Enzymes

Question 1

What are enzymes

Answer 1

Enzymes lower the activation energy required for a chemical reaction by stabilising the transition state

Question 2

RATE ENHANCEMENT

Answer 2

= catalysed rate/uncatalysed rate

Question 3

Disease

Answer 3

Enzyme deficiencies caused by “inborn errors” cause disease (e.g. Phenylketonuria – lack of phenylalanine hydroxylase)

Question 4

Diagnosis

Answer 4

Myocardial Infarction or Indigestion? - Need to be able to measure the concentration and activity of many enzymes

Question 5

Drug Therapy

Answer 5

Many drugs are enzyme inhibitors. The specific dosage is related to the inhibition mechanism

Question 6

Define the function of a lysosome

Answer 6

Catalyses the cutting of polysaccharide chains
Lysozyme binds to the polysaccharide chain, catalyzes the cleavage of a specific covalent bond and releases the cleaved products

Question 7

What is the only valid parameter

Answer 7

vo (initial reaction velocity) is the only valid parameter

Question 8

What is enzyme activity measured by

Answer 8

increasing the substrate concentration and measuring the accumulation of products over time.

Question 9

What does the activity of an enzyme depend on

Answer 9

how rapidly it can process a substrate

Question 10

What is the rate of reaction trend

Answer 10

The rate of reaction increases as the substrate concentration increases until a maximum is reached (Vmax)

Question 11

How can Vmax be calculated

Answer 11

Plotting a double reciprocal of the data (Lineweaver-Burke Plot = this gives us a lot more information about the enzyme

Question 12

What is Michaelis-Menten Kinetics

Answer 12

This is represented as Km (the constant) and is used in conjunction with Vmax to measure the activity of an enzyme using the Michaelis-Menten equation

Question 13

What is the Michaelis- Menten equation

Answer 13

Vo = Vmax . [S] / Km + [S]

Question 14

What does Km = [S] represent

Answer 14

The substrate concentration required for half maximum velocity

Question 15

What is the biological significance of Km

Answer 15

Blood Glucose Levels:
Hexokinase has a lower Km and works at low concentrations of glucose. It is found in all tissues and is required for energy production in cells. A low Km ensures the utilisation of glucose even at very low concentrations

Question 16

What is glucokinase

Answer 16

Its predominantly found in the liver and will only phosphorylate glucose when concentrations are high allowing the production of glycogen (storage).

Question 17

What does a high Km ensure for glucose

Answer 17

it ensures its not removed from the blood for storage when the concentration is low

Question 18

Where is PHOSPHO1 found

Answer 18

it is an enzyme found in mineralising cells such as osteoblasts and chondrocytes
Enzyme had unknown substrate/function
Amino acid sequence suggested it to be a phosphatase (enzyme which removes phosphate groups from molecules)

Question 19

What are enzyme inhibitors

Answer 19

chemicals that interfere with enzyme reactions Many drugs are inhibitors of enzymes

Question 20

What are irreversible enzyme inhibitor called

Answer 20

inactivators

Question 21

what are reversible enzyme inhibitors called

Answer 21

Competitive

Allosteric (non competitive)

Question 22

What is irreversible inhibition

Answer 22

Irreversible inhibitors react with the enzyme and form a covalent adduct with the protein.

Question 23

What is competitive inhibition

Answer 23

A competitive drug can compete with the substrate for the active site of the enzyme.
• The substance has a similar structure to that of the normal substrate.
• The drug will occupy the active site and then leave it unchanged.
• The action of the enzyme is slowed down by the presence of a competitive inhibitor.
• A competitive inhibitor is like a key that gets though the keyhole but cannot unlock the door.

Question 24

How do Competitive Inhibitors Alter the Kinetics of an Enzyme

Answer 24

•The inhibitor binds reversibly to the active site
•The inhibitor and substrate compete for the active site
Therefore, a higher concentration of substrate is needed to reach Vmax, resulting in an increase in Km (Vmax is unchanged)

Question 25

What is allosteric inhibition

Answer 25

Allosteric inhibitors can bind to the enzyme at the same
time as the substrate i.e. they never bind to the active site.
• They render the enzyme and enzyme-substrate complex inactive because the inhibitor cannot be driven from the enzyme by higher substrate concentration (in contrast to competitive inhibition)

Question 26

What does allosteric inhibition cause

Answer 26

Vmax decreases.

• Km often (but not always) increases

Question 27

What are the two types of allosteric inhibition

Answer 27

mixed inhibition and non-competitive inhibition

Question 28

Mixed inhibition

Answer 28

Allosteric change affects substrate binding and activity
Inhibition can be reduced but not overcome by increasing [S].
Vmax decreases due to inhibition
Km increases as substrate affinity is reduced

Question 29

Non-competitive Inhibition

Answer 29

Allosteric change affects activity but not substrate binding
Vmax decreases due to reduced efficiency of the reaction
Km (and substrate affinity) is unchanged

Question 30

Whats the function of phosphofructokinase

Answer 30

Catalyses transfer of phosphate from ATP to fructose 6-phosphate
Important step in glycolysis
• PFK binds ATP at two sites (active site and inhibitory site)
• In the presence of high levels of ATP, the inhibitory site is occupied and fructose 6-phosphate binding is affected
• Glycolysis does not proceed when ATP is not needed!