Peptides are made of **amino acids** linked together by special chemical bonds called **peptide bonds**. Amino acids are organic molecules that serve as the building blocks of peptides and proteins. Each amino acid has a central carbon atom (called the alpha carbon) bonded to four different groups: an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (often called the R group) that distinguishes one amino acid from another.
When amino acids join to form peptides, the amino group of one amino acid reacts with the carboxyl group of another, releasing a molecule of water and forming a peptide bond. This bond is a type of amide bond that links the carbon atom of the carboxyl group to the nitrogen atom of the amino group. The resulting chain has a free amino group at one end (called the N-terminus) and a free carboxyl group at the other end (called the C-terminus). This directionality is important in how peptides and proteins are synthesized and function.
Peptides can be very short, consisting of just two amino acids (a dipeptide) or three (a tripeptide), or they can be longer chains of dozens of amino acids. When the chain grows beyond about 50 amino acids, it is usually called a polypeptide or protein. The sequence of amino acids in the chain is called the **primary structure**, and it determines the peptide’s properties and biological function.
The peptide bond itself is planar and rigid due to resonance stabilization, which means it does not freely rotate. This rigidity influences how the peptide chain folds and arranges itself in three-dimensional space. The folding leads to higher levels of structure:
– The **secondary structure** involves local shapes like alpha helices and beta sheets formed by hydrogen bonding between backbone atoms.
– The **tertiary structure** is the overall 3D shape of a single peptide chain, stabilized by interactions among the side chains (R groups), such as hydrophobic interactions, ionic bonds, and disulfide bridges.
– The **quaternary structure** occurs when multiple peptide chains (subunits) come together to form a functional protein complex.
The side chains of amino acids vary widely in size, charge, polarity, and chemical reactivity, which gives peptides and proteins their incredible diversity and specificity. Some amino acids have nonpolar side chains that avoid water, while others have polar or charged side chains that interact with water or other molecules. This variety allows peptides to fold into precise shapes and perform a vast array of biological functions, from signaling molecules to enzymes.
In summary, peptides are chains of amino acids linked by peptide bonds. The amino acids themselves consist of a central carbon attached to an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain. The sequence and chemical nature of these amino acids determine the peptide’s structure and function.
