Proteins

Proteins are the acid-insoluble macromolecules built from amino acids, and they are among the most important and abundant compounds in living things.

Polymers of amino acids

A protein is a polypeptide — a linear chain of amino acids joined one after another by peptide bonds. Every protein is therefore a polymer whose building blocks (monomers) are amino acids.

Because there are twenty different types of amino acid available (alanine, cysteine, proline, tryptophan, lysine and so on), a protein is a heteropolymer — a polymer made from more than one kind of monomer. This is different from a homopolymer, in which a single type of monomer simply repeats a certain number of times.

Essential and non-essential amino acids

The fact that proteins are made of amino acids matters for nutrition. Amino acids fall into two groups:

• Non-essential amino acids — those the body can manufacture for itself.
• Essential amino acids — those the body cannot make and must obtain from the diet.

Since the amino acids in food come from dietary proteins, proteins in our food are the source of the essential amino acids we need for health.

What proteins do

Proteins carry out a wide variety of jobs in living organisms. Some transport nutrients across the cell membrane, some fight infectious organisms, some act as hormones, and some act as enzymes. A few representative examples: