Carbohydrate Metabolism–I
Glycolysis,
TCA Cycle
&
Hexose Monophosphate Shunt
Pathway
Metabolism of Carbohydrate
1. Energy (ATP) producing pathways/reactions
(in all cells)
•Glycolysis
•Oxidation of pyruvate
•Tricarboxylic acid cycle (TCA cycle)
2. Production of NADPH and ribose
• Hexose monophosphate(HMP) shunt pathway
3.Pathways involved in blood glucose homeostasis
•Gluconeogenesis
•Metabolism of glycogenglycogenesis & glycogenolysis
Clinical Importance/Disorders of
Carbohydrate Metabolism
•Obesity
•Diabetes mellitus
•Glycogen storage diseases
Glycolysis
• Definition
• Tissue location
• Intracellular location
• Types
• End product
• Reactions (Pathway)
• Energetics
• Functions
Glycolysis
Synonym: Embden-Meyerhof pathway
• Definition
Glycolysis is an oxidative pathway for the
catabolism of glucose to pyruvate or lactate
accompanied by the formation of ATP.
Tissue location
All the cells of the body. RBC
Intracellular location:
Cytosol
Types:
Aerobic in the presence of oxygen
Anaerobic
lack of oxygen
End product:
Pyruvate, during aerobic conditions
Lactate/lactic acid,RBC under anaerobic
conditions in skeletal muscles during
prolonged contraction
Reaction Sequence/Pathway
It can be divided in to 2 phases
I. Energy consuming/investment phase
glucose → glyceraldehyde-3-phosphate
II. Energy generation/payoff phase –
glyceraldehyde-3-phosphate → pyruvate
I. Energy consuming/investment phase
Glucose (6 C)
ATP
Hexokinase
Mg2+
ADP
glucokinase (in liver cell)
Glucose-6-phosphate (6 C)
Phospho hexose isomerase
Fructose-6-phosphate (6 C)
ATP
Mg2+
Phosphofructokinase
ADP
Fructose-1, 6-bisphosphate (6 C)
Aldolase
Glyceraldehyde-3-Phosphate
Dihydroxyacetone-P (3 C)
Triose phosphate isomerase
(3 C)
II. Energy generation/payoff phase –
Glyceraldehyde-3-Phosphate (3 C)
Pi
NAD+
Glyceraldehyde-3- phosphate dehydrogenase
NADH + H+
1, 3-bisphosphoglycerate (3 C)
ADP
Mg2+ 1, 3-bisphospho-glycerate kinase Substrate level
phosphorylation
ATP
3-phosphoglycerate (3 C)
Phosphoglycerate mutase
2-phosphoglycerate (3 C)
Mg2+
H2O Enolase
Phosphoenol pyruvate (3 C)
Phosphoenol pyruvate (3 C)
Cytosol
ADP
Pyruvate kinase Substrate level
Mg2+
phosphorylation
ATP
Lactate dehydrogenase
Lactate (3 C)
Pyruvate (3 C)
Anaerobic condition
+
NAD
+
NADH+H
Aerobic condition
Pyruvate
Acetyl CoA
TCA cycle
glucose
hexokinase/ glucokinase
glucose-6- phosphate
Cytoplasm
Phosphohexose isomerase
Fructose -6- phosphate
Phosphofructo kinase
Fructose-1,6-bisphosphate
Aldolase
Glyceraldehyde – 3- phosphate
Glyceraldehyde-3- phosphate dehydrogenase
1,3-bisphosphoglycerate
Dihydroxy acetone
1, 3-bisphospho-glycerate kinase
phosphate
3-phosphoglycerate
Phosphoglycerate mutase
2-phosphoglycerate
Enolase
Phosphoenolpyruvate
Pyruvate kinase
Lactate dehydrogenase Lactate
Pyruvate
Energetics of Aerobic Glycolysis
Energy produced
Reaction
catalysed by
Glyceraldehyde 3-phosphate
dehydrogenase
Phosphoglycerate
kinase
Pyruvate kinase
Method of ATP
formation
ATP per
mol of
glucose
Respiratory chain
oxidation of 2
NADH
Substrate level
phosphorylation
Substrate level
phosphorylation
5
2
2
Total = 9
Energy utilised
Reaction catalysed by ATP per mol of
glucose utilised
Hexokinase
Phosphofructokinase
1
1
Total =2
Net energy produced in Aerobic Glycolysis
= energy produced – energy utilized
= 9- 2
= 7 ATP
Energetics of Anaerobic Glycolysis
Energy produced
Reaction
catalysed by
Method of ATP
formation
Phosphoglycerate Substrate level
kinase
phosphorylation
Pyruvate kinase
Substrate level
phosphorylation
ATP per
mol of
glucose
2
2
Total= 4
Energy utilised
Reaction catalysed by ATP per mol of
glucose utilised
Hexokinase
Phosphofructokinase
1
1
Total= 2
Net energy produced in Anaerobic Glycolysis
= energy produced – energy utilized
= 4- 2
= 2 ATP
Functions/Significance of Glycolytic Pathway
1. Glycolysis is a major pathway for ATP synthesis
a)In all the cells of the body.
