Lecture 11 - Glycolysis

Question 1

Describe the payoff phase of glycolysis.

Answer 1

At the end of the investment phase of glycolysis we are left with two molecules of G3P therefore the payoff phase steps occurs for both of these molecules.

Step 6 - G3P is converted to 1,3-Biophosphoglycerate and NAD+ is reduced to NADH. By adding a second phosphate a high energy phosphate compound is formed meaning the phosphate is easily lost
Step 7 - 1,3-biphophoglycerate loses a phosphate to ADP producing 3-phophoglycerate and ATP as the products. The reaction is catalysed by phosphoglycerate kinase
Step 8 - Phosphoglyceromutase catalyses the rearrangement of 3-phophoglycerate to 2-phophoglycerate.
Step 9 - 2-phophoglycerate is converted to phosphoenolpyruvate (PEP) in a dehydration reaction catalysed by enolase
Step 10 - Phosphoenolpyruvate is converted to pyruvate as the phosphate is lost to ADP forming Pyruvate and ATP as the products. The reaction is catalysed by Pyruvate kinase

Question 2

Describe the possible fates of pyruvate.

Answer 2

1. Pyruvate is completely oxidised to CO2 in the Krebs cycle
   During glycolysis only 147kJ of energy are released from one mole of glucose which is only 3% of the total free energy of glucose. The remaining energy is release in the TCA cycle and the electron transport chain which produce far more ATP than glycolysis
2. Conversion of pyruvate to lactate
   In anaerobic conditions the cell must produce all its ATP via glycolysis. However in step 6 NAD+ is consumed and NADH is formed. In order for glycolysis to continue the NADH must be re-oxidised to NAD+.
   Therefore pyruvate is converted to lactase to produce NAD+. The reaction is catalysed by lactate dehydrogenase which allow glycolysis in oxygen deprived conditions.

Question 3

What other substrate may be used in glycolysis?

Answer 3

Glucose is not the only substrate in glycolysis, many other compounds can enter the pathway at various stages.
E.g. glycerol is produced by the hydrolysis of triglycerides. This can be converted to the glycolytic intermediate dihydroxyacetone phosphate (DHAP)

Question 4

What are the energetics behind glycolysis allowing it to occour?

Answer 4

The conversion of glucose to two molecules of pyruvate is exergonic = -147 kJ/mol
However the conversion of ADP to ATP is endergonic = +30 x2 kJ/mol
Therefore glycolysis as a whole is exergonic = -87 kJ/mol
Therefore the conversion of glucose to pyruvate is still favoured even though it is coupled to the energy-requiring production of two molecules of ATP.

Four steps in glycolysis are exergonic and are therefore favoured reactions. These steps involve either the production or hydrolysis of ATP.
Although the overall pathway thermodynamically favours pyruvate production, the free energy change of each individual reaction is not great. This means that most of the steps can be reversed without needing large amounts of energy.

Question 5

How is glycolysis regulated and controlled?

Answer 5

Irreversible Steps:
Glycolysis is regulated by three irreversible steps: 1, 3, and 10.
These steps are catalyzed by specific enzymes:
* Step 1: Hexokinase
* Step 3: Phosphofructokinase (PFK)
* Step 10: Pyruvate kinase

Control of Step 3 (Phosphofructokinase, PFK):
Step 3 is the major control point of glycolysis.
Phosphofructokinase (PFK) catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
PFK is regulated by allosteric regulation.
PFK is downregulated by:
ATP
NADH
Citrate
Long-chain fatty acids
H+
PFK is upregulated by:
AMP
Fructose-2,6-bisphosphate (F-2,6-BP)
Fructose-2,6-bisphosphate acts as a potent activator of PFK.
Tissue-specific regulation:
Muscle: Regulation primarily by the ATP:AMP ratio and inhibition by H+ during intense exercise.
Liver: Regulation by ATP, citrate, and F-2,6-BP; low pH is not a regulator.

Control of Step 1 (Hexokinase):
Hexokinase is inhibited by glucose-6-phosphate (G6P) through product inhibition.
Functions of G6P inhibition:
Ensures that phosphorylation of glucose decreases when sufficient G6P is present.
* Allows glucose in the blood to be available to liver glucokinase, which has a lower affinity for glucose than hexokinase and is not inhibited by G6P.