Peptides are intermediate between amino acids and proteins. There’s no distinct boundary separating them from proteins. Generally, compounds with fewer than 50 amino acids are referred to as peptides. Two amino acids compose the simplest peptide named dipeptides. There’re only 2 to 20 amino acids in oligopeptides, while polypeptides contain 20 to 50.
Structure of Peptide Bonds and Polypeptide
The covalent peptide bonds are formed enzymatically between two amino acids. One contributes a carboxyl group and lose a hydroxyl, another gives an amino group and lose a hydrogen, consequently a water molecule is produced during this dehydration reaction. In a polypeptide chain, all amino and carboxyl groups linked to the α-carbon participate in peptide bonds, except those at the termini. The end with free amino group is termed N-terminus, while another end that has a free carboxyl group is called C-terminus.
Within the peptide bond (-CO-NH-), carbon and oxygen are linked by a double bond, and other atoms are linked by single bonds. The free rotation of single bonds allows them in different plane theoretically, but resonance among the carbon, nitrogen, and oxygen atoms creates a partial double-bond character to keep all atoms in peptide and the two connected α-carbons in a same plane. The α-carbons are positioned in a trans configuration to minimize steric hindrance from their R-groups. The C-N bond in a peptide measures 0.133 nm. It’s shorter than a typical C-N single bond (0.145 nm) but longer than a C=N double bond (0.125 nm). It reflects a partial double-bond nature.
Physical and Chemical Properties of polypeptide
The small molecular size and simple structure grant oligopeptides properties similar to amino acids. They exhibit definite melting points that above 200°C. In longer peptide, side chains determine their properties. They can coil and fold into various shapes akin to proteins. Hydrophobic groups tend to be hidden internally, while hydrophilic groups are exposed to the external environment. Polypeptides share similar physical properties with proteins. There’s no distinct melting point. The high temperatures will disrupt their native conformation to render them inactive biologically.
Peptides are generally water-soluble because of their polar peptide bonds. The polar or charged side chains give them an excellent water or polar organic solvents solubility. Hydrophobic R-groups make peptides less water-soluble but more soluble in organic solvents. Like amino acids and proteins, peptides are amphoteric molecules. Carboxyl groups on the R-groups release hydrogen ions, to impart a negative charge, while amino groups absorb hydrogen ions to create a positive charge. The net charge is pH-dependent, and zero at a specific pH that is called isoelectric point (PI). As pH values below pI, the net charge is positive. If pH values above pI, the peptide carries negative charges.
Functions of Bio-Peptide, oligopeptide
Animals' small intestines utilize proteins not limited to free amino acids. Oligopeptides containing 2-3 amino acids are most easily absorbed. This absorption pathway avoids competition among different amino acids for the same transport proteins, so short peptides are transferred from the intestine to cells more quickly and easily. Inside intestinal cells, peptides are broken down by enzymes into amino acids that are released into blood for protein synthesis and other physiological functions. Animals fed with peptide-supplemented diets grow more lean muscle than those given amino acid.
Oligopeptides can also complex with metal ions to enhance absorption. For example, trace elements such as iron and zinc are attached to oligopeptides that are transported into the circulatory system. Otherwise, these metal ions will easily precipitate in the alkaline digestive tract environment and result in decreased bioavailability. This characteristic helps improve nutritional intake in organisms.
Additionally, some polypeptides exhibit antimicrobial properties against bacteria, fungi, viruses, and even cancer cells. Typically, antimicrobial peptides rich in lysine and arginine have fewer than 50 amino acids and are. Lipophilicity allows them to adhere to cell membranes to drill pores. Then the intracellular substances leakage causes microbial death. These widely distributed short peptide are a vital defense mechanism for many organisms. They are considered potential new antibiotic candidates, since they have rapid and broad-spectrum antimicrobial action without drug resistance.