L8: Biochemistry of enzymes

Question 1

What is the definition of enzymes?

Answer 1

En= inside Zyme=yeast

Enzymes are organic thermo-labile catalysts.

Question 2

What is a catalyst?

Answer 2

catalyst is a substance that increases the chemical reaction without change (not consumed, not affect the end product).

They accelerate the chemical reactions inside the biological systems (living cells).

Question 3

What is the chemical nature of enzymes?

Answer 3

All enzymes are protein in nature except ribozymes (RNA in nature).

Question 4

What are the types of enzymes?

Answer 4

1) Simple protein enzymes: They are formed of protein only.
2) Complex (conjugated) protein enzymes: formed of two parts

Question 5

What are the parts of the complex enzyme?

Answer 5

The protein part is called apoenzyme

The Non-protein part is called the cofactor.

The whole enzyme is called a holoenzyme.

Question 6

What is the active site?

Answer 6

A restricted region of an enzyme molecule that binds to the substrate.

Question 7

How is the active site formed?

Answer 7

It is formed from Amino acids sequences in the polypeptide chain.

Question 8

What is the mechanism of the enzyme action?

Answer 8

1) The substrate (S) binds to the enzyme (E) to form an activated intermediate enzyme-substrate complex (ES).
2) The activated complex (ES) cleaved to the products (P) and the original enzyme (E).

Question 9

What does the cofactor do in complex enzymes?

Answer 9

In complex enzymes, the coenzyme helps the cleavage of the substrate.

Question 10

What are the enzymatic reaction steps?

Answer 10

1) Substrate approaches active site.
2) Enzyme-substrate complex forms.
3) Substrate transformed into products.
4) Products released.
5) Enzyme recycled.

Question 11

What are the theories that explain the E-S complex?

Answer 11

Lock and key theory. (Proposed by Fischer in 1894)
Induced fit theory.

Question 12

Lock and key theory (Proposed by Fischer in 1894)

Answer 12

❖ In this model, the active sites of the unbound enzyme are complementary (fit) in shape to the substrate.
❖ The substrate fits in this catalytic site in a similar way to lock and key. The key will only fit its own lock.

Question 13

Induced fit theory

Answer 13

 It is a more flexible model.

 The catalytic site of the enzyme is not complementary to the substrate.

 The binding of the substrate to the enzyme induces changes in the shape of the catalytic site making it fitter for the substrate.

Question 14

What are the factors affecting the rate of enzyme action?

Answer 14

1- Effect of enzyme conc.
2- Effect of substrate conc.
3- Effect of temperature
4- Effect of PH
5- Concentration of coenzymes
6- Concentration of ion activators
7- Presence of enzyme inhibitors
8- Effect of time

Question 15

Th effect of enzyme conc.

Answer 15

The rate of enzyme action is directly proportional to the concentration of enzyme provided sufficient supply of substrate & constant conditions.

Question 16

The effect of the substrate conc.

Answer 16

The rate of reaction increases as the substrate concentration increases up to a certain point at which the reaction rate is maximal (Vmax.)

At Vmax, the enzyme is completely saturated with the substrate, then any increase in substrate concentration doesn’t affect the reaction rate.

Question 17

What is the Michael is constant (Km)?

Answer 17

It is the substrate concentration that produces half the maximum velocity of enzyme (1/2
Vmax).

Question 18

What are enzymes with low Km?

Answer 18

have high affinity to the substrate i.e. they act at maximal velocity at low substrate concentration.

Question 19

What are examples of enzymes with low Km?

Answer 19

Hexokinase which acts on glucose at low concentration (fasting state)

Question 20

What are enzymes with high Km? And what are their examples?

Answer 20

have low affinity to the substrate i.e. they act at maximal velocity at high substrate concentration.

E.g. Glucokinase enzyme acts on glucose at high concentrations (fed state).

Question 21

What is the effect of temperature on the rate of the reaction?

Answer 21

 The rate of reaction increases gradually with the rise in temperature until reaches a maximum at a certain temperature, called optimum temperature.

 The optimum temperature is around 37°C in humans

 After the optimum temperature, the rate of reaction decreases

Question 22

What does the enzyme activity decrease after a certain point of temperature?

Answer 22

due to the denaturation of the enzyme (60-65).

Question 23

Why does the increase in temperature (up to a certain limit) affect the enzyme activity?

Answer 23

The effect of temperature on reaction rate is due to:
a)Increase in temperature increases the initial energy of the substrate and thus decreases the activation energy.

b) Increase of collision of molecules.

Question 24

Effect of pH on the enzyme activity

Answer 24

Each enzyme has an optimum PH at which its activity is maximal.

Change of PH above or below optimum PH decrease rate of enzyme action

Question 25

Why does Change of PH above or below optimum PH decrease the rate of enzyme action?

Answer 25

due to:
1) The enzyme activity depends on the ionization state of both enzyme and substrate which is affected by PH.

2) Marked change in PH will cause denaturation of the enzyme.

Question 26

Concentration of coenzymes

Answer 26

In the conjugated enzymes that need coenzymes, the increase in the coenzyme concentration will increase the reaction rate.

Question 27

The concentration of ion activators

Answer 27

The increase in metal ion activator increases the reaction rate.

Many enzymes are activated by ions:
1- Chloride ion activate salivary amylase
2- Calcium ions activate the thrombokinase enzyme

Question 28

Presence of enzymes inhibitor

Answer 28

presence of enzyme inhibitor decreases or stops the enzyme activity

Question 29

Effect of time

Answer 29

In an enzymatic reaction, the rate of reaction is decreased over time.

Question 30

Why does the rate of enzymatic reaction decrease with time?

Answer 30

This is due to:
a) The decrease in substrate concentration
b) The accumulation of the products.
c) The change in PH than optimum PH.