Peptides are naturally occurring short chains of molecules called amino acids. Peptides are widely present in all living organisms and play a role in various biological processes. These molecules are considered smaller versions of proteins, contributing to the body’s overall function and regulation.
Amino Acids: The Fundamental Units
Amino acids serve as the basic structural units that combine to form peptides. Each amino acid shares a common core structure, which includes a central carbon atom, known as the alpha-carbon. This alpha-carbon is covalently bonded to four different groups: an amino group (–NH2), a carboxyl group (–COOH), a single hydrogen atom, and a variable side chain.
The unique R-group distinguishes the 20 commonly occurring amino acids. This side chain can vary in its chemical composition, size, and electrical charge. For instance, some R-groups are simple hydrogen atoms, while others are complex carbon-based rings or chains containing various functional groups.
The diversity of these R-groups imparts distinct chemical and physical properties to each amino acid. Some amino acids have nonpolar, hydrophobic R-groups, meaning they tend to repel water and are often found buried within the interior of a peptide structure. Other R-groups are polar and hydrophilic, readily interacting with water and located on the exterior of a peptide.
Additionally, some R-groups carry a positive or negative charge, making them acidic or basic, respectively. These charged R-groups are important for forming ionic bonds and interacting with other charged molecules within biological systems. The specific arrangement and properties of these R-groups within a peptide chain dictate its overall shape, function, and interactions with other molecules.
Connecting Amino Acids: The Peptide Bond
Amino acids link together to form peptides through a covalent bond known as a peptide bond. This bond forms between the carboxyl group of one amino acid and the amino group of an adjacent amino acid. The formation of a peptide bond involves a chemical reaction called dehydration synthesis, where a molecule of water is released as a byproduct.
During this reaction, the hydroxyl group (–OH) from the carboxyl end of one amino acid combines with a hydrogen atom (–H) from the amino end of another amino acid. This results in the formation of a new carbon-nitrogen bond and the expulsion of a water molecule. This process can be repeated, allowing amino acids to connect sequentially and form a growing chain.
As more amino acids are added, the chain lengthens, creating a peptide. Depending on the number of amino acids linked, these chains are given specific names:
A dipeptide consists of two amino acids.
A tripeptide has three.
An oligopeptide typically contains between two and twenty amino acids.
When the chain extends beyond approximately 20 amino acids, it is often referred to as a polypeptide.
Peptide Diversity and Biological Roles
The variety in peptides stems from the combinations and sequences of the 20 distinct amino acids. Even a small change in the order of amino acids can lead to a peptide with different properties and functions. This sequential arrangement determines the peptide’s unique three-dimensional structure, which is directly related to its biological activity.
Peptides are shorter chains of amino acids, typically ranging from 2 to 100 amino acids in length. In contrast, proteins are much longer, more complex chains, consisting of 50 or more amino acids, and they fold into intricate three-dimensional structures necessary for their biological roles. While the terms are used interchangeably, proteins have higher molecular weights and more elaborate structures than peptides.
Peptides perform diverse functions throughout the body. Many act as hormones, such as insulin, which helps regulate blood sugar by allowing cells to absorb sugars. Other peptides function as neurotransmitters, transmitting signals between nerve cells in the brain and nervous system. Some peptides exhibit antimicrobial properties, helping to defend the body against pathogens. Peptides also serve as signaling molecules, facilitating communication between cells and tissues, and can even be components or modulators of enzymes, influencing biochemical reactions.