Proteins and enzymes — Edexcel International AS Level Biology
Amino acids, peptide bonds and protein structure; globular and fibrous proteins; how enzymes work and the factors that change their rate.
Revision notes
6 short notes, in the order of the specification. Each one in short:
An amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. Amino acid monomers are joined by condensation reactions, which release water and form peptide bonds, to make polypeptides. A protein is one or more polypeptides folded into a specific shape.
The primary structure of a protein is its sequence of amino acids. It determines how the chain coils or folds into the secondary structure (alpha helix or beta pleated sheet) and then the tertiary structure, held by hydrogen, ionic and disulfide bonds. Several polypeptides together form a protein's quaternary structure.
Globular proteins such as haemoglobin are compact, rounded and soluble, with hydrophilic R groups outside, so they carry out metabolic roles. Fibrous proteins such as collagen are long and rope-like, insoluble and strong, with many cross-links, so they provide structural support. Each structure suits its function.
Enzymes are biological catalysts that reduce the activation energy of reactions. A substrate binds to the active site, a region whose three-dimensional shape is complementary to it, forming an enzyme–substrate complex. Intracellular enzymes work inside cells and extracellular enzymes are secreted to work outside cells.
The initial rate of an enzyme-catalysed reaction depends on temperature, pH, enzyme concentration and substrate concentration. Rate rises with temperature to an optimum and then falls as the enzyme denatures. Each enzyme has an optimum pH. More enzyme or substrate raises the rate until another factor becomes limiting.
In core practical 4, you change one factor (temperature, pH, enzyme concentration or substrate concentration) and measure the initial rate of an enzyme-catalysed reaction, for example the volume of oxygen produced by catalase. The initial rate is found from the gradient of the tangent at time zero on a graph of product against time.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
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18 cards: flip them, mark what you knew, and practise the rest.
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The whole of membranes, proteins, dna and gene expression on one page, so you can see where this subtopic fits.
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Quick check questions
Name the bond that links two amino acids.
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A peptide bond (covalent), formed in a condensation reaction that releases water.
Which bonds hold the tertiary structure of a protein?
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Hydrogen bonds, ionic bonds and disulfide bonds (plus hydrophobic interactions).
Why is haemoglobin soluble?
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Hydrophilic R groups face outwards, so it dissolves in the cytoplasm of red blood cells.
What does an enzyme do to the activation energy of a reaction?
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It lowers it.
How is the initial rate of reaction found from a graph?
Show answer
From the gradient of the tangent drawn at time zero on a graph of product against time.
Frequently asked questions
What is the difference between primary, secondary, tertiary and quaternary protein structure?
Primary structure is the sequence of amino acids joined by peptide bonds. Secondary is coiling into an alpha helix or folding into a beta pleated sheet, held by hydrogen bonds. Tertiary is the overall 3D folding, held by hydrogen, ionic and disulfide bonds. Quaternary is two or more polypeptides together.
How are the structures of haemoglobin and collagen related to their functions?
Haemoglobin is globular and soluble, with four polypeptides each holding an iron-containing haem group that binds oxygen. Collagen is fibrous and insoluble: three polypeptides form a triple helix with hydrogen bonds and cross-links, giving great tensile strength for support in tendons and skin.
Why do enzymes denature at high temperatures?
High temperatures give the enzyme molecule more kinetic energy, which breaks the hydrogen and ionic bonds holding the tertiary structure. The active site changes shape, so the substrate can no longer fit, and the enzyme-substrate complex cannot form. This denaturation is not reversible.
What is the activation energy and how do enzymes lower it?
Activation energy is the energy needed to start a reaction. An enzyme lowers it by binding the substrate in its active site and putting strain on its bonds, so the reaction can happen at body temperature. The enzyme is not used up and can catalyse many more reactions.
What are intracellular and extracellular enzymes?
Intracellular enzymes work inside cells, for example catalase and the enzymes of respiration. Extracellular enzymes are made in cells and secreted so that they act outside them, for example amylase and trypsin, which digest starch and protein in the digestive system.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026