Major Metabolic Pathways of Glucose
By
Reem M. Sallam, MD, PhD.
Clinical Chemistry Unit, Pathology Dept.
College of Medicine, KSU
Metabolic Pathway
 A pathway is made out of multistep sequences of reaction.
The product of one reaction is the substrate of the
subsequent reaction.
 Metabolism is the sum of all the chemical changes
occurring in a cell, a tissue or the body.
 Pathways can be either catabolic (degradative) or anabolic
(synthetic), or both (amphibolic)
 The pathways of metabolism must be regulated. Regulation
occurs at certain steps called rate-limiting reactions.
Metabolic Pathway
 Regulatory mechanisms are either rapid & short term or slow and
long term.
 Rapid regulation can be by covalent modification (e.g.
phosphorylation/dephosphorylation) or allosteric.
 Slow regulation occurs at the gene level by induction or repression.
Metabolic Pathways of Glucose:
Production and Utilization
Glycogenolysis
Hexose interconversion
Gluconeogenesis
Production
Glucose
Utilization
Glycolysis
HMP/PPP
Hexose interconversion
Glycogenesis
Krebs cycle
Metabolic Pathways of Glucose:
Catabolic and Anabolic
Catabolic cycles
Anabolic cycles
Glycolysis (Mainly)
Gluconeogenesis
Krebs (Mainly)
Glycogenolysis
Glycogenesis
HMP
Glycogenesis and Glycogenolysis
Glycogenesis:
Synthesis of glycogen from glucose
Mainly liver and muscle, Cytosol
Glycogenolysis
Degradation of glycogen into glucose
Mainly liver and muscle, Cytosol
Hexose Monophosphate Pathway (HMP) or
Pentose Phosphate Pathway (PPP)
1- Important source for NADPH
Which is used in reductive syntheses
2- Source for metabolically active ribose
Which is used for production of nucleotides:
For nucleic acids
For co-enzymes
Glucose Transport
Na+-Monosaccharide Cotransporter:
Against concentration gradient
Energy dependent
Carrier-mediated
Coupled to Na+ transport
Small intestine, renal tubules
Na+-Independent Facilitated Diffusion:
With concentration gradient
Energy Independent
Glucose Transporters (GLUT 1-14)
Glucose Transport:
Facilitated Diffusion
Glucose Transporters
• Tissue-specific expression pattern
GLUT-1
GLUT-2
GLUT-3
GLUT-4
GLUT-5
GLUT-7
RBCs and brain
Liver, kidney & pancreas
Neurons
Adipose tissue & skeletal
muscle
Small intestine & testes
Liver (ER-membrane)
• Functions:
GLUT-1, 3 & 4
GLUT-2
GLUT-5
Glucose uptake from blood
Blood & cells (either direction)
Fructose transport
Glycolysis: Objectives
 Major oxidative pathway of glucose
 The main reactions of glycolytic pathway
 The rate-limiting enzymes/Regulation
 ATP production (aerobic/anaerobic)
 Pyruvate kinase deficiency hemolytic
anemia
Glycolysis: An Overview
 Glycolysis, the major pathway for glucose oxidation,
occurs in the cytosol of all cells.
 It is unique, in that it can function either aerobically
or anaerobically, depending on the availability of
oxygen and intact mitochondria.
 It allows tissues to survive in presence or absence of
oxygen, e.g., skeletal muscle.
 RBCs, which lack mitochondria, are completely reliant
on glucose as their metabolic fuel, and metabolizes it
by anaerobic glycolysis.
Reactions of Aerobic Glycolysis
 The conversion of glucose to pyruvate occurs in 2 stages.
 The first 5 reactions is the energy investment phase (ATP is
used to synthesize phosphorylated intermediates).
 The second phase is the energy generation phase in which 4
molecules of ATP are made by the substrate-level
phosphorylation/glucose molecule, and 2 molecules of
NADH are formed when pyruvate is produced (aerobic
glycolysis), whereas NADH is reconverted to NAD+ when
lactate is the end product (anaerobic glycolysis).
Reactions of Glycolysis
1. Phosphorylation of glucose into glucose 6-P by
hexokinase (in most tissues) or glucokinase (in liver).
This is irreversible reaction. It is one of the regulatory
enzyme. The enzyme’s gene in induced by insulin and
repressed by glucagon
2. Isomerization of glucose 6-P to fructose 6-P by
phosphoglucose isomerase. Reversible reaction, so
not regulated step.
Reactions of Glycolysis
3. Phosphorylation of Fructose 6-P to fructose 1,6
bisphosphate by phosphofructokinase1 (PFK-1).
Irreversible reaction. It is the most important regulatory
step and the rate limiting step of glycolysis. It is regulated:
•
•
Allosteric inhibition by elevated levels of ATP, and of citrate.
Allosteric activation by high level of AMP, and by fructose 2,6
bisphosphate (the most potent activator of PFK-1)
4. Cleavage of fructose 1,6 bisphosphate by aldolase into
dihydroxyacetone phosphate (DHAP) and glyceraldehyde
3-P. This is reversible reaction and not regulated.
Reactions of Glycolysis
5. Isomerization of dihydroxyacetone phosphate (DHAP) and
glyceraldehyde 3-P by triose-P isomerase. This is reversible
reaction and not regulated. Because DHAP has to be
isomerized to glyceraldehyde 3-P for further metabolism
by glycolysis, there is a net production of 2 molecules of
glyceraldehyde 3-P .
6. Oxidation of glyceraldehyde 3-P to 1,3-bisphosphoglycerate
(1,3-BPG) by glyceraldehyde 3-P dehydrogenase. 1,3-BPG
contains high-energy P group.
Reactions of Glycolysis
7. Synthesis of 3-phosphoglycerate from 1,3-BPG by
phosphoglycerate kinase. There is formation of 1 ATP
molecule. This is a substrate-level phosphorylation. Because 2
molecules of 1,3-BPG are formed from each glucose molecule,
there is formation of 2 ATP molecule in this step.
8. Shift of P group from carbon 3 to carbon 2 by phosphoglycerate
mutase. Reversible reaction.
9. Dehydration of 2-P glycerate by enolase resulting in the
formation of phosphoenolpyruvate (PEP) that has high energy
P- bond. It is reversible reaction.
Reactions of Glycolysis
10. Formation of pyruvate from PEP by pyruvate kinase (PK). It is
irreversible step, so it is regulated. It results in the formation of
ATP by substrate level phosphorylation. It is activated by
fructose 1,6 bisphosphate (feed-forward regulation). The
enzyme is covalently regulated by
phosphorylation/dephosphorylation: hypoglycemia  glucagon
release from  cells of pancreas  increase intracellular level
of cAMP activation of cAMP-dependent protein kinases
phosphorylation and inactivation of PK.
Dephosphorylation of PK by phosphoprotein phosphatase 
reactivation of the enzyme.
Glycolysis in RBC: formation of 2,3-BPG
In RBCs, some of the 1,3-BPG is converted to 2,3-BPG by the
enzyme bisphosphoglycerate mutase.
 2,3-BPG is an important regulator of the binding of oxygen to
hemoglobin, because the presence of 2,3-BPG reduces the
affinity of hemoglobin for oxygen enabling hemoglobin to
release oxygen efficiently to the tissues.
 2,3-BPG is then hydrolyzed by a phosphatase to 3phosphoglycerate, which can continue glycolysis.

Pyruvate Kinase Deficiency
Hemolytic Anemia
Pyruvate Kinase Deficiency
Hemolytic Anemia
 Normal mature RBC lacks mitochondria and is completely
dependent on glycolysis for production of ATP.
 If there is genetic deficiency in glycolysis enzymes, the rate of
glycolysis will decrease in RBCs decreased ATP production 
alteration in RBCs membranes and cell shape  premature death
& lysis of RBCs  hemolytic anemia.
 Pyruvate kinase deficiency is the most common genetic defects of
glycolytic enzymes.
 The severity of the hemolytic anemia depends on the degree of
enzyme deficiency and the degree of RBC compensation by
increasing the level of synthesis of 2,3-BPG to facilitate the release
of Oxygen from hemoglobin to tissues.
Substrate-level phosphorylation Vs.
Oxidative phosphorylation
 Phosphorylation is the metabolic reaction of introducing