Enzymes

Question 1

Importance of enzymes

Answer 1

• Metabolism - sum of all chemical reactions occurring in an organism
• Enzymes control metabolic reactions, which can either be anabolic (formation of molecules from smaller units) or catabolic (breaking down of molecules)

Question 2

Example of an anabolic reaction?

Answer 2

DNA replication, which is controlled by the enzyme DNA polymerase

Question 3

Example of a catabolic process?

Answer 3

Digestion - the enzyme maltase breaks down maltose in two molecules of glucose

Question 4

Intracellular, extracellular (enzymes)

Answer 4

• Intracellular - working inside cells

- Extracellular - working outside cells

Question 5

Example of extracellular enzymes

Answer 5

• Most extracellular enzymes are catabolic
• In multicellular organisms, digestive enzymes are secreted from cells into the digestive system
• Unicellular organisms, such as yeast and bacteria, secrete digestive enzymes into their immediate environment

Question 6

How do enzymes work?

Answer 6

1. Substrates collide with the active site of the enzyme
2. The shape of the active site is complementary to the substrate
3. The substrate binds to the active site to form an enzyme-substrate complex (ESC)
4. Bonds in the substrates are placed under strain and break; the enzyme provides an alternative reaction pathway that reduces the activation energy required for the reaction
5. An enzyme-product complex is formed and the product/s is/are released

Question 7

Substrate

Answer 7

A reactant in an enzyme-catalysed reaction

Question 8

Active site

Answer 8

The region of an enzyme to which substrates bind

Question 9

Enzyme

Answer 9

• Biological catalysts that facilitate chemical reactions

- Enzymes lower the activation energy of a reaction

Question 10

Lock and Key hypothesis

Answer 10

This suggests that the shape of the active site is an ideal fit for the substrate molecule and therefore is specific to one substrate

Question 11

Induced-fit hypothesis

Answer 11

This suggests that initially weak binding by the substrate will alter the enzyme’s tertiary structure. This strengthens the temporary bonds between the substrate and the enzyme, and weakens bonds within the substrate

Question 12

Effect of temperature on enzyme activity

Answer 12

• A rise in temperature will increase enzyme activity up to an optimum temperature - enzyme and substrate molecules gain kinetic energy and move faster, thereby increasing the chance of successful collisions
• Above optimum temperature, further increases in temperature reduce and eventually stop enzyme activity - weak bonds (e.g. hydrogen bonds) in the active site vibrate more, strain, and break (i.e. the enzyme denatures)

Question 13

Effect of pH on enzyme activity

Answer 13

• Enzyme activity decreases when pH moves away from the optimum (either increasing or decreasing) - changes to H⁺ ion concentrations alter the changes on amino acids in the active site, which can prevent substrate molecules from binding
• A further change in pH can stop all enzyme activity - hydrogen and ionic bonds in the active site are broken, which causes a permanent change in the enzyme’s tertiary structure (i.e. the enzyme is denatured)

Question 14

Effect of substrate concentration on enzyme activity

Answer 14

Reaction rate increases as substrate concentration rises but eventually plateaus (when Vmax, the maximum rate of reaction, is reached). Enzyme concentration becomes a limiting factor when Vmax is reached - a higher collision rate between substrates and active sites results in enzyme- substrate complexes (ESCs) forming at a greater rate. Reaction rate plateaus when one of the factors becomes limiting

Question 15

Effect of enzyme concentration on enzyme activity

Answer 15

An increase in enzyme concentration will raise the reaction rate to a higher Vmax (at which point substrate concentration becomes the limiting factor) - a higher collision rate between substrates and active sites results in enzyme- substrate complexes (ESCs) forming at a greater rate. Reaction rate plateaus when one of the factors becomes limiting

Question 16

Denaturation

Answer 16

• The (usually permanent) change in the secondary/tertiary structure of a protein, due to the breaking of bonds - these include hydrogen bonds, ionic bonds, and disulfide bridges
• A change in structure usually results in a loss of function

Question 17

What are not examples of denaturation

Answer 17

• A small decrease in temperature - this slows enzyme activity due to a restriction of kinetic energy
• Enzyme inhibition
• The breaking of peptide bonds in a protein’s primary structure

Question 18

Which factors would you choose to record when investigating factors which affect enzyme activity?

Answer 18

• Temperature (e.g. investigating the effect of temperature on the activity of lipase, using a water bath set at a range of temperatures - independent variable)
• pH (e.g. the rate at which lipids are broken down, using the conversion of triglycerides in milk to fatty acids, which can be monitored using a pH indicator - dependent variable)
• Substrate and enzyme concentration
• cofactor concentration

Question 19

Competitive enzyme inhibitors - binding site, mechanism, reversible/irreversible, examples, effect on reaction rate

Answer 19

• Binding site = active site
• Mechanism = competes with the substrate for the enzyme’s active site and blocks the substrate from entering
• Reversible (usually)
• Examples = Penicillin - an antibiotic that inhibits the active site of the bacterial enzyme transpeptidase, thereby preventing cell wall formation, Statins - inhibit HMG-CoA reductase (an enzyme involved in cholesterol production)
• Effect on reaction rate = Slows ate, but Vmax can still be reached if substrate concentration is increased

Question 20

Non-competitive enzyme inhibitors - binding site, mechanism, reversible/irreversible, examples, effect on reaction rate

Answer 20

• Binding site = allosteric site (i.e. a region of the enzyme that is not the active site)
• Mechanism = tertiary structure of the enzyme (and therefor active site) changes shape, meaning the active site is no longer a complementary shape to the substrate
• Sometimes reversible, sometimes not
• Examples = Cyanide ions (CN⁻) - a metabolic poison that inhibits cytochrome c oxidase (a respiratory enzyme), Organophosphates - they are used as insecticides; these chemicals inhibit acetylcholinesterase, which is an enzyme necessary for the correct functioning of insect (and mammalian) nervous systems
• Effect on rate of reaction = slows rate and lowers Vmax

Question 21

End-product inhibition

Answer 21

This occurs at the conclusion of metabolic reaction pathways. The final product in a series of reactions will often inhibit one of the enzymes in the reacion pathway from which it has been produced. This regulates the rate at which the product is made (i.e. it acts as a negative feedback mechanism). End-product inhibition can be competitive or non-competitive

Question 22

Cofactor

Answer 22

• A non-protein substance required for enzymes to function
• Inorganic
• They form temporary bonds with the enzyme but leave following the reaction
• Examples = Cl⁻ ions (enzyme = amylase), Mg²⁺ (enzyme = DNA polymerase)

Question 23

Coenzyme

Answer 23

• Organic
• They form temporary bonds with the enzyme but leave following the reaction
• Examples = Coenzyme A (enzyme = acetyl CoA carboxylase), NAD⁺ (enzyme = lactate dehydrogenase)

Question 24

Prosthetic group

Answer 24

• Can be metal ions, lipids, carbohydrates or molecules derived from vitamins
• Can be organic or inorganic
• Permanently bond to the enzyme
• Examples = Zn²⁺ (enzyme = carbonic anhydrase), FAD (enzyme = succinate dehydrogenase, an enzyme used in respiration)

Question 25

Precursor activation

Answer 25

Often enzymes are produced as inactive precursor proteins. The addition of a cofactor can activate an enzyme. The activation is usually achieved by altering the shape of the enzyme’s active site