b)In tissues lacking mitochondria,egRBC.
c)During strenuous exercise, anaerobic glycolysis is the
source of energy for muscles.
2. 2,3- bisphosphoglycerate formed from 1,3bisphosphoglycerate in RBC help to release oxygen
from Hb in the peripheral tissues.
3. Glycolysis serves as pathway for catabolism of
fructose, galactose and some pentoses
derived from the diet
4. It provides carbon skeletons for the synthesis of
certain non-essential amino acids
5. Most of the reactions of glycolysis are reversible
used for gluconeogenesis in liver and kidneys.
Rapaport Luebering Cycle in RBC
In this pathway the reaction catalyzed by 1,3bisphosphoglycerate kinase is bypassed involves no
net yield of ATP. It takes place in RBC.
Pathway
Glucose (6 C)
GLYCOLYSIS
1, 3-bisphosphoglycerate (3 C)
Bisphosphoglycerate mutase
2,3 Bisphosphoglycerate
(2,3 BPG)
Phosphatase
3-phosphoglycerate (3 C)
Pi
lactate
Function
2,3-BPG binds to Hb decreases its affinity for oxygen leading
to release of oxygen in the peripheral tissues.
Metabolic Fate of Pyruvate
Pyruvate
CoASH
NAD+
Lipoamide, TPP, FAD
NADH + H+
Pyruvate dehydrogenase
complex
CO2
Acetyl CoA
Energetics
1 molecule of glucose produces 2 molecules of
pyruvate and hence, 5 molecules of ATP at this step.
Tricarboxylic Acid Cycle (TCA Cycle)
• Contents:
•
•
•
•
•
•
•
Definition
Tissue location
Intracellular location
Reactions (Pathway)
Energetics
Functions
Clinical significance
Tricarboxylic Acid Cycle (TCA Cycle)
Synonym: Krebs cycle or Citric acid cycle
• Definition
TCA cycle is the final common pathway for
oxidation of carbohydrate (glucose), fat (fatty acid)
and protein (amino acid) for production of energy.
Tissue location
All tissues
Intracellular location:
Mitochondrial matrix
Acetyl CoA
NADH+H Oxaloacetate
H2O
NAD+
Malate dehydrogenase
Malate
H2O
Aconitase
2+
Fe
H2O
Cis–aconitate
H2O
Fumarase
Fumarate
CoASH
Citrate
Aconitase
Fe2+
Isocitrate
FADH2
FAD
Succinate
DehydrogenaseSuccinate
ADP
ATP
2+
Mn
CO2
NAD+
NADH+H+
-Ketoglutarate
CO2
NAD+
CoASH
NADH+H+
Succinate thiokinase Succinyl CoA
Energetics of TCA Cycle
No. of ReducedMethod of
Reaction
catalyzed by Coenzyme
ATP production
Oxidative
phosphorylation
Oxidative
-Ketoglutarate 1 NADH
phosphorylation
DH
Substrate level
–
–
–
–
–
Succinate thiokinase
phosphorylation
Oxidative
1 FADH2
Succinate
phosphorylation
DH
1 NADH Oxidative
Malate DH
phosphorylation
Isocitrate DH
1 NADH
No. of ATP
molecules
formed
2.5
2.5
1
1.5
2.5
Total 10
Amphibolic Role of TCA Cycle
TCA cycle has a dual role .
