Lecture 9

Question 1

What is the role of enzymes in chemical reactions?

Answer 1

Catalysts - increase rate of reaction

Question 2

How does Gibbs energy determine the abundance of substrates and products?

Answer 2

Gibbs energy < 0 means energy is release; products dominate.
Gibbs energy > 0 means energy is required; substrate dominates
Gibbs energy = 0 means at equilibrium concentration of substrate and products are equal.

Question 3

What reaction kinetics are involved in Gibbs energy?

Answer 3

Enthalpy and Entropy

Question 4

How is reaction rate increased?

Answer 4

The increase in reaction rate is achieved by lowering the free energy of activation of the reaction, thereby allowing the transition state to be reached more easily.

Question 5

What are the features of enzyme catalysed reactions?

Answer 5

• Much faster reaction rates
• Much milder reaction conditions
• Reaction specificity
• Tightly Regulated

Question 6

What does “milder reaction conditions” mean?

Answer 6

Enzyme catalysed reactions are efficient at the mild conditions of temperature and pH found inside biological cells. In contrast, chemically catalysed or uncatalyzed reactions often require high temperatures and extremes in pH and/or pressure.

Question 7

What is meant by reaction specificity?

Answer 7

Enzymes are specific to their substrates (reactants). This is useful in intracellular conditions where hundreds and thousands of different substrates and reactions coexist.

Question 8

What is meant by tightly regulated?

Answer 8

There are many mechanisms that ensure the tight control of enzyme catalysed reactions, ensuring that their activity is efficient when and where required.

Question 9

What is the type of reaction catalysed by Oxidoreductases?

Answer 9

Transfer of electrons

Question 10

What is the type of reaction catalysed by Transferases?

Answer 10

Group-transfer reactions

Question 11

What is the type of reaction catalysed by Hydrolases?

Answer 11

Hydrolysis reactions (transfer of functional groups to water)

Question 12

What is the type of reaction catalysed by Lyases?

Answer 12

Addition of groups to double bonds or the reverse

Question 13

What is the type of reaction catalysed by Isomerases?

Answer 13

Transfer of groups within molecules to yield isomeric forms

Question 14

What is the type of reaction catalysed by Ligases?

Answer 14

Formation of C-C,C-S,C-O and C-N bonds by condensation reactions coupled to ATP cleavage.

Question 15

Explain enzyme stereospecificity?

Answer 15

Enzymes display both geometrical specificity and stereospecificity. Enzymes are specific in binding chiral substrates and in catalysing their reactions and bind their substrates through several weak bonds.

Question 16

Why is enzyme stereospecificity important?

Answer 16

the multiple weeks bonds provide specificity and allows the resulting product(s) to be released.

Question 17

Forming an Enzyme-substrate complex?

Answer 17

There may be one or more substrates bound to the enzyme. The formation of this complex leads to the transition state species, which then forms the product(s).

Question 18

Where does the enzyme bind to the substrate?

Answer 18

The enzyme binds the substrate(s) in a region of the enzyme called the active site. However, for the amino acids side chains in the active site to be able to react with its substrate, they must be close together and in the right orientation to each other.

Question 19

The main features of an active site are?

Answer 19

It takes up a relatively small portion of the total volume of enzyme.
• It is a 3-dimensional entity.
• Binding of substrate can be a direct fit or an induced fit (see below).
• Most substrates are bound to enzymes by relatively weak forces.
• Active sites are clefts or crevices.

Question 20

How is the active site cleft formed?

Answer 20

by folding the polypeptide chain into a specific shape. The parts of the protein not directly involved in the active site are needed to stabilise the tertiary structure.

Question 21

What happens once the substrate is bound and the transition state is formed?

Answer 21

the bonds are rearranged. This happens with the assistance of certain amino acid side chains that form the active site. Bonds are broken, new bonds are formed, and the substrate is transformed into the product(s). Once the product is released, the enzyme is free to catalyse the reaction of new substrate to product.

Question 22

How do enzymes participate in the chemistry of reactions?

Answer 22

through its catalytic groups, which are amino acid side chains that can act as acids, bases, nucleophiles and electrophiles

Question 23

What is covalent catalysis?

Answer 23

one type of catalytic mechanism that some enzymes employ. A covalent enzyme- substrate intermediate is briefly formed. It is highly reactive or unstable and has a high probability of entering the transition state and completing the reaction

Question 24

What is acid-base catalysis?

Answer 24

involves the donation or acceptance of a proton by acidic or basic groups within the protein. This mechanism can function instead of or together with covalent catalysis.