Enzymes I

Question 1

What are enzymes?

Answer 1

Proteins that catalyse (speed up) specific chemical reactions

Question 2

What are the 5 different functions of enzymes?

Answer 2

1. Digestion
   - carbs, fats, proteins
2. Blood Clotting
   - fibrin clot catalysed by thrombin
3. Defence
   - immune system- activation of complement
4. Movement
   - Muscle actomyosin is an ATPase
5. Nerve Conduction
   - membrane ion pumps for Na+ and Ca2+

Question 3

How are enzymes drug targets?

Answer 3

Antibiotics:
-penicillins inhibit cell wall synthesis
Anti-inflammatory agents:
-aspirin blocks prostaglandins
Anticancer drugs:
-methotroxate interferes with synthesis of DNA precursors

Question 4

What are 5 key properties of enzymes?

Answer 4

• increase reaction rate
• show specificity
• unchanged at end of reaction
• do not alter reaction equilibrium
• facilitate reaction by decreasing activation energy of reaction

Question 5

What is the enzyme’s active site?

Answer 5

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

Question 6

What was used to find evidence of enzyme active sites?

Answer 6

• X-ray crystallography
• kinetic studies of enzyme activity
• helps to identify where substance binds to active site
• learn about affects of mutations

Question 7

What are the two models used to describe the binding of a substrate to an enzyme’s active site?

Answer 7

• Lock and Key model

- Induced Fit model

Question 8

Describe the Lock and Key model

Answer 8

The enzyme’s active site is complementary to the substrate

-but this isn’t true for all enzymes

Question 9

Describe the Induced Fit model

Answer 9

The enzyme’s active site is not complementary to the substrate but as binding starts, the enzyme is made to fit around the substrate molecule

Question 10

What is enzyme-substrate binding used for?

Answer 10

• to bring molecules together in active site
• to constrain substrate movement, and so make reaction more likely
• stabilise positive and negative charges in transition state
• provide a reaction pathway for lower charges
• to strain a particular bonds in the substrate, makes breakage easier as substrate is distorted on binding to resemble transition state
• use of cofactors

Question 11

What is the effect of enzyme-substrate concentration, chemically and graphically?

Answer 11

• increasing the conc of enzyme and substrate= an increase in rate of reaction
• curve increases rapidly but then plateaus as it reaches max. velocity
• when looking at substrate conc, active sites eventually get saturated= max. velocity
• when looking at enzyme conc, substrates eventually gets used up= max. velocity

Question 12

What is the equation to work out the turnover number of an enzyme?

Answer 12

Max. velocity/conc. of enzyme

Question 13

What is the Michaelis Constant, Km?

Answer 13

Measure used to find out substrate conc. when the reaction is at half the rate

Question 14

What is competitive inhibition and what does it cause?

Answer 14

• inhibitor binds to same active site as substrate
• reduces rate of reaction
• max. velocity is unaltered but Michaelis constant increases

Question 15

What is non-competitive inhibition and what does it cause?

Answer 15

• binds to allosteric site on enzyme causing a conformational change
• enzyme is denatured due to change in tertiary structure
• max. velocity decreases but Michaelis constant is unaltered

Question 16

How are enzymes regulated?

Answer 16

Compartmentalisation:
-sequences in enzyme polypeptide chain target enzyme to ER/nucleus
Allosteric regulation:
-regulatory molecules control protein shape
Covalent modification of enzyme:
-change enzyme shape and activity
-e.g. phosphorylation

Question 17

What are the properties of allosteric enzymes?

Answer 17

• multisubunit complexes
• regulatory sites and catalytic sites on diff subunits
• regulation occurs via conformational changes
• regulated by feedback inhibition
• found at the beginning of metabolic pathways