1. Catabolic
Oxidation of acetyl-CoA produced
from metabolism of carbohydrates, fats
and proteins to CO2,H2O and energy .
2. Anabolic
Intermediates of TCA are utilised for the
synthesis of various compounds like glucose, non
–essential amino acids, fatty acid,heme.
Anabolic Pathways:
Intermediates of TCA cycle involved
Gluconeogenesis
Oxaloacetate
Fatty acid synthesis
Citrate
Heme synthesis
Succinyl CoA
Hexose Monophosphate Shunt
Pathway
(HMP Shunt Pathway)
•
•
•
•
•
Definition
Tissue location
Intracellular location
Functions
Clinical significance
Hexose Monophosphate Shunt
Pathway
(HMP Shunt Pathway)
Synonyms: Pentose phosphate pathway; Direct oxidative
pathway
• Definition
This is a pathway for oxidation of glucose
alternative to glycolysis, for production of NADPH
and ribose (ribose-5-P).
Tissue location
RBC, adrenal cortex, liver, testis, thyroid,
adipose tissue, etc.
Intracellular location: Cytosol
Reactions/Pathway:
HMP shunt pathway has two phases.
1.Oxidative phase (irreversible)
2. Non-oxidative phase (reversible
Pathway Glucose (6 C)
ATP
Mg2+
Hexokinase
ADP
Glucose-6-phosphate (6 C)
1. Oxidative phase
Cytosol
NADP +
Glucose-6-Phosphate dehydrogenase
NADPH + H
6-phosphogluconolactone
Hydrolase
H2O
6-phosphogluconate
NADP+
6-Phosphogluconate dehydrogenase
CO2
NADPH + H+
Ribulose-5-P
2. Non-Oxidative phase
cytosol
Ribulose-5-P
3-Epimerase
Isomerase
Ribose-5-P
Xylulose-5-P
Mg2+
Transketolase
TPP,
Sedoheptulose-7-P Glyceraldehyde-3-P
Transaldolase
Fructose-6-P
Erythrose-4-P
Transketolase TPP,2+
Mg
Fructose-6-P
Glyceraldehyde-3-P
Glycolysis
Functions/Significance of HMP pathway
1. Generation of ribose → For the synthesis of
nucleotides and then nucleic acids
2. Production of NADPH
•For reductive synthesis of Fatty acids, Cholesterol
and Steroid hormones
•For detoxification of H2O2 in RBC and lens of the eye.
(Protection of erythrocytes against hemolysis)
Clinical Significance of HMP pathway
Glucose-6-phophate dehydrogenase (GPD) deficiency
There is a defective of generation of NADPH
This is characterized by jaundice and severe anemia.
Thank you
Alanine transaminase
2. Pyruvate +
PLP
glutamate
Alanine
+-ketoglutarate
Glutamate
Glutamate dehydrogenase
-ketoglutarate +
ammonia
3.Glutamate
NADP+
NADPH+ H+
Anaeplerosis or Anaplerotic
Reactions
• Definition
Metabolic reactions, which replenish the intermediates of TCA
cycle, are called anaplerotic reactions or anaplerosis.
• Reactions
1.Pyruvate+CO2Pyruvate carboxylase
Biotin
ATP
ADP +Pi
Oxaloacetate
Anabolic Pathways:
Intermediates of TCA cycle involved
Gluconeogenesis
Oxaloacetate
Fatty acid synthesis
Citrate
Heme synthesis
Succinyl CoA
Synthesis of Glutamate by -Ketoglutarate,
transamination
Synthesis of Aspartate by
transamination
Oxaloacetate
Catabolic Pathways:
Oxidation of acetyl-CoA produced from metabolism
of carbohydrates, fats and proteins to CO2,H2O
and energy
Regulation of Glycolysis
Glycolysis is controlled by the energy state of
the cell and by hormones insulin and glucagon.
The 3 key glycolytic enzymes hexokinase,
Phosphofructo kinase, Pyruvatekinase
are regulated.
Enzyme
Inducer Repressor Activator Inhibitor
liver
liver
Hexokinase
Glucose-6phosphate
glucokinase Insulin glucagon
Phospho
fructo
kinase
Insulin
glucagon
Pyruvate
kinase
Insulin
glucagon
AMP,
ATP,
Fructose- Citrate,
6phosphat
eFructose- ATP
1,6bisphosp
hate
Pathway
Glucose (6 C)
ATP
Hexokinase
Mg2+
ADP
Glucose-6-phosphate (6 C)
Cytosol
Phospho hexose isomerase
Fructose-6-phosphate (6 C)
ATP
Mg2+
Phosphofructokinase
ADP
Fructose-1, 6-bisphosphate (6 C)
Aldolase
Glyceraldehyde-3-Phosphate
Dihydroxyacetone-P (3 C)
Triose phosphate isomerase
(3 C)
Glyceraldehyde-3-Phosphate (3 C)
Pi
NAD+
Glyceraldehyde-3- phosphate dehydrogenase
NADH + H+
1, 3-bisphsophoglycerate (3 C) Bisphosphoglycerate mutase
2,3 Bisphosphoglycerate
(2,3 BPG)
Phosphatase
3-phsophoglycerate (3 C)
Pi
lactate
Clinical Significance of Glycolysis
1.Hemolytic anemia
Deficiency of pyruvate kinase leads to decreased synthesis
of ATP in RBCs in turn leading to instability of membrane
and hemolysis and hence, also anemia
2.Muscle cramp
seen during excessive physical exercise due to anaerobic
glycolysis leading to accumulation of lactic acid in muscles.
Metabolic Fate of Pyruvate
•Pyruvate generated in the cytoplasm is transported to the
mitochondria by pyruvate transporter.
•Pyruvate is converted to acetyl CoA by oxidative
decarboxylation by pyruvate dehydrogenase complex.
•This is an irreversible reaction .
•Pyruvate dehydrogenase complex is a multi-enzyme
complex containing 3 enzymes activities. It also contains 5
coenzymes.
thiamine pyrophosphate (TPP),
lipoamide,
coenzyme A (CoASH),
FAD
and
Regulation
1.Allosteric mechanism
Acetyl CoA
NADH
Pyruvate
Pyruvate dehydrogenase complex
Acetyl CoA
2.Covalent modification
Pyruvate dehydrogenase complex is regulated by
covalent modification.
The enzyme active in dephosphorylated form
Clinical Aspects
Lactic acidosis:
Pyruvate is converted to lactate and its accumulation
in the blood leads to lactic acidosis.
Causes :
Inherited deficiency of pyruvate dehydrogenase
Arsenite,and mercuric ions, inhibit the enzyme
Regulation of of TCA Cycle
• The cellular needs of ATP .
When energy charge of the cell is low,
indicated by high level of NAD+ and FAD,
cycle operates at a faster rate.
• Sites for regulation
• Citrate synthase
• Isocitrate dehydrogenase
• -Ketoglutarate dehydrogenase
Enzyme
Citrate synthase
Activator
Inhibitor
ATP
Isocitrate
Dehydrogenase
-ketoglutarate
Dehydrogenase
ADP
ATP, NADH
ATP, NADH
Inhibitors (synthetic) of TCA cycle
Enzyme
Inhibitor
mechanism
Citrate
Fluroacetate Suicide
synthase
inhibition
Succinate
Malonate
Competitive
dehydrogen
inhibition
ase
1. Gluconeogenesis
glucose
glucose- 6-phosphatase
glucose-6- phosphate
Cytoplasm
phosphoenolpyruvate
oxaloacetate
malate
aminoacids
Pyruvate
oxaloacetate
malate
citrate
fumarate
 -ketoglutarate
Succinyl coA
2.Fatty acid synthesis
Oxaloacetate
Acetyl CoA
CoASH
H2O Citrate
Fatty acid
synthesis
3.Heme synthesis
Succinyl CoA
+glycine
ALA synthase
PLP
Heme
4.Transamination
Alanine transaminase
Alanine +
PLP
 ketoglutarate
Pyruvate +
glutamate
Regulation of HMP Shunt Pathway
Glucose-6-phosphate dehydrogenase is the ratelimiting enzyme of this pathway.
This enzyme is inhibited by NADPH and
induced by insulin.