Amino Acids and Peptide Bonds — Definition

Definition

Imagine proteins as long, intricate chains, much like a string of beads. Each individual bead in this chain is an 'amino acid'. So, amino acids are the basic units, or monomers, that make up proteins, which are large, complex molecules essential for virtually every function in living organisms.

Let's break down an amino acid's structure. At its heart is a central carbon atom, often called the 'alpha-carbon'. Attached to this alpha-carbon are four different groups:

1
An amino group ($-\text{NH}_2$): This group contains nitrogen and acts as a base, meaning it can accept a proton ($H^+$).
2
A carboxyl group ($-\text{COOH}$): This group contains carbon and oxygen and acts as an acid, meaning it can donate a proton ($H^+$).
3
A hydrogen atom ($-\text{H}$): A simple hydrogen atom.
4
A side chain or R-group: This is the most crucial part because it's what makes each of the 20 common amino acids unique. The R-group can be as simple as a hydrogen atom (in glycine) or a complex ring structure. It determines the amino acid's chemical properties – whether it's polar, nonpolar, acidic, or basic – and thus influences how the protein folds and functions.

Because of the amino and carboxyl groups, amino acids can exist as 'zwitterions' at physiological pH, meaning they carry both a positive and a negative charge simultaneously, making them electrically neutral overall.

Now, how do these amino acid beads link up to form a protein chain? They do so through a special type of chemical bond called a 'peptide bond'. A peptide bond is formed when the carboxyl group of one amino acid reacts with the amino group of another amino acid. This reaction is a 'dehydration synthesis' or 'condensation reaction' because a molecule of water is removed in the process.

Once formed, the peptide bond is a very strong and stable covalent bond. It has a unique characteristic: it behaves somewhat like a double bond, meaning there's limited rotation around the C-N bond within the peptide linkage.

This rigidity is incredibly important because it restricts the possible shapes a protein can take, playing a vital role in determining its three-dimensional structure and, consequently, its biological activity.

A chain of amino acids linked by peptide bonds is called a polypeptide, and a functional polypeptide (or multiple polypeptides) is what we call a protein.