ProteinsEdexcel A-Level Biology B: Revision notes
Section 1
Amino acids and peptide bonds
An amino acid has a central carbon atom bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable R group. There are 20 different R groups, so 20 different amino acids.
Two amino acids join in a condensation reaction: the carboxyl group of one reacts with the amino group of the other, releasing water and forming a peptide bond. Many amino acids joined form a polypeptide; one or more polypeptides folded make a protein. Hydrolysis breaks peptide bonds by adding water.
Worked example: a polypeptide of 120 amino acids has 119 peptide bonds, so 119 water molecules were released.
The R group is not involved in the peptide bond. The bond forms between the amino and carboxyl groups.
Section 2
Bonds that hold protein shape
Three types of bond between R groups hold the folded shape:
- Hydrogen bonds: weak, but very numerous.
- Ionic bonds: between oppositely charged R groups; weak and broken by changes in pH.
- Disulfide bonds: strong covalent bonds between the sulfur atoms of two cysteine residues.
Hydrogen bonds between groups in the main chain also hold secondary structure.
Section 3
Four levels of structure
Primary: the sequence of amino acids, joined by peptide bonds.
Secondary: regular coiling or folding of the chain into α-helices and β-pleated sheets, held by hydrogen bonds between main-chain groups.
Tertiary: the overall three-dimensional folding of one polypeptide, held by hydrogen, ionic and disulfide bonds between R groups.
Quaternary: two or more polypeptides associated, sometimes with a non-protein (prosthetic) group.
The primary structure determines everything above it, because the sequence decides which R groups can bond.
Quaternary structure only exists if the protein has more than one polypeptide chain.
Section 4
Fibrous and globular proteins
Fibrous proteins have long, narrow molecules, are usually insoluble, have repetitive primary structure and are structural (e.g. collagen).
Globular proteins are folded into compact, spherical shapes. Hydrophilic R groups face outwards and hydrophobic ones inwards, so they are soluble. They have metabolic roles such as enzymes, hormones and transport (e.g. haemoglobin).
Section 5
Collagen and haemoglobin
Collagen: three polypeptide chains wound into a triple helix. Every third amino acid is glycine, which is very small, so the chains pack tightly. Hydrogen bonds between chains and covalent cross-links between molecules make fibres with very high tensile strength. It is found in tendons, skin and artery walls.
Haemoglobin: a globular protein with four polypeptide chains, each containing a haem group with an iron ion (Fe²⁺) that binds one oxygen molecule. Its hydrophilic outer surface makes it soluble, so it can be packed into red blood cells to carry oxygen.
Collagen is not globular and not soluble. Do not describe haemoglobin as having a triple helix.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Proteins
- A biochemist synthesises a short polypeptide in the laboratory by joining amino acids together one at a time. She is checking that the bonds formed are the same as those found in proteins made in cells.Explain what is meant by the primary structure of the polypeptide and why a change in the primary structure could change the function of the protein.2 marks
- Haemoglobin is the protein in red blood cells that carries oxygen. It is soluble in the cytoplasm of the red blood cell and each molecule can carry up to four oxygen molecules.Explain why haemoglobin is soluble in the cytoplasm of a red blood cell.2 marks
- Tendons connect muscle to bone and must resist large pulling forces without stretching. The main protein in a tendon is collagen, which is a fibrous protein, whereas haemoglobin in the blood is a globular protein.Explain how the structure of collagen makes it suitable for its role in a tendon.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).