Amino acids and protein structureAQA A-Level Biology: Revision notes
Section 1
Amino acids
Amino acids are the monomers from which proteins are made. Every amino acid has a central carbon bonded to an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and an R group (side chain).
The twenty amino acids common to all organisms differ only in their R group. The R group gives each amino acid its own size, charge and ability to bond.
Section 2
Peptide bonds
A condensation reaction between the carboxyl group of one amino acid and the amine group of another forms a peptide bond and eliminates a molecule of water.
- Dipeptide: two amino acids joined.
- Polypeptide: many amino acids joined.
A functional protein may contain one or more polypeptides. Hydrolysis breaks the peptide bonds, using water.
A chain of n amino acids has n − 1 peptide bonds. Haemoglobin has 574 amino acids in four chains, so 574 − 4 = 570 peptide bonds.
Count peptide bonds per chain: each separate polypeptide has one fewer bond than it has amino acids.
Section 3
Primary and secondary structure
- Primary structure: the sequence of amino acids in the polypeptide, joined by peptide bonds. It determines all the higher levels of structure.
- Secondary structure: the polypeptide coils into an α-helix or folds into a β-pleated sheet, held by hydrogen bonds between the peptide groups (C=O and N–H) along the chain.
Section 4
Tertiary and quaternary structure
- Tertiary structure: the polypeptide folds further into a specific three-dimensional shape. Three types of bond hold it in place: hydrogen bonds (many, weak), ionic bonds (between oppositely charged R groups, stronger, broken by pH change) and disulfide bridges (covalent bonds between the sulfur atoms of two cysteine R groups, strong).
- Quaternary structure: two or more polypeptide chains associate to make one functional protein, for example haemoglobin (four chains).
The sequence of amino acids (primary) decides which R groups can bond, so a change in primary structure can alter the shape and function.
Section 5
Structure and function of proteins
The precise three-dimensional shape of a protein allows it to do its job. Proteins have a variety of functions in all living organisms:
- enzymes: an active site complementary to the substrate
- antibodies: binding sites shaped to fit antigens
- transport proteins: for example haemoglobin carries oxygen
- structural proteins and hormones.
Because the shape depends on the bonds, anything that breaks them changes the shape and can stop the protein working.
Section 6
Biuret test for proteins
- Add sodium hydroxide solution to the sample.
- Add a few drops of dilute copper(II) sulfate solution and mix. (Biuret reagent combines both.)
- Purple (lilac) shows protein; blue means no protein.
No heating is needed. The test detects peptide bonds, so free amino acids give no colour change.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Amino acids and protein structure
- Alanine is an amino acid whose R group is a methyl group, CH₃. Cysteine is an amino acid whose R group is CH₂SH, which contains sulfur.Explain why two cysteine molecules in a polypeptide can form a bond between their R groups that two alanine molecules cannot.2 marks
- Haemoglobin is a protein made from four polypeptide chains: two α chains of 141 amino acids each and two β chains of 146 amino acids each.Explain why a change in the sequence of amino acids in a protein can change its function.2 marks
- Ribonuclease is an enzyme made from a single polypeptide of 124 amino acids, folded into a compact three-dimensional shape. Its tertiary structure is held together by hydrogen bonds, ionic bonds and four disulfide bridges.Describe the primary, secondary and tertiary structure of ribonuclease.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).