Glycolysis

Question 1

What happens during glycolysis?

Answer 1

One molecule of glucose (6C molecule) is degraded into two molecules of pyruvate (3C molecule)

Question 2

How is free energy released in glycolysis stored?

Answer 2

As 2 molecules of ATP and 2 molecules of NADH

Question 3

In standard condition of the exergonic reaction glycolysis, why does it tend to be irreverisble?

Answer 3

Because of negative delta G naught

Question 4

How much total free energy is released by glucose in glycolysis?

Answer 4

5.2%

Question 5

How is glucose stored?

Answer 5

Glycogen, starch, sucrose

Question 6

How does glucose eventually become pyruvate?

Answer 6

Oxidation via glycolysis

Question 7

How does glucose eventually become ribose 5-phosphate?

Answer 7

Oxidation via pentose phosphate pathway

Question 8

Historical perspective of glycolysis: Louis Pasteur (1854-1864)

Answer 8

• Fermentation caused by microorganisms
• Aerobic growth requires less glucose than anaerobic growth (consumes sugar faster)
  –> Consumes only 1/16th of sugar with O2 present

Question 9

Historical perspective of glycolysis: Buchner (1897):

Answer 9

Reactions of glycolysis can be carried out in cell-free yeast extract

Question 10

Historical perspective of glycolysis: Harden and Young (1905):

Answer 10

1) Inorganic phosphate required for fermentation
2) Yeast extract could be separated in small molecular weight, essential coenzymes/cozymase, and bigger molecules called enzymes/zymase

Question 11

Historical perspective of glycolysis: Inhibitor studies

Answer 11

• Iodoacetate treatment resulted in the accumulation of fructose-1,6-bisphosphate
• Similarly, fluoride caused accumulation of 2-phosphoglycerate and 3-phosphoglycerate

Question 12

How does the system convert under anaerobic conditions?

Answer 12

1) Alcoholic fermentation in yeast (2 pyruvate –> 2 ethanol + 2CO2
2) Lactic acid fermentation in vigorously contracting muscle, erythrocytes, some other cells, and in some microorganisms (2 pyruvate –> 2 lactate)

Question 13

What is the first stage of glycolysis and what reactions are involved?

Answer 13

• Preparatory stage in which glucose is phosphorylated and converted to fructose, which is then phosphorylated to fructose-1,6-bisphosphate, which gets cleaved into two molecules of glyceraldehyde-3-phosphate
• Investment of 2 ATP molecules
• Reactions 1 to 5

Question 14

What is the second stage of glycolysis and what reactions are involved?

Answer 14

• Two molecules of glyceraldehyde-3-phosphate are eventually converted to pyruvate
• Generation of 4 ATP molecules and 2 NADH molecules, therefore, there is a net gain of 2 ATP molecules per molecule of glucose in glycolysis
• Reaction 6 to 10

Question 15

What is oxidized in mitochondria under aerobic conditions?

Answer 15

2 NADH molecules

Question 16

How is the free energy released from oxidized NADH synthesized to 6 molecules of ATP?

Answer 16

By oxidative phosphorylation

Question 17

What happens to pyruvate under aerobic conditions?

Answer 17

It is catabolized further in mitochondria through pyruvate dehydrogenase and citric acid cycle where all the carbon atoms are oxidized to CO2 (free energy released used to synthesize ATP, NADH, and FADH2)

Question 18

What is the importance of phosphorylated intermediates?

Answer 18

1) Possession of negative charge which inhibits their diffusion through membrane
2) Conversion of free energy in high energy phosphate bond

Question 19

Hexokinase (HK)

Answer 19

Phosphorylation of hexoses (mainly glucose)
- Enzyme present in most cells
- Requires Mg-ATP complex as substrate - ATP alone = competitive inhibitor
- Glucokinase is main hexokinase in liver and prefers glucose as substrate
- Catalyzed by proximity effects
- Enzyme undergoes large conformational change upon binding with glucose (allosterically inhibited by G6P)