Proteins play countless roles throughout the biological world, from catalyzing chemical
reactions to building the structures of all living things.
Despite this wide range of functions, all proteins are made out of the same twenty building
blocks, called amino acids. The way these twenty amino acids are arranged
dictates the folding of the protein into its unique final shape and its function.
Amino acids are made of carbon, oxygen, nitrogen, hydrogen, and sulfur atoms.
These atoms form an amino group, a carboxyl group and a side chain attached to a central
carbon atom. The side chain is the only part that varies from amino acid to amino acid
and determines its properties. Hydrophobic amino acids such as leucine and
isoleucine have carbon rich side chains, which don’t interact well with water.
Hydrophilic amino acids such as serine, or threonine interact well with water. Charged amino acids like
glutamic acid or arginine interact with oppositely charged amino acids or with water.
The primary structure of the protein is the linear sequence of amino acids as encoded
by DNA. The amino acids are joined by peptide bonds,
which link an amino group and a carboxyl group. A water molecule is released each time a bond
is formed. Specific amino acid sequences give proteins
their distinct shapes and chemical characteristics. These protein chains often folds into two
types of secondary structures stabilized by hydrogen bonds.
A protein chain can fold into a rigid alpha helix, forming regular patterns of hydrogen
bonds between the backbone atoms of nearby amino acids.
Backbone atoms of the chain can interact side-by-side to form beta sheets.
Many proteins fold into a compact globular shape, with hydrophobic side chains sheltered
inside away from the surrounding water. The functions of many proteins rely on this
folded structure. For instance, hemoglobin forms a pocket to hold heme, a small molecule
with an iron atom in the center that binds oxygen.
Two or more polypeptide chains can come together to form one functional molecule with several
subunits. The four subunits of hemoglobin cooperate so that the complex can pick up
more oxygen in the lungs and release it in the body.
Many proteins rely on the ability to recognize the shape of specific molecules in order to
function correctly. The flexible arms of antibodies protect the
body from disease by recognizing and binding to foreign molecules and thus preventing the
viral RNA or DNA to enter the cell. Collagen forms a strong triple helix that
is used throughout the body for structural support.
The calcium pump moves ions across cell membranes allowing the synchronized contraction of muscle
cells. The hormone insulin is a small, stable protein
that can easily maintain its shape while traveling through the blood to regulate blood sugar
levels. Alpha amylase is an enzyme with a catalytic
site that begins the breakdown of carbohydrates in our saliva.
Ferritin forms a hollow shell that stores iron from our food.
Learn more about the functions and 3D structures of nucleic acids, proteins, and molecular
machines at the RCSB Protein Data Bank.