Importance, phases, reactions, energetics and regulation of glycolysis

1. Glycolysis
R. C. Gupta
Professor and Head
Department of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Glycolysis is also known as Embden-
Meyerhof pathway
It is the main pathway for oxidation of
glucose
Its tissue distribution is nearly
universal

3. Enzymes of glycolysis are present in
cytosol
One glucose molecule is oxidised to two
molecules of pyruvate or lactate
In aerobic conditions (adequate oxygen),
the end product of glycolysis is pyruvate

4. One molecule of NAD is reduced per
molecule of pyruvate formed
The reduced NAD is reoxidised in the
respiratory chain by molecular oxygen
Thus, a continuous supply of NAD is
assured for continuation of glycolysis

5. Reduced NAD cannot be reoxidised under
anaerobic conditions due to lack of oxygen
When all the available NAD is reduced,
glycolysis will stop
To prevent this, pyruvate is reduced to
lactate in anaerobic conditions

6. Reduction of pyruvate is coupled with
oxidation of NADH to NAD
The purpose of conversion of pyruvate into
lactate is to ensure availability of NAD
Availability of NAD facilitates continuation
of glycolysis

7. When the conditions become aerobic
again, lactate is oxidised to pyruvate
Pyruvate is either converted into glucose
or is oxidised

8. The glycolytic reactions may
be divided into three phases:
Priming or preparatory phase
Splitting phase
Oxidative phase

9. Priming phase
Glucose is prepared for
splitting and oxidation
It is converted into
fructose-1,6-biphosphate
Two high-energy bonds of ATP
are utilised

10. Splitting phase
The hexose is split into two
trioses
The final product is glyceraldehyde-
3-phosphate

11. Oxidative phase
Glyceraldehyde-3-phosphate is
oxidised to pyruvate (or lactate)
Energy is liberated during oxidative
reactions

12. Reactions of the priming phase
Glucose is first phosphorylated to
glucose-6-phosphate
The phosphate group is provided by ATP
The reaction is catalysed by hexokinase
or glucokinase

13. Hexokinase
Widely distributed
Can act on all
hexoses
Has a low Km
Allosteric enzyme
Glucokinase
Present in liver
and b cells
Acts on glucose
only
Has a high Km
Inducible enzyme

14. Some free energy is released in the
phosphorylation reaction
Therefore, the reaction is functionally
irreversible

16. Hexokinase acts when the intracellular
glucose concentration is low e.g. during
fasting
Glucokinase acts when the intracellular
glucose concentration is high as after a
meal

17. Conversion of glucose into glucose-6-
phosphate is important because:
After its conversion into glucose-
6-phosphate, glucose cannot leave
the cell
Glucose enters most of the
metabolic pathways in the form of
glucose-6-phosphate

18. Glycolysis can begin with glycogen also
Glycogenolysis liberates glucose-1-
phosphate
This can be isomerised to glucose-6-
phosphate
This would conserve one high-energy
phosphate bond

19. In the second reaction, glucose-6-
phosphate is isomerised to fructose-6-
phosphate
This is a reversible reaction
It is catalysed by phosphohexose
isomerase

21. Fructose-6-phosphate is phosphorylated
to fructose-1,6-biphosphate
The reaction is catalysed by phospho-
fructokinase-1 (PFK-1)
The reaction is functionally irreversible
due to release of free energy

23. Reactions of the splitting phase
The second phase begins now
Fructose-1,6-biphosphate is cleaved into
two molecules of glyceraldehyde-3-
phosphate
The conversion occurs by two reactions

24. Fructose-1,6-biphosphate is split into two
triose molecules
The trioses are dihydroxyacetone
phosphate and glyceraldehyde-3-
phosphate

26. Dihydroxyacetone phosphate is
isomerised to glyceraldehyde-3-
phosphate
Thus, two molecules of glyceraldehyde-
3-phosphate are formed from one
molecule of glucose

28. Glyceraldehyde-3-phosphate is oxidised
Energy is released during its oxidation
This energy is used to introduce inorganic
phosphate into the substrate by a high-
energy bond
Reactions of the oxidative phase

30. This is the only reaction of glycolysis in
which NAD is reduced
The reaction can be inhibited by arsenate

31. The high-energy phosphate of 1,3-bi-
phosphoglycerate is transferred to ADP
1,3-Biphosphoglycerate is converted into
3-phosphoglycerate

33. In the next reaction, 3-phosphoglycerate is
isomerised to 2-phosphoglycerate
The reaction is catalysed by phospho-
glycerate mutase

35. 2-Phosphoglycerate is dehydrated to
phosphoenol pyruvate (PEP) by enolase
The intramolecular re-arrangement
converts the low-energy phosphate into
high-energy phosphate
Fluoride ions inhibit enolase

37. The high-energy phosphate of phospho-
enol pyruvate is transferred to ADP
The reaction is functionally irreversible

38. Enol pyruvate is a transient intermediate
It is spontaneously converted into pyruvate
Pyruvate is the end product of aerobic
glycolysis

