Biological Molecules: Enzymes

Question 1

Enzymes as Catalysts

Answer 1

• Enzymes are protein molecules that act as biological catalysts
- A catalyst is a chemical that speeds up the rate of reaction and remains unchanged and reusable at the end of the reaction

Question 2

What are substrates and how are enzyme-substrate complexes formed?

Answer 2

• Any molecule that can have a reaction catalyzed by an enzyme is called a substrate

• Substrates bind to the reactive part of the enzyme, the active site
• A substrate will only bind to the active site if it is complementary in shape
• Therefore, every substrate will have a specific enzyme that it can bind with

• Complementary binding refers to when two molecules fit together to complete each other

Question 3

What determines an enzyme’s properties?

Answer 3

• Enzyme properties are related to their tertiary structure

• The active site’s shape is determined by the enzyme’s tertiary structure
• Each different enzyme has a different tertiary structure and so a different shaped active site
• If substrate shape doesn’t fit active site, an enzyme-substrate won’t be formed and the reaction won’t be catalysed

• If tertiary structure of enzyme is changed, the shape of the active site will change
- Shape of active site can be changed by factors like pH, temperature and the presence of inhibitors or cofactors

Question 4

How Enzymes Speed Up Reactions

Answer 4

• Activation energy is the energy required to start a reaction
• Enzymes catalyse reactions by lowering the activation energy
• If two substrate molecules need to be joined, being attached to the enzyme holds them together, reducing any repulsion between the molecules so they can bond more easily
• If enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily

Question 5

Models of Enzyme Action

Answer 5

• The ‘lock and key’ model

• Suggests that the enzyme is like a lock, and that the substrate is like a key, that fits into it due to their complementary shapes
• Suggests that the enzyme active site is a fixed shape and that due to random collision the substrate can collide and attach to the enzyme. This forms an enzyme-substrate complex

• The ‘induced fit’ model

• When the enzyme active site is induced, or slightly changes shape, to mould around the substrate
• The products are then removed, and the enzyme active site returns to its original shape

Question 6

Enzyme Inhibitors

Answer 6

• Enzyme activity can be reduced or stopped with inhibitors
• Competitive inhibitors
• They fit into the active site of an enzyme instead of the substrate
• This forms a physical barrier that prevents the formation of enzyme-substrate-complexes
• Because both inhibitor and substrate can fit into the active site they compete with each other for the enzyme
• Weaker inhibition effect
• Often bind reversibly

• Non-competitive inhibitors

• These inhibitors bind to a separate part of an enzyme called its allosteric site
• Binding of a non-competitive inhibitor causes a conformational change in the enzyme
• This change alters the shape of the enzyme’s active site so that it is not complementary with its substrate
• Because these inhibitors do not bind to the active site they do not compete with substrates
• More powerful inhibition effect
• Often bind permanently

Question 7

Factors Affecting Enzyme Activity: Temperature

Answer 7

• Rate of reaction increases up to the optimum temperature as the kinetic energy of the enzyme increases
- More enzyme-substrate complexes are formed per unit of time, as more successful collisions occur - more molecules have sufficient activation energy

• Above the optimum temperature rate of reaction decreases as the enzyme becomes denatured

• The hydrogen and ionic interactions in the tertiary structure begin to break
• This changes shape of active site so an enzyme-substrate complex can no longer be formed

Question 8

Factors Affecting Enzyme Activity: pH

Answer 8

• The pH of a solution is a measure of the hydrogen ion concentration
• Enzymes are denatured at extremes of pH
• Small changes to pH can disrupt the ionic and hydrogen bonds in the enzyme’s tertiary structure
• If these bonds break, the active site changes shape and can no longer form enzyme-substrate complexes - denatured

Question 9

Factors Affecting Enzyme Activity: Substrate Concentration

Answer 9

• As concentration of substrate increases, rate of reaction increases as more enzyme-substrate complexes are formed

• However, beyond a certain point (saturation point) the rate of reaction no longer increases as enzyme concentration becomes the limiting factor
• Too many substrates for the enzymes to cope with

Question 10

Factors Affecting Enzyme Actvity: Enzyme Concentration

Answer 10

• The rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to

• However increasing the enzyme concentration beyond a certain point has no effect on the rate of reaction
• There are more active sites than substrates so substrate concentration becomes the limiting factor

Question 11

How can the pH of a solution be calculated?

Answer 11

• The pH of a solution can be calculated using the following formula: pH = -log10[H+]