Amino acids and protein structureEdexcel International A Level Biology: Revision notes
Section 1
Amino acid structure
An amino acid has a central carbon atom bonded to four groups: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable R group. There are 20 amino acids in proteins and they differ only in their R group, for example H in glycine and CH₃ in alanine.
R groups can be hydrophobic, hydrophilic, charged or able to form disulfide bridges, and these properties decide how a protein folds.
Section 2
Peptide bonds and condensation
Amino acids join by a condensation reaction between the carboxyl group of one and the amino group of another, forming a peptide bond (a covalent C–N bond) and releasing a molecule of water. Two amino acids make a dipeptide; many make a polypeptide. A chain of n amino acids has (n − 1) peptide bonds.
Peptide bonds are broken by hydrolysis, which adds water.
Section 3
Levels of protein structure
- Primary: the sequence of amino acids in the polypeptide, held by peptide bonds.
- Secondary: coiling into an α-helix or folding into a β-pleated sheet, held by hydrogen bonds between the C=O and N–H groups of the peptide links.
- Tertiary: the further folding into a specific 3D shape, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups.
The primary structure decides the tertiary structure, so a change in one amino acid can change the shape and function of the protein.
Peptide bonds are the primary structure. Heating breaks the weaker bonds that hold the secondary and tertiary structure, not the peptide bonds.
Section 4
Globular and fibrous proteins
Globular proteins are compact and folded into a ball, with hydrophilic R groups on the outside and hydrophobic ones in the middle, so they are soluble. They have metabolic roles such as enzymes and transport. Haemoglobin is globular: it has four polypeptide chains, each with a haem group that binds oxygen.
Fibrous proteins are long and insoluble, with a structural role. Collagen has three polypeptide chains wound into a triple helix, with a glycine at every third position and cross-links between chains, giving high tensile strength in tendons, skin and bone.
Section 5
Recommended practical: biuret test
Add sodium hydroxide solution, then a few drops of dilute copper(II) sulfate solution (or biuret reagent), to the sample. A violet colour shows peptide bonds, so a protein; the solution stays blue if there are none. A single amino acid or dipeptide does not give a positive result, as at least two peptide bonds are needed.
Boiled protein still gives violet, because denaturing does not break peptide bonds. The test is qualitative, not quantitative, unless a colorimeter and standards are used.
Must Know
- Amino acid: amino group, carboxyl group, H and R group
- Condensation forms a peptide bond and water
- Primary: sequence; secondary: α-helix or β-sheet with hydrogen bonds; tertiary: 3D folding
- Globular (haemoglobin): soluble, metabolic. Fibrous (collagen): insoluble, structural
- Biuret: violet means peptide bonds
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Amino acids and protein structure
- All amino acids share a common structure: a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. The simplest amino acid, glycine, has a hydrogen atom as its R group. Alanine has a methyl group, CH₃, as its R group.Describe how a dipeptide is formed from glycine and alanine.2 marks
- Collagen and haemoglobin are both proteins. Collagen forms tough fibres in tendons: each molecule has three polypeptide chains wound round each other, and about one third of its amino acid residues are glycine. Haemoglobin is found inside red blood cells; each molecule has four polypeptide chains, each with a haem group that binds oxygen.Compare the structure and function of a fibrous protein and a globular protein, using collagen and haemoglobin as examples.2 marks
- In sickle-cell disease, a single amino acid in the β polypeptide chain of haemoglobin is changed. A hydrophilic glutamic acid is replaced by a hydrophobic valine, at a position on the outside of the folded chain. The abnormal haemoglobin is much less soluble and, when oxygen concentration is low, its molecules stick together into long fibres.Describe the primary, secondary and tertiary structure of a protein.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).