Enzymes #2


Biological Catalysts

Enzymes are biological catalysts that speed up metabolic reactions and remain unchanged at the end of the reaction. The number of reactions that an enzyme can catalyse per second is known as the turnover number

Active site– the indented area on the surface of the enzyme which has a complementary shape to the substrate molecule

Metabolic reactions – chemical reactions inside living cells/organisms

Catalysts can speed up reactions as they can speed up reactions without altering the pH or temperature and instead function at temperatures and pressures that sustain life. They are more specific than chemical catalysts because of their complementary shape to the substrate so rarely produce unwanted by-products.

Enzyme Structure

  • Enzymes are proteins and have protein structure (primary, secondary, tertiary, quaternary) and therefore are coded for by genes
  • Some use cofactors
  • Metabolic disorders are the result of deficient enzymes in metabolic pathways
  • Enzymes also catalyse the formation of an organism’s structural components e.g. collagen in bone and cartilage in blood vessel walls
  • Active site contains about 6 – 10 amino acids and the tertiary structure is essential as the shape of the active site must be complementary to the substrate molecule
  • Active sites are changed by temperature and pH as this alters the bonds that hold together the bonds of the tertiary structure

Intracellular – inside the cell

  • Metabolic pathways contain a series of consecutive reactions each catalysed by a specific enzyme
  • The reactants and intermediates act as substrates and are known as metabolites
  • Catabolic – metabolites broken down, Anabolic – larger metabolites synthesised from smaller ones
  • g. respiration and photosynthesis
  • Catalase breaks down hydrogen peroxide, has four polypeptide chains and a haem group, is the fastest acting enzyme with the highest turnover number known, found in peroxisomes in eukaryotes, white blood cells use it to kill microbes

Extracellular – outside the cell

  • Amylase is produced in salivary glands to digest polysaccharides
  • Trypsin is made in the pancreas and digests proteins in the small intestine


Prosthetic groups – a cofactor that is permanently bound by covalent bonds to an enzyme molecule.

E.g. Zinc ion bound to the active side of carbonic anhydrase

Some cofactors act as co-substrates and some change the distribution of charge on the enzyme’s surface and make temporary bonds to aid the substrate in binding to the enzyme.

Coenzymes – these bind temporarily to the active site of the enzyme with the substrate and are organic non-protein molecules which are chemically changed during the reaction

Lock-and-key hypothesis

Induced-fit hypothesis

Enzymes lower the activation energy for a reaction.

Effect of temperature on enzymes

  • Extra heat energy causes the molecules to vibrate which increases the rate of successful collisions between molecules and increases the force with which they collide
  • Both enzyme and the substrate move faster, therefore the rate of formation of ES complexes increases and the rate of reaction increases.
  • Rate of reaction is at its maximum at optimum temperature
  • Heat also makes molecules vibrate which may break some of the weaker bonds in the tertiary structure of the active site, denaturing it (bonds like the ionic bonds and hydrogen bonds)
  • As the active side changes the substrate will no longer fid
  • As more heat is applied the enzyme active site changes shape completely and irreversible so the reaction cannot proceed at all.

Effect of pH on enzymes

Acidic – 0-6 pH

Neutral – 7 pH

Alkaline – 8-14 pH

Acids are proton (H+) donors

Buffers – Can donate or accept protons, resist changes to pH

Enzymes work at different pH’s as they work in different areas and extra/intracellularly.

Effect of substrate concentration

Initially rate of reaction increases as more enzyme substrate complexes while the substrate concentration is the limiting factor but when the reaction reaches Vmax the limiting factor is the enzyme concentration as all the enzyme active sites are occupied and the maximum turnover rate is reached.

Enzyme synthesis – genes for synthesising particular enzymes can be switched on or off depending on a cells needs

Enzyme degradation – Cells degrade old enzymes to regulate the metabolism in the cell and to ensure abnormal proteins are not accumulated in the cell

The initial rate can be measured by finding the gradient of the tangent of the steepest part of the curve of any given reaction. Initial rates for both A and B can then be compared.





End product inhibition – when enzyme catalysed reactions are regulated by the final product molecule acting as an inhibitor for the first enzyme in the sequence. This is an example of negative feedback. In metabolic pathways this is non-competitive and reversible