Enzymes I

Question 1

What is an enzyme?

Answer 1

Proteins that speed up (catalyse) specific chemical reactions

Question 2

Examples of the functions of an enzyme

Answer 2

Digestion: carbohydrates, fats, proteins

Blood clotting: fibrin clot catalysed by thrombin
Defence-immune system

Movement: muscle actomyosin is an ATPase (enzyme that breaks down ATP into ADP and inorganic phosphate)

Nerve conduction: Acetylcholinesterase breaks down acetylcholine to prevent overstimulation of cholinergic nerves

Question 3

Examples of different types of enzyme and the reactions they catalyse

Answer 3

Proteases – Hydrolyse peptide bonds in other proteins causing proteolysis

Nucleases – Breaks the phosphodiester bond between the amino acids of nucleic acids

Polymerases – Brings about the formation of polymers, usually RNA or DNA, by bringing together the monomers of these polymers

Kinases – Able to transfer phosphate groups from phosphate-donating molecules, usually ATP, to specific substrates

Question 4

Name one disease that can be caused by a defect in an enzyme

Answer 4

Phenylketonuria

Question 5

What is Phenylketonuria?

Answer 5

Inherited disease caused by a mutation in the phenylalanine hydroxylase (PAH) gene

Question 6

What are the consequences of having Phenylketonuria?

Answer 6

Low levels of the enzyme phenylalanine hydroxylase (found in the liver)

This results in the person’s body not being able to convert phenylalanine into tyrosine causing a build-up of phenylalanine to toxic levels

In high levels Phenylalanine can be broken down into toxins which can affect brain maturation

Question 7

What are some examples of drugs that use enzymes as there targets? What are those enzymes?

Answer 7

Penicillins: Inhibit enzymes of bacterial cell wall synthesis

Anti-inflammatory agents: aspirin blocks prostaglandin biosynthesis by inhibiting the activity of the enzyme cyclooxygenase

Question 8

Key features of enzymes

Answer 8

Increase reaction rate

Show specificity

Unchanged at end of reaction

Do not alter reaction equilibrium (allows reaction to reach equilibrium by catalysing reaction in both directions)

Facilitates reaction by decreasing the free energy of activation of the reaction

Question 9

What is the free energy of activation?

Answer 9

The energy which must be provided to a system with potential reactants in order to produce a chemical reaction - reactant must reach transition state in order for reaction to occur

Question 10

What is the transition state?

Answer 10

The state corresponding to the point in a reaction with the highest free energy. At this point the molecule is neither a reactant or product and is very unstable.

Question 11

What is an active site of an enzyme?

Answer 11

3-D cavity or cleft that binds substrate(s) using electrostatic, hydrophobic, hydrogen bonding and van der Waals interactions

Question 12

Where did evidence for enzyme having active sites come from?

Answer 12

1. X-ray crystallography

2. Kinetic studies of enzyme activity

Question 13

What are the 2 models of enzyme activity?

Answer 13

1. Lock and key

2. Induced fit

Question 14

How does the Lock and key model explain enzyme activity?

Answer 14

States that shape of active site of enzyme is completely complimentary to shape of substrate

Question 15

How does the induced fit model explain enzyme activity?

Answer 15

States that active site of enzyme not complimentary to shape of substrate however, when substrate starts to bind enzyme undergoes conformational change which cause active site to change shape and become complimentary to shape of substrate

Question 16

Which model is more accurate induced fit or lock and key?

Answer 16

Induced fit model more accurate

Question 17

How does the activity of the enzyme Hexokinase show that the induced fit model is more accurate?

Answer 17

Changes shape when it forms an enzyme-substrate complex with glucose - The 2 lobes of the hexokinase squeeze out the water from the glucose

Question 18

What is the enzyme-substrate binding energy?

Answer 18

Free energy that is released by the formation of weak interactions between a complementary substrate and enzyme

Question 19

How does an enzyme reduce the free energy of activation?

Answer 19

Enzyme uses the enzyme-substrate binding energy to:
Bring molecules together in active site

Constrain substrate movement (brings substrates together within one space – this is called approximation)

Stabilise positive and negative charges in transition state
Provide a reaction pathway of lower energy:

To strain particular bonds in the substrate- making breakage easier.

Use cofactors (bound within active site of enzyme): bring new chemistry to active site

Question 20

What is the Michaelis-menten equation?

Answer 20

An equation used to calculate the rate of a reaction catalysed by an enzyme

Equation: V= Vmax x (Substrate conc./substrate conc. + Km)

Question 21

What is Vmax?

Answer 21

The rate of reaction when the enzyme is saturated with substrate

Question 22

What is Km (Michaelis constant)?

Answer 22

Concentration of substrate needed for an enzyme to reach half its Vmax (measure of affinity of active site for its substrate)

Question 23

What is the Turnover number?

Answer 23

The maximum amount of substrate that can be converted to product by a single active site per unit of time.
Equation: Vmax/enzyme conc.

Question 24

What is the linewaver-burke equation?

Answer 24

Rearrangement of Michaelis-Menten equation which allows you to plot straight line graph of substrate conc. against reaction velocity

Question 25

On a linewaver-burke graph what is the y-intercept equal to?

Answer 25

1/Vmax

Question 26

On a linewaver-burke graph what is the x-intercept equal to?

Answer 26

-1/Km

Question 27

What is competitive inhibition?

Answer 27

Competition for same active site on enzyme between substrate and inhibitor
Causes less enzyme – substrate complexes to form

Question 28

What is the effect of competitve inhibition on Vmax and Km?

Answer 28

Causes Vmax to remain unchanged but does increase Km

Question 29

What is non-competitive inhibition?

Answer 29

Substrate and inhibitor bind to different active sites on the enzyme

Question 30

What is the effect of non-competitive inhibition on Km and Vmax?

Answer 30

Causes Vmax to decrease but Km to remain unchanged

Question 31

What are the mechanisms that control enzyme activity?

Answer 31

Control of gene expression - Controls amount of enzyme produced by protein synthesis

Compartmentation: Signal sequence in enzyme polypeptide chain target enzyme to ER, mitochondrion, nucleus etc

Allosteric regulation: regulatory molecules
control enzyme shape - thus controlling substrate accessibility to active site

Covalent modification of enzyme - e.g. phosphorylation

Question 32

What is an allosteric enzyme?

Answer 32

An enzyme that is able to change its shape upon binding of a ligand thus preventing access to its active sites by its substrate

Question 33

What is the process of allosteric regulation?

Answer 33

Inhibitor (usually product of catalysed reaction) can bind to sites on regulatory peptide on allosteric enzyme

This causes a conformational change within the allosteric enzyme thus causing it to change shape

This makes the active sites of the allosteric enzyme no longer accessible to the substrate meaning enzyme can no longer catalyse reaction

Question 34

Features of an allosteric enzyme

Answer 34

1. Multisubunit complexes
2. Regulatory sites and catalytic sites on different subunits
3. Regulation occurs via conformational changes
4. Exhibit non-Michaelis-Menten kinetics: reaction velocity vs substrate conc. plots are sigmoidal
5. Involved in feedback inhibition of metabolic pathways