Peptides are short strings of amino acids that form an important component of the cell membrane. They are also considered to be “building blocks” of proteins. However, they are less complex than proteins. The structure of a peptide includes both hydrophilic and hydrophobic regions. Depending on the size, shape, and chemistry, the peptide can be transported across the membrane either directly or through a variety of mechanisms.
Aside from their basic functions, peptides are used in various medicines to treat various diseases. Although they carry risks, they can be effective in some instances. If they are administered incorrectly, they can cause harm. Consequently, they are only recommended by certified peptide specialists.
Many peptides are categorized according to the type of chemical cross-links they form. These include covalent and noncovalent. Covalent cross-links are formed with the positively charged groups of the peptide attached to acidic phospholipid headgroups. Noncovalent cross-links are formed with the phosphate group of the lipid.
In addition to their primary function, peptides can be classified according to the type of endocytic pathway they follow. Proline-rich peptides have been known to be highly efficient at cellular uptake, and have been found to be non-cytotoxic. This type of peptide has also been associated with caveolae-mediated endocytosis.
Arginine-rich peptides can be transported through direct translocation, or through clathrin-mediated uptake. This type of peptide has a guanidine group on its arginine that forms bidentate hydrogen bonds with other cell surface components.
One of the most studied peptides is penetratin. It is known to form inverted micelles that facilitate cellular uptake. Recent studies suggest that it can be internalized via a route that involves direct translocation.