Proteins

Biology AS · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Proteins are polymers of amino acids joined by peptide bonds. Every amino acid has the same backbone and a different R group. The order of amino acids decides how the chain folds, and the folded shape decides what the protein can do.

There are four levels of structure. Primary is the sequence of amino acids. Secondary is the coiling or folding of the backbone. Tertiary is the overall 3-D shape of one polypeptide. Quaternary is two or more polypeptides together.

Globular proteins such as haemoglobin are compact, generally soluble and have physiological roles. Fibrous proteins such as collagen are long, generally insoluble and have structural roles.

Rules to remember

Amino acid: a central carbon joined to an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and an R group
Peptide bond: condensation between the carboxyl group of one amino acid and the amine group of the next, releasing one water molecule; hydrolysis breaks it
Secondary structure: α-helix or β-pleated sheet, held by hydrogen bonds between –C=O and –N–H groups of the backbone
Tertiary structure is held by interactions between R groups: hydrogen bonds, ionic bonds, disulfide bonds (covalent, between two cysteines) and hydrophobic interactions
Haemoglobin: two α-globin and two β-globin chains, each with a haem group containing one Fe²⁺ ion; each Fe²⁺ binds one O₂ reversibly, so one molecule carries up to four O₂
Haemoglobin is soluble because hydrophilic R groups face outwards and hydrophobic R groups point inwards
Collagen: three polypeptides in a triple helix with glycine as about every third amino acid; molecules are staggered and joined by covalent cross-links to form fibrils and fibres with high tensile strength

Common mistake

Saying that denaturing a protein changes its primary structure. Heat and pH break the weak bonds of the tertiary structure; the sequence of amino acids and the peptide bonds stay the same.

Worked example

A protein is made of two polypeptide chains, one of 21 amino acids and one of 30 amino acids. How many peptide bonds does it contain, and which level of structure do the two chains together show?

A chain of n amino acids has n − 1 peptide bonds.
First chain: 21 − 1 = 20. Second chain: 30 − 1 = 29.
Total = 20 + 29 = 49 peptide bonds.
More than one polypeptide in a protein is quaternary structure.

Answer: 49 peptide bonds; the two chains together show quaternary structure.

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