What can you recall from IGCSE?
Define the following terms:
1. Anabolic reactions: Reactions that build up molecules
Catabolic reactions: Reactions that break down molecules
Combination of anabolic and catabolic reactions
A substance that speeds up reactions without changing the produced substances
Metabolic pathway: Sequence of enzyme controlled reactions
Only able to catalyse specific reactions
The molecule(s) the enzyme works on Molecule(s) produced by enzymes
• Intracellular enzymes
Work inside cells eg.DNA polymerase
• Extracellular enzymes Secreted by cells and work outside cells eg.
• Recommended names Short name, often ending in ‘ase’ eg. creatine kinase
• Systematic name
Describes the type of reaction being catalysed eg. ATP:creatine phosphotransferase Eg. 220.127.116.11
• Classification number
Timeline of enzyme discovery
1835: Breakdown of starch to sugar by malt 1877: Name enzyme coined to describe chemicals in yeast that ferment sugars
1897: Eduard Buchner extracted enzyme from yeast and showed it could work outside cells
1905: Otto Rohm exyracted pancreatic proteases to supply enzymes for tanning
1926: James B Sumner produced first pure crystalline enzyme (urease) and showed enzymes were proteins
1930-1936: Protein nature of enzymes finally established when digestive enzymes crystallised by John H Northrop 1946: Sumner finally awarded Nobel prize
Enzymes lower the activation energy of a reaction
Energy levels of molecules
Initial energy state of substrates
Activation energy of enzyme catalysed reaction
Activation energy of uncatalysed reactions
Final energy state of products
Progress of reaction (time)
Enzymes lower activation energy by forming an enzyme/substrate complex
Substrate + Enzyme
Product + Enzyme
In anabolic reactions enzymes bring the substrate molecules together.
In catabolic reactions the enzyme active site affects the bonds in substrates so they are easier to break
Lock-and-key hypothesis assumes the active site of an enzyme is rigid in its shape
How ever crystallographic studies indicate proteins are flexible.
The Induced-fit hypothesis suggests the active site is flexible and only assumes its catalytic conformation after the substrate molecules bind to the site. When the product leaves the enzyme the active site reverts to its inactive state.
Enzymes are globular proteins
• Active site has a specific shape due to tertiary structure of protein. • A change in shape of the protein affects shape of active site and the function of the enzyme.
Click to link to jmol interactive representation courtesy of University of Arizona
Characteristics of enzymes
• Only change the rate of reaction. They do not change the equilibrium or end products. • Specific to one particular reaction
• Present in very small amounts due to high molecular activity:
Turnover number = number of substrate molecules transformed per minute by one enzyme molecule Catalase turnover number = 6 x106/min
How would you measure the effect of an enzyme?
• Compare uncatalysed rate with catalysed. • Enzymes can increase rate by a factor of between 108 to 1026
Characteristics of enzymes
• Rate of enzyme action is dependent on number of substrate molecules present
Vmax = maximum rate of reaction Rate of Reaction (M)
Vmax approached as all active sites become filled Some active sites free at lower substrate concentrations
Why do scientists measure the initial rate of reaction of enzyme-catalysed reactions?
Initial rate of reaction Rate of Reaction (M)
They measure rate at start of reaction before any factors, eg. substrate concentration, have had time to change.
Rate of enzyme –catalysed reactions are affected by temperature.
Temperature coefficient Q10:
rate of reaction at (x + 10) oC
Q10 = ----------------------------------------rate of reaction at x oC Q10
for between 0 - 40 oC is 2
Enzymes denature at 60oC
Rate of reaction
Rate doubles every 10oC
Enzyme denaturing and losing catalytic abilities
Some thermophilic bacteria have enzymes with optimum temperatures of 85oC
pH affects the formation of hydrogen bonds and sulphur bridges in proteins and so affects shape.
Rate of Reaction (M)
Enzymes in medicine
Glucose oxidase + peroxidase + blue dye on dipsticks to detect glucose in urine:
Dye changes according to amount of glucose
Enzyme-linked immunosorbent assays (ELISAs) detect antibodies to infections.
Now answer the exam questions
• The first question you will complete and then swap with your partner. You will then mark each others work using the provided mark-scheme. You must agree each others marking. • The second question you will complete and mark your own and then I will mark it and see if I agree with your marking. • The third question you will complete and I will mark it. You will then check my marking and we will agree a score. • The total score for all three questions will be recorded.