Regulation of Enzyme Activity

Question 1

What is the function of a coenzyme?

What type of molecule typically acts as a coenzyme?

Answer 1

They function as carriers of reaction components

e.g. electrons or acyl units

Many water-soluble vitamins are essential coenzymes

Question 2

Which coenzymes act as carriers of acyl units or carbon dioxide units?

Answer 2

Coenzyme A carries acyl units

Biotin and thiamine pyrophosphate carry CO2 units

Question 3

What is the coenzyme function of riboflavin (B2)?

What does a deficiency result in?

Answer 3

FAD, FMN

It results in normochromic-normocytic anaemia

Question 4

What is the coenzyme function of niacin?

What does a deficiency result in?

Answer 4

NAD+, NADP+

Deficiency results in pellagra

Question 5

What is the coenzyme function of thiamine (B1)?

What does a deficiency result in?

Answer 5

Thiamine pyrophosphate

Deficiency results in beriberi

Question 6

What is the coenzyme function of Vitamin C?

What does a deficiency result in?

Answer 6

Prolyl and lysly hydroxylase

Deficiency results in scurvy

Question 7

What is the coenzyme function of biotin?

What does a deficiency result in?

Answer 7

Biotinylated carboxylases

Deficiency results in alopecia, ataxia and seizures

Question 8

What is the coenzyme function of cobalamin (B12)?

What does a deficiency result in?

Answer 8

Cobamide coenzymes

Deficiency results in pernicious anaemia

Question 9

What is the coenzyme function of folic acid?

What does a deficiency result in?

Answer 9

Tetrahydrofolate

Deficiency results in megaloblastic anaemia

Question 10

What is the coenzyme function of pyridoxal (B6)?

What does a deficiency result in?

Answer 10

Pyridoxal phosphate

Deficiency results in peripheral neuropathy, depression, seizures and glossitis

Question 11

What is the coenzyme function of pantothenic acid?

What does a deficiency result in?

Answer 11

coenzyme A

Deficiency results in hypoglycaemia

Question 12

What is the difference in the pathways that NADH and NADPH are involved in?

Answer 12

They are both carriers of reducing power

NADH is involved in energy metabolism

NADPH is involved in biosynthetic reactions

Question 13

What type of reactions is NADH involved in?

Answer 13

Redox reactions

It carries electrons from one species to another

Question 14

How does the molecular structure of NADPH differ to NADH?

Answer 14

NADPH has an additional phosphate group on the ribose sugar of the adenosine nucleoside component

Question 15

Why will no enzyme work on both NAD and NADP?

Answer 15

The presence of the phosphate group on NADP ensures specificity

Question 16

In which reaction does NAD+ act as a cofactor?

Answer 16

The conversion of ethanol to acetaldehyde

This is catalysed by alcohol dehydrogenase

NAD+ is converted to NADH

Question 17

What happens to acetaldehyde after it has been formed from ethanol?

What cofactor is required?

Answer 17

It is metabolised to pyruvate by pyruvate decarboxylase

Pyruvate decarboxylate uses biotin as a cofactor

Question 18

What 2 molecules are essential for the metabolism of alcohol to pyruvate?

Answer 18

An input of NAD+ and a biotin cofactor

Question 19

What is the most common mutation leading to enzyme defect in humans?

Answer 19

Mutations leading to defects in the G6PDH enzyme

This is the glucose-6-phosphate dehydrogenase enzyme

Question 20

What is the role of the G6PDH enzyme?

Answer 20

it is part of the first step in the Pentose Phosphate Pathway

This pathway involves making a large amount of the body’s NADPH and ribose-5-phosphate

Question 21

Why must the Pentose Phosphate Pathway be fully active during growth?

Answer 21

Active growth requires:

1. more nucleic acids - this requires ribose-5-phosphate
2. active biosynthetic pathways - this requires NADPH

Question 22

What does a mutation in G6PDH lead to?

Answer 22

The activity of G6PDH is limited, leading to a restricted ability to make NADPH

Question 23

When do people with mutations in G6PDH detect symptoms?

What can this lead to?

Answer 23

Carriers do not detect symptoms until there is an increased demand for NADPH

This can trigger a haemolytic crisis and anaemia as red blood cells have a dependence on G6PDH for their NADPH

Question 24

What is the reaction that G6PDH is involved in?

How can this reaction be increased?

Answer 24

It converts NADP+ to NADPH

Activity of G6PDH is increased by high levels of NADP+

This increases production of NADPH

Question 25

What are red blood cells reliant on to generate NADPH?

Answer 25

They lack mitochondria so are entirely reliant on the PPP to generate NADPH

Question 26

Which drug should be avoided in patients with G6PDH deficiency? Why?

Answer 26

Primaquine - an antimalarial drug It causes increased generation of peroxides This damages the RBC membrane and leads to haemolytic crisis

Question 27

In which 3 ways can a G6PDH mutation affect the enzyme?

Answer 27

1. mutations affecting the active site disrupt activity 2. mutations affecting the regulative site disrupt NADP+ binding 3. mutations may destabilise the dimer

Question 28

Why can treatment with primaquine induce a haemolytic crisis in some people?

Answer 28

It arises as NADPH has an important role in maintaining the levels of reduced glutathione in cells

Question 29

What is the structure of glutathione?

Answer 29

It is a tripeptide of glycine, cysteine and glutamic acid

Question 30

What is the role of glutathione in its reduced form?

Answer 30

It acts as a sulfhydryl buffer It keeps the cysteine residues of haemoglobin and other proteins in the reduced state

Question 31

How does glutathione act to stabilise cell membranes?

Answer 31

It reduces lipid hydroperoxides These are produced by the reaction of unsaturated bonds in lipids with hydrogen peroxide

Question 32

What happens to cell membranes if lipid hydroperoxides are not reduced?

Answer 32

The hydroperoxides react and cause cleavage of acyl chains This leads to lysis of red blood cells

Question 33

What is the reaction performed by glutathione to reduce hydroperoxides? What enzyme is involved?

Answer 33

Glutathione reduces the hydroperoxides to alcohols This is catalysed by glutathione peroxidase It also produces oxidised glutathione

Question 34

How can oxidised glutathione be converted back to reduced glutathione?

Answer 34

By NADPH This is catalysed by glutathione reductase

Question 35

How can the cell membranes become damaged when there is no NADPH present?

Answer 35

Reduced glutathione mops up damaging molecules such as free radicals Without NADPH, there is no reduced glutathione The membrane becomes damaged as the molecules are not removed

Question 36

Why does primaquine cause RBC lysis in patients with G6PDH mutations?

Answer 36

Primaquine stimulates peroxide formation This increases the demand for NADPH to a level that the mutant enzyme cannot provide

Question 37

When are thiopurine drugs used?

Answer 37

They are used as anti-cancer agents