Forms and functions of enzymes

Question 1

What is an enzyme?

Answer 1

An enzyme is a protein (biological) catalyst that increases the velocity of a chemical reaction and it is not consumed at the end of a reaction (regenerated). It increases the velocity of the reaction by providing an alternative pathway with lower activation energy for the reaction to occur (lower the free energy barrier to allow the reaction to attain equilibrium faster)

Question 2

Features of enzymes

Answer 2

1. Higher reaction rates
2. Milder reaction conditions are needed (almost neutral pH, temperatures below 100 deg, atmospheric pressure)
3. Greater reaction specificity (complementary fit of the substrate with enzyme controls the selectivity for the substrate and the product formed) (high yield and rarely produce side products)
4. capacity for regulation (catalytic activities of many enzymes can change in response to the concentrations of substances besides their substrates and products
5. Enzymes do not change the thermodynamic properties of the substrate and the products - no effect on the change in Gibbs Free energy of a reaction
6. Accelerates both the forward and backward reaction

Question 3

Why is it important for enzymes to have high yield?

Answer 3

Many biological reactions take place with many steps. If the enzyme does that have a high yield, there will be very little product at the end of a series of reactions. Not be able to support cellular function.

Question 4

What is the induced-fit model?

Answer 4

Binding of the substrate induces a conformational change in the enzyme’s active site and results in an enhanced complimentary fit once the substrate is bound. (analogy: catching a tennis ball)

Question 5

What are cofactors?

Answer 5

non-protein components which are required for enzyme activity.
Include:
-Metal ions (inorganic)
-Co-enzymes (organic molecules usually derived from vitamins) which are further classified into:
-Co-substrates - can dissociate and bind onto the active site of the enzyme
-Prosthetic group - binds tightly through covalent bonds at the active site of an enzyme

Question 6

How does metal ions act as a cofactor?

Answer 6

It helps the enzyme to orientate the substrate so that it can interact well with the active site

Question 7

Why does the lock and key hypothesis not able to explain how an enzyme accelerates a reaction?

Answer 7

This is because if the enzyme and the substrate are a perfect fit, they will form a stable enzyme-substrate (ES) complex. This stabilisation will increase the activation energy of the reaction. Thus, it will impede catalysis.

Question 8

How does the induced-fit model explain how an enzyme accelerates a reaction?

Answer 8

An enzyme with an active site more complementary to the transition state helps to stabilise the transition state and lowers the activation energy, thereby enhancing the catalysis of the reaction

Question 9

How can metal ions help in catalysis?

Answer 9

1. Binding to substrates to orientate them properly to fit into the active site of an enzyme for a reaction
2. Electrostatically stabilising or shielding negative charges (reduce repulsion -> making the reaction more favourable)
3. Polarisation of substrates

Question 10

Advantages of using metal ions instead of positively charged amino acid side groups for catalysis?

Answer 10

1. The positive charge on the metal ions are not dependent on pH (+ve charge on the amino acid side groups will be affected by pH changes)
2. Highly charged (can have charges greater than +1)

Question 11

Effects of pH on enzyme activity

Answer 11

1. Changes in pH can affect ionisable groups of the substrate which can alter its binding to the enzyme (disruption of the conformation of the enzyme)
2. Changes in pH alter ionisation of the amino acid residues involved in the catalytic activity of the enzyme, such as ionisable side chains at the active site that participates directly in the reaction or contribute to the proper orientation of the substrate. Results in a non-complementary fit of the substrate to the active site of the enzyme.
3. Changes in pH can generate and break intra and inter-molecular bonds, thereby changing the conformation of the enzyme and hence rendering it ineffective. Results in a non-complementary fit of the substrate to the active site of the enzyme.

Question 12

Effects of temperature on enzyme activity

Answer 12

Enzymes function at an optimal range of temperature
An increase in temperature will increase the kinetic energy of the substrates -> increase the rate of collision between the substrate and the active site of the enzyme -> increase the rate of effective collision, hence increasing the rate of reaction.

However, at temperatures above the optimal temperature, the inter and intra molecular interactions that maintains the conformation of the enzyme will be disrupted (e.g. H bonds, van der Waal’s forces and hydrophobic interactions) -> no effective collision will occur -> enzyme is denatured and enzyme activity decreases significantly.