Proteins

Section 3 of 4

Your muscles, skin and hair, and the enzymes that run your cells, are made of proteins. A protein is a long chain of amino acids, the molecules with an amine group and an acid group that you met among the functional groups.

Every amino acid has an amine group, -⁠NH₂, and a carboxylic acid group, -⁠COOH, on the same carbon. That carbon also carries a side group, and the side group makes each amino acid different. Glycine's side group is a hydrogen atom. Alanine's is a methyl group, -⁠CH₃. Your body builds its proteins from about 20 kinds of amino acid.

Glycine, C₂H₅NO₂The simplest amino acid. One carbon atom carries both an -NH₂ group and a -COOH group.HOHNCCOHHH
Glycine, C₂H₅NO₂: its side group is a hydrogen atom.
Alanine, C₃H₇NO₂An amino acid. One carbon atom carries an -NH₂ group, a -COOH group, a -CH₃ group and a hydrogen atom. This is L-alanine, the form found in proteins.HOHNCCOHHCHHH
Alanine, C₃H₇NO₂: its side group is a methyl group, -⁠CH₃. Lesson 7 showed it comes in two hands.

The peptide bond

The -⁠COOH of one amino acid reacts with the -⁠NH₂ of the next. The -⁠OH from the acid and an H from the amine leave as a water molecule, and the two amino acids join through an amide link, -⁠CONH⁠-, the same link that holds nylon's chains together. Between amino acids it is called a peptide bond. It is another condensation, like the links in starch and in fats.

Glycine, C₂H₅NO₂The simplest amino acid. One carbon atom carries both an -NH₂ group and a -COOH group.HOHNCCOHHH plus Glycine, C₂H₅NO₂The simplest amino acid. One carbon atom carries both an -NH₂ group and a -COOH group.HOHNCCOHHH
make
Glycylglycine, C₄H₈N₂O₃Two glycine units joined by a peptide bond, an amide link, -CONH-, from the C=O carbon of the first unit to the nitrogen of the second.HOHOHNCCNCCOHHHHH plus Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
Two glycine molecules join into glycylglycine and give off water. The peptide bond is the amide link, -⁠CONH⁠- in the middle.

The new molecule still has an -⁠NH₂ at one end and a -⁠COOH at the other, so more amino acids can join on, one at a time. A chain of many amino acids is called a polypeptide.

From chain to protein

A protein is one or more of these chains, from a few dozen to thousands of amino acids long. The order of the amino acids decides how the chain folds up, and the way it folds decides the protein's job. Insulin, which controls the sugar in your blood, has 51 amino acids. Haemoglobin, which carries oxygen in your blood, has 574, in four chains.

A chain of amino acids folds into a protein Left, twelve coloured beads on a string, each bead an amino acid, the colours standing for different kinds. An arrow labelled folds points right, to the same twelve beads folded up into a compact clump. a chain of amino acids a folded protein folds joined by peptide bonds ready for its job
A chain of amino acids folds into a protein. Each bead stands for one amino acid, and the order of the beads decides how it folds.

Enzymes are proteins that speed up the chemistry in your body. Amylase, which cuts starch into maltose in your mouth, is one. Keratin is a protein whose long chains twist together into strong ropes; your hair and nails are made of it.

Quick check

Glycine and alanine join through a peptide bond. Which groups react to make it?