40. Normally pyruvate is converted into acetyl
CoA
Acetyl CoA is oxidised in the citric acid
cycle

41. The end product is different in anaerobic
conditions
Conditions can become anaerobic in
muscles during exercise
Oxygen consumption can outstrip the
supply during exercise

42. Pyruvate is reduced to lactate in
anaerobic conditions
NADH is oxidised to NAD in this
reaction
NAD is then used for oxidation of
glyceraldehyde-3-phosphate

44. Conversion of pyruvate into lactate also
occurs in cells lacking mitochondria
Such cells include RBCs, corneal cells
and lens cells
NADH cannot be oxidised in these cells
due to absence of respiratory chain

46. An alternate intermediate is formed from
1,3-biphosphoglycerate in RBCs
This is 2,3-biphosphoglycerate (2,3-BPG)
2,3-BPG is formed by isomerisation of
1,3-biphosphoglycerate

47. 2,3-Biphosphoglycerate has an important
role in RBCs
It binds to Hb and helps in the
release of oxygen from oxyhaemoglobin
No ATP is formed when 2,3-biphospho-
glycerate is converted into 3-phospho-
glycerate

49. Two high-energy phosphate bonds of
ATP are utilised in the priming phase:
For conversion of glucose into
glucose-6-phosphate
For conversion of fructose-6-phos-
phate into fructose-1,6-biphosphate
Energetics

50. Oxidation of glyceraldehyde-3-phosphate
to 1,3-biphosphoglycerate reduces one
NAD molecule to NADH
One NADH will form three ATP equi-
valents upon its oxidation in the
respiratory chain

51. One ATP is formed when 1,3-biphospho-
glycerate is converted into 3-phospho-
glycerate
Another ATP is formed when phosphoenol
pyruvate is converted into pyruvate
Thus, 5 ATP equivalents are formed from
each molecule of glyceraldehyde-3-
phosphate

52. Two molecules of glyceraldehyde-3-
phosphate are formed from one molecule
of fructose-1,6-biphosphate
Ten ATP equivalents are formed from two
molecules of glyceraldehyde-3-phosphate

53. Thus, the total gain of energy is ten ATP
equivalents
Total consumption of energy is two ATP
equivalents
Therefore, the net gain is eight ATP equi-
valents from each molecule of glucose

54. NADH cannot be oxidised in the
respiratory chain in the absence of
oxygen
The energy present in NADH would not
be captured in anaerobic conditions

55. In anaerobic conditions, oxidation of
NADH is coupled with reduction of
pyruvate to lactate
Therefore, the net gain of energy is only
two ATP equivalents

56. In aerobic conditions, pyruvate is
oxidized to acetyl CoA
This reaction forms 3 ATP equivalents
Oxidation of one acetyl CoA in citric
acid cycle forms 12 ATP equivalents

57. Thus, eight ATP equivalents are formed
from oxidation of one molecule of
glucose to two molecules of pyruvate
30 ATP equivalents from oxidation of two
molecules of pyruvate to carbon dioxide
and water

58. Thus, 38 ADP molecules are converted to
ATP on complete oxidation of one
molecule of glucose
Energy of hydrolysis of terminal phosphate
bond of ATP is 7.3 kcal/mol
So, 38 high-energy phosphate bonds of
ATP represent a capture of 38 X 7.3 =
277.4 kcal per mol of glucose

59. Potential energy present in glucose is
686 kcal per mol
So, the efficiency of oxidation of glucose
is 277.4  686 X 100% or nearly 40%

60. Hydrolysis of ATP to ADP gives 7.3 kcal
per mol in standard laboratory conditions
In living cells, it may be more
Thus, efficiency of oxidation of glucose in
living cells may be more than 40%

61. Three reactions of glycolysis are
functionally irreversible
These are catalysed by allosteric
enzymes
These are hexokinase, phosphofructo-
kinase-1 and pyruvate kinase
Regulation

62. Hexokinase is inhibited by glucose-6-
phosphate
However, this reaction is not unique to
glycolysis
The committed reaction of glycolysis is
catalysed by phosphofructokinase-1

63. Thus, phosphofructokinase-1 is the main
regulatory enzyme
ATP and citrate are allosteric inhibitors of
phosphofructokinase-1
AMP and fructose-2,6-biphosphate are
its allosteric activators

64. Intracellular concentrations of ATP and
AMP indicate the energy status of the cell
Abundance of energy, indicated by a high
ATP concentration, decreases glycolysis
Lack of energy, indicated by a high AMP
concentration, increases glycolysis

65. Pyruvate kinase is inhibited by ATP and
alanine
It is activated by fructose-1,6-biphosphate

66. Glucokinase too has a role in regulation
It is activated by fructose-1-phosphate
and inhibited by fructose-6-phosphate
In liver, glucokinase is induced by insulin