Amino Acids

Core Concepts

In this article, you will learn about amino acids, what they are used for, and how they contribute to the formation of proteins.

Topics Covered in Other Articles

Key Terms

Peptide bond: a bond resulting from a condensation reaction between two amino acids. The carboxyl group of one amino acid reacts with the amino group of an adjacent amino acid, releasing a molecule of water.
Peptide: a short chain of amino acids linked together by peptide bonds
Polypeptide: a polymer with a defined sequence of amino acids linked together through covalent peptide bonds
Amino acid: the monomer of a protein
Monomer: a single subunit of a chain
Polymer: many subunits linked together
Conformation: the three-dimensional shape of a molecule

What are amino acids?

Amino acids are the building blocks of a larger macromolecule, proteins. They consist of a carbon atom bonded to a hydrogen atom, carboxyl group, amino group, and a variable group.

The COOH on the right side of the image above is the carboxyl group. The carboxyl group is usually important in enzymatic protein transport. On the left side of the image, the H₂N is what is known as the amino group. The large letter R in the center of the image represents the amino acid’s variable group. This group is what makes each amino acid unique from one another It also determines the amino acid’s properties. There exist only 20 different R variations, so there are only 20 types of amino acids.

Twenty Standard Amino Acids — The 20 different amino acids. Note the variable groups, depicted in gray at the bottom of each amino acid.

Polymers and Monomers

In chemistry and biology, a broad term for a single subunit in a chain is a monomer. Monomers are the smallest building blocks. Two monomers put together create a dimer. Thus, dimers are two subunits of a chain. Trimers, likewise, link three subunits (monomers) together. Finally, the polymer consists of many of those subunits linked together.

Each time two monomers link together to form a dimer, they go through a process known as dehydration synthesis. Similarly, two dimers (or larger chains) linking together go through the same process. Dehydration synthesis links two monomers together to form one water molecule.

Amino acids are the monomers of proteins. By removing water through dehydration synthesis reactions, they form long chains of amide bonds.

Peptide bonding is when amino acids link together. When just two amino acids bond, they share a peptide bond. When multiple amino acids bond together, they form a polypeptide.

Amino Acids to Protein

When amino acids form polypeptides, they are on the path to becoming a protein. However, they are not a protein yet. A polypeptide is a protein when it has a three-dimensional shape. There are four levels of protein structure, as detailed below.

Primary Structure

The primary structure is the order of amino acids in the chain, acquired by the DNA. A slight change in the primary structure can change the function of the protein.

Secondary Structure

The secondary structure involves coiling and folding a chain of amino acids. There are multiple possible secondary structures.

A common secondary structure is the alpha helix. The alpha helix is stabilized by a hydrogen bond between every fourth peptide bond. Below is the characteristic shape of an alpha helix.

Secondary Structure of Amino Acids - Alpha Helix — An alpha helix.

The beta sheet is another type of secondary structure. In the beta sheet, non-parallel polypeptide chains are pleated (folded) in an accordion style.

Beta Pleated Sheet of Amino Acids — A beta sheet.

Tertiary Structure

The next level of structure is the tertiary structure. The tertiary structure of a protein is the protein’s three dimensional shape. This shape occurs due to the bonding of side chains, hydrophobic interactions, and van der Waal interactions.

Quaternary Structure

The quaternary structure results from the interactions between several polypeptide chains. Usually, this is a protein with one or more amino acid chains.

In the image below, the quaternary strucuture occurs because of the different color polypeptide chains interacting.

202101 Protein quaternary structure containing amino acid chains — In this diagram, the blue portion and the pink portion represent different amino acid chains.

Functions of Amino Acids and the Proteins They Form

A protein’s shape determines how and if it is going to function. A protein can only work properly if its shape is correct for its function.

The process of denaturation alters the native conformation of a protein. As a result, its function can be dramatically altered. Adding a chemical agent that disrupts hydrogen bonding, getting too hot, or changing pH are all ways to potentially denature proteins.