Isoleucine is one of the amino acids, which are the building blocks of proteins. The proteins in the food you eat and the proteins in your body all contain variable amounts of isoleucine. Your cells use the isoleucine you eat for several different purposes, including helping to provide your cells with energy.
Isoleucine is very similar in structure to the amino acid leucine. Like all amino acids, it has an acidic group called a carboxylic acid, and a basic group called an amine as part of its structure. Its identity is determined by its side chain, which is a group that varies from one amino acid to the next and determines each amino acid's chemical properties. Isoleucine's side chain has the formula CH(CH3)CH2CH3, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry."
One of the major functions of isoleucine is proteinogenesis in the body, which means it is a building block for the proteins your body cells make. You can't make the structural and functional proteins your body depends on -- including the protein that makes up muscle, the antibodies used by your immune system, and a variety of hormones -- without isoleucine. Isoleucine is an essential amino acid, meaning you have to consume it -- you can't make it from other molecules, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Another important use of isoleucine in the body is the production of energy. You break down isoleucine into a molecule called acetyl-CoA, which is the same molecule you make as you break down sugars. You then further burn acetyl-CoA to produce carbon dioxide, oxygen and a large quantity of energy. You can also make fat from acetyl-CoA, meaning excess isoleucine in the diet -- like other excess energy-providing nutrients -- can lead to body fat storage.
You can also use isoleucine to make two other kinds of molecules: glucose and ketone bodies. Glucose, or blood sugar, typically comes from carbohydrates in your diet. If you're very low on blood sugar, however, your cells engage in a process called gluconeogenesis. You can't make glucose from fats, but you can make it from some amino acids, isoleucine included. Ketone bodies are energy-providing molecules commonly made in some body organs, including the heart.