Prof. Roshada Hashimroshadahashim@gmail.com

GLYCOLYSIS

www.lowcarbluxury.com/newsletter/lclnewsvol03...

General features of Glycolysis

1. Anaerobic degradation of hexose sugar

2. Conversion of a 6-carbon molecule (glucose, fructose) to a 3-carbon molecule ( dihydroxyacetone phosphate, glyceraldehyde 3-phosphate; pyruvate

3. One 6-carbon molecule will give two 3-carbon molecules

4. All the intermediates are phosphorylated; -vely charged at pH 7

5. Pi bonded by either an ester or anhydride bond

6. 2 phases: activation phase and energy production phase

1. 10 steps in glycolysis2. First 5 steps is the preparation or activation of glucose3. Uses 2 molecules of ATP4. 6-carbon degraded to 2 3-carbon molecules

1. Step 1: Phosphorylation. Glucose converted to glucose 6 phosphate

1. Coupling reaction Glucose Glucose 6-P G°’ = 13.8kJ/mol (3.3kcal/mol) ATP + H2O ADP + Pi G°’ = -30.5kJ/mol (-7.3kcal/mol)

Glucose + ATP Glucose 6-P +ADP G°’ = -16.7kJ/mol (-4.0kcal/mol)

2. Reaction catalysed by hexokinase (remember kinase – ATP dependent enzyme

3. Substrate can be any hexose sugar (fructose, mannose, glucose)

4. Glucose 6-P inhibits hexokinase

5. Keq for this reaction is high (2000) rxn is reversible but this does not happen in the cell b’coz:

a. Hexokinase affinity for glucose and ATP is higher than for ADP and G 6-P. hexokinase tends to be saturated with glucose and ATP

b. Hexokinase is inhibited by G 6-P

Hexokinase is an allosteric enzyme:

Activator: ADPInhibitor: ATP and Glucose 6-Phosphate

Liver glucokinase requires a higher glucose concentration to achieve saturation

Glucokinase: lowers blood glucose

Glucokinase Hexokinase

High activity in the liver

Low activity in the liver

Not found in muscle Found in the muscle

Specific for glucose Hexoses are substrates

Km(glucose) = 10mM Km(glucose) = 0.1

Step 2: Isomerization. Glucose 6 phosphate to fructose 6-phosphate

Glucose 6-P Fructose 6-P G°’ = 1.67kJ/mol (0.4kcal/mol)

1. The enzyme that catalyses the reaction is glucose phosphate isomerase2. Acid-base catalysis: Lys and His in the active site: Lys acts as the acid and His

as the base

(-3.4 kcal/mol)1. Endergonic reaction of phosphorylation of fructose 6-P is coupled with the

hydrolysis of ATP. 2nd ATP; 2nd activation step

2. This is the step which commits glucose to glycolysis (G 6-P and F 6-P involved in other pathways. The only way for F 1,6 bisP to be metabolised is via glycolysis

3. Highly exergonic & irreversible

4. PFK – key regulatory enzyme in glycolysis; ALLOSTERIC ENZYME

5. ATP: negative modulator

Step 3: Phosphorylation of fructose 6-phosphate to Fructose 1,6bisphosphate

www.rpi.edu/.../MBWeb/mb1/part2/gluconeo.htm

(5.7 kcal/mol)

1. The last of the activations step

2. Cleavage takes place between carbon-3 and carbon-4

3. Rxn moves towards triose sugar formation although G’ is positive

Step 4: Cleavage of Fructose 1,6bisphosphate to glyceraldehyde 3-P & dihydroxyacetone phosphate

1. Amino acids participating in the active site: Lys, Cys (thiol grp acts as a base) and His

2. Aldol cleavage

( 1.8kcal/mol)

1. 2nd glyceraldehyde 3-phosphate formed from this rxn

2. G under physiological conditions is slightly positive: 2.41kJ/mol or 0.58kcal/mol

3. Reaction favours formation of glyeraldehyde 3-phosphate because G for subsequent reactions in glycolysis are very negative and drives the rxn forward. (Overall G for glycolysis is negative)

Step 5: Isomerization of Dihydroxyacetone phosphate to glyceraldehyde 3-P

courses.cm.utexas.edu/.../Lecture-Ch14-1.html

glucose C1 and C6 becomes glyceraldehyde 3-phosphate C3

glucose C2 and C5 becomes glyceraldehyde 3-phosphate C2

glucose C3 and C4 becomes glyceraldehyde 3-phosphate C1 

2nd phase of glycolysis: production of energy

1. Involves 5 steps

2. Production of ATP

(1.5kcal/mol)

1. Involves 2 sets of reactions: i) Electron transfer rxn, from Glyceraldehyde 3-P to NAD+

ii) The addition of a phosphate

2. G 3-P to 3-Phosphoglycerate G’ = -43.1kJ/mol (-10.3kcal/mol) (oxdn) 3-PG to 1,3 bisPG G’ = 49.3kJ/mol ( 11.8kcal/mol) (phosln) Overall G’ = 6.2kJ/mol (1.5kcal/mol)

Step 6: Oxidation of Glyceraldehyde 3-P to 1,3 bisphosphoglycerate

Oxidation of glyceraldehyde 3-phosphate to a carboxylic acid

EXERGONIC

1

2

3

Electron transfer from G3-P to NAD+

3-phophoglycerate

ENDERGONIC

Step 7: Conversion of 1,3 bisphosphoglycerate to 3-phosphoglycerate

(-4.5kcal/mol)

1. A phosphate grp is transferred frm 1,3bPG to ADP

2. First ATP formed in glycolysis

3. Substrate-level phosphorylation

Question:If the G’ for the hydrolysisof 1,3bPG = -49.3kJ/mol and the G’ the hydrolysis of ATP is – 30.5kJ/mol, what is the G’ for the formation of 3-phosphoglycerate and ATP?

Step 8: Conversion of 3-PG to 2-PG

(1.1 kcal/mol)

Step 9: Dehydration of 2-PG to phosphoenolpyruvate (PEP)

(0.4 kcal/mol)

Step 10: Transfer of phosphate grp. from phosphoenolpyruvate (PEP) to ADP

(-7.5 kcal/mol)

1. PEP high energy compd. with high phosphate-grp transfer potential

2. Another example of substrate level phosphorylation

3. Pyruvate kinase is an allosteric enzyme

4. Pyruvate kinase is inhibited by high levels of ATP

Conversion of pyruvate to lactate in the muscle

1. Rxn is catalysed by lactate dehydrogenase

2. NAD+ is the co-factor

3. Rxn highly exergonic: G’ =25.1kJ/mol (6kcal/mol)

4. Lactate can be recycled in the liver to form pyruvate and glucose by gluconeogenesis

www.biologie.uni-hamburg.de/b-online/e19/19d.htm

Overall conversion of glucose to 2 moles of pyruvate:

G’ = -73.3 kJ/mol (-17.5kcal/mol)

www.nd.edu/~aseriann/glyreg.html

Regulation of glycolysis

1. Hexokinase

2. Phosphofructokinase

3. Pyruvate kinase

www-medlib.med.utah.edu/NetBiochem/tabletit.htm

Substrate To ATP

Glucose Glucose 6-phosphate -1

Fructose 6-phosphate Fructose 1,6 bisphosphate -1

2 x 1,3 phophoglycerate 2 x 3-phosphoglycerate +2

2 x PEP 2 x pyruvate +2

Net 2

ATP production and Efficiency of Glycolysis

Glucose + 2 ADP + 2Pi 2 Lactate + 2 ATP G’ = -184.5kJ/mol(-44.1 kcal/mol)

But in glycolysis only 2 ATPs are formed when glucose is oxidised to lactate. To form the ATP molecules would require : 161.1kJ/mol(-14.6 kcal/mol)

2ADP + 2Pi 2ATP G’ = 61.1kJ/mol(-14.6 kcal/mol)

% of energy conserved is 61.1/184.5 x 100 = 33.1%