2.4. Enzymes

Question 1

What are enzymes

Answer 1

• Globular proteins capable of acting as biological catalysts
• They allow reactions to occur at conditions naturally present within organisms

Question 2

What reactions do enzymes play a role in catalysing?

Answer 2

• Reactions necessary to life that affect the structure and function of an organism
• These include anabolic reactions, such as muscular hypertrophy, and catabolic reactions, like respiration
• Enzymes catalyse both intracellular reactions (like catalase breaking down hydrogen peroxide) and extracellular reactions (like amylase breaking down starch)

Question 3

How does the lock and key hypothesis explain the mechanism of enzyme action?

Answer 3

• Enzymes, as proteins, have a specific shape, whether that be a unique quaternary structure or a unique tertiary structure
• The part of the enzyme where the substrate (the substances involved in the reaction the enzyme is catalysing) binds to, called the active site, therefore also has a specific shape
• This shape is complementary to the shape of the substrate; the substrate is a key and the active site is the lock

Question 4

How does the induced-fit hypothesis explain the mechanism of enzyme action?

Answer 4

• The shape of the active site of an enzyme is not initially complementary to the shape of the substrate, but becomes so as the substrate enters
• This means the inital interaction between the enzyme and substrate is weak, but strengthens after the conformation of the active site’s shape

Question 5

How does an enzyme catalyse a reaction after a substrate enters?

Answer 5

• Regardless of which substrate entry hypothesis is abided by, after the substrate enters, a enzyme-substrate complex forms
• The enzyme lowers the activation energy between substances in the substrate by bringing them closer together, providing a platform for the reaction, removing intermediary steps and forming temporary interactions with the substrate to strain its bonds
• After the reaction has finished, an enzyme-product complex forms, from which the products are released

Question 6

What are the factors affecting enzyme activity?

Answer 6

• Temperature
• pH (as hydrogen ions interact with polar and charged R-groups in the enzyme’s active site)
• Substrate concentration
• Enzyme concentration

• The mechanisms behind their influence are largely GCSE knowledge
• These factors change enzyme activity until a limiting factor stops any further increase; the Vₘₐₓ is the maximum rate of reaction at a given enzyme concentration, regardless of other factors

Question 7

What is the temperature coefficient (Q₁₀) and how can it be calculated?

Answer 7

• The temperature coefficient (Q₁₀) measures the proportional rise in rate of reaction after a rise in temperature of 10 degrees Celsius
• It is calculated by dividing the rate of the reaction at a given temperature with the rate of the reaction at a temperature 10 degrees Celsius lower
• A Q₁₀ value of 2 indicates that the rate doubles with every increase in 10 degrees

Question 8

What is a cofactor?

Answer 8

• A non-protein component of an enzyme that aids it in its role as a catalyst
• It can help move atoms from one reaction to another or form an integral part of the enzyme’s active site
• Sometimes they change the shape of an enzyme’s active site by binding to it in a process called precusor activation

These can be organic or inorganic. The chloride ion, which is essential for the function of amylase, is an example of an inorganic cofactor

Question 9

What are coenzymes and what is an example?

Answer 9

• Coenzymes form a subcategory of cofactors and are always organic
• Many vitamins are sources of coenzymes; for example, vitamin B5 is used to make coenzyme A, which is crucial in the breakdown of carbohydrates for respiration

Question 10

What are prosthetic groups and what is an example?

Answer 10

• Prosthetic groups form a subcategory of cofactors that are always permanently bound to the enzyme
• An example includes the zinc ion, which is essential for the function of carbonic anhydrase

By contrast, some other cofactors are only temporarily bound to an enzyme

Question 11

What is enzyme inhibition?

Answer 11

• Inhibitors are substances that prevent or hinder enzymatic function
• They are essential for regulating rates of biological reactions
• Inhibition is split into two subsets: competitive and non-competitive
• Within these subsets, inhibition is further categorised as either reversible or irreversible

Question 12

What is the mechanism behind competitive inhibition?

Answer 12

• A wholly or partially complementarily-shaped inhibitor will bind to the active site of an enzyme in place of the substrate
• This prevents the substrate from binding to the active site, stopping the enzyme’s facilitation of any metabolic activity

Question 13

How does competitive inhibition affect the rate of an enzyme-controlled reaction with increasing substrate concentration when compared to a non-inhibited reaction?

Answer 13

• At a given substrate concentration, the rate of reaction will be lower in the presence of a competitive inhibitor, as some active sites are unavailable for substrate-binding
• However, since most (but not all) competitive inhibitors are reversible, the rate of reaction will eventually reach the same maximum as it would in a non-inhibited reaction if substrate concentration is increased enough, provided enzyme concentration remains constant

Question 14

What is the mechanism behind non-competitive inhibition?

Answer 14

• The inhibitor binds to an allosteric site on the enzyme, which is a non-active site part of the enzyme that the substrate is unable to bind to
• Following this, the active site changes shape and the substrate is unable to bind to it

Question 15

How does non-competitive inhibition affect the rate of an enzyme-controlled reaction with increasing substrate concentration when compared to a non-inhibited reaction?

Answer 15

• Introducing a non-competitive inhibitor reduces the Vₘₐₓ at a given enzyme concentration as non-competitive inhibitors are usually non-reversible and have equal affinity for ESCs and empty enzymes
• Thus, with increasing substrate concentration, the rate of reaction will increase until all enzymes are saturated with either substrates or non-competitive inhibitors, and this maximum rate of reaction will be lower than for non-inhibited reactions

Question 16

What is end-product inhibition?

Answer 16

• When the product of an enzymatic reaction acts as an inhibitor
• This causes a negative feedback loop, preventing the concentration of a product from becoming too high, the concentration of reactants from falling too low and the rate of reaction from exceeding a certain value
• An example of an end-product inhibited reaction is the production of ATP in respiration, which is catalysed by phosphofructokinase