin, cyclotide, and polymyxin.

3. Peptides containing one to several disulfide bridges:

Hepcidin has a beta-sheet-rich structure with four 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 eight conserved cysteine residues.

4. Peptides rich in certain amino acids:

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

Antimicrobial peptides have biological significance and are promising candidates for novel therapeutic agents. Many peptides are reported to have a wide range of biological activities, including use as polypeptide antibiotics, anti-HIV agents, insecticides, and anti-tumor, anti-fouling, anti-microbial, trypsin inhibition, and uterotonic agents 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 these 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 Lys substitution or fluorinated amino acids for regular amino acid substitution have been found useful in breaking the secondary structure and thus to reducing 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 solid-phase synthesis and thereby optimize synthetic conditions.

Short synthetic peptides can be used as nutritional aids and health foods. Compared to intact-protein diets, free-amino-acid diets, and parenteral nutrition support, peptides have advantage in the 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 that bioactive peptides can enhance antioxidant status, antisepsis capacity, immune function, anti-inflammatory capacity, mineral absorption, and appetite.