Glycolysis

Question 1

Glycolysis definition

Answer 1

the first step in the breakdown of glucose to extract energy for cellular metabolism

Question 2

Glucose

Answer 2

• The most convenient
• Glucose is the most important carbohydrate
• More than half of all organic compounds is in starch and cellulose, two polysaccharides
• It is our primary source of energy and is oxidised to carbon dioxide and water
• Remember: GOCOWE!
• Glucose + O2 → CO2 + H2O + Energy
• Sometimes it takes another path and ends up as substances e.g. fat
• Cells such as erythrocytes, retina and renal medulla use only glucose as an energy source
• Need glucose for brain to function
• Think: need chocolate to study!

Question 3

Glycolysis reaction

Answer 3

Glucose + O2 → CO2 + H2O + Energy

Question 4

Intracellular stages of metabolism

Answer 4

1. glycolysis
2. citric acid cycle
3. electron transfer chain

Question 5

GLUT 1

Answer 5

• brain

- low Km

Question 6

GLUT 2

Answer 6

• liver, pancreatic beta cells

- insulin independent

Question 7

GLUT 3

Answer 7

• brain

- low Km

Question 8

GLUT 4

Answer 8

• muscle, adipose tissue

- insulin dependent

Question 9

GLUT 5

Answer 9

• gut

- fructose dependent

Question 10

How is glucose transported into cells

Answer 10

via passive transport a.k.a. facilitated diffusion, through glucose transporters

Question 11

Action of GLUT 1

Answer 11

1. A conformational change is triggered by the binding of glucose to the outside of the cell. This causes the binding site to face inwards.
2. Glucose is then released into the cell.
3. This causes another conformational change which regenerates the binding site back onto the outside of the cell

Question 12

Glycolysis reaction

Answer 12

glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP + 2 H2O + 2 NADH + 2 H+

Question 13

Stages of glycolysis

Answer 13

1. Glucose is trapped and destabilised.
2. Two 3-carbon molecules are formed.
3. Generation of ATP

Question 14

Two main cellular requirements

Answer 14

• ATP production

- Provision of building blocks for synthetic reactions

Question 15

Control points in glycolysis

Answer 15

Enzymes act as potential control points by catalysing irreversible reactions.

Question 16

Hexokinase

Answer 16

Glucose + ATP → glucose 6-phosphate + ADP + H+

△ G = -33.5 kJ mol-1

Question 17

Pyruvate Kinase

Answer 17

phosphoenolpyruvate + ADP + H+ → pyruvate + ATP

△ G = -16.7 kJ mol-1

Question 18

Phosphofructokinase

Answer 18

fructose 6-phosphate + ATP → fructose 1,6-bisphosphate + ADP + H+
△ G = -22.2 kJ mol-1

Question 19

The key enzyme in control of glycolysis

Answer 19

Phosphofructokinase

Question 20

Negative modulators

Answer 20

• ATP
• Citrate (early intermediate in the citric acid cycle; biosynthetic precursors are abundant)
• H+ (prevents excessive lactic acid formation)
• a negative feedback loop to maintain homeostasis

Question 21

Positive modulators

Answer 21

• AMP

- Fructose 2, 6-bisphosphate

Question 22

Energy charge

Answer 22

• energy charge = the ATP/AMP ratio

Question 23

The cell is fully charged if…

Answer 23

all adenylate nucleotides are in the shape of ATP

Question 24

The cell is discharged if…

Answer 24

the cell only contains AMP and Pi

Question 25

So why is AMP and not ADP the positive regulator?

Answer 25

BECAUSE if ATP is readily used up (ATP → ADP + Pi) then adenylate kinase can use some of the energy in ADP (2 ADP → ATP + AMP).

Question 26

Glucose transport into cell

Answer 26

• Glucose is not readily diffusible through plasma membrane, it is too big!
• Not lipophilic enough
• Therefore, requires a transporter, specifically, a symporter
• It utilises a Na gradient
• Inside cell high concentration of potassium and low concentration of sodium
• Outside cell low concentration of potassium and high concentration of sodium
• NA/K pump is an antiporter, transports Na out against concentration gradient and K in against its concentration gradient
• Energetically unfavourable reaction
• Opposite of osmosis
• Maintains this balance at an energy cost
• Glucose and Na move together with concentration gradient – doesn’t require energy.

Question 27

Glucose transport into cells: passive or active transport?

Answer 27

• passive (facilitated diffusion)

Question 28

What does Low Km mean?

Answer 28

the receptor has a high affinity (binds readily)

Question 29

Why does the brain have a low Km?

Answer 29

it needs to be able to bind to even very low concentrations of glucose for survival

Question 30

Stages of glucose transport into cell

Answer 30

1. A conformational change is triggered by the binding of glucose to the outside of the cell. This causes the binding site to face inwards.
2. Inside has a low concentration of glucose therefore glucose will be released
3. The off-rate is higher than the on-rate
4. Glucose is then released into the cell.
5. This causes another conformational change which regenerates the binding site back onto the outside of the cell.

Question 31

Stage 1 of glycolysis

Answer 31

• Glucose is trapped and destabilised.
• C6 of Glucose is phosphorylated to form glucose 6-phosphate
• Isomerised to produce fructose 6-phosphate (more symmetrical molecule than glucose)
• Symmetry is favourable
• Phosphorylate C1 to make more symmetrical, forming fructose 1,6 – triphosphate
• Kinase phosphorylates
• A hexokinase
• Got a phosphate, rearrange it, you get another phosphate!

Question 32

Stage 2 of glycolysis

Answer 32

• Two 3-carbon molecules are formed
• Carbon 2 and carbon 3 are swapped around
• Can be readily swapped by an enzyme called isomerase
• G3P very rapidly converted into something else and so it disappears
• All of this reaction is driven to completion by G3P

Question 33

Stage 3 of glycolysis

Answer 33

• Generation of ATP
• Control Points in Glycolysis
• G3P gets further phosphorylated
• Phosphates used to make ATP
• Isomerisation reactions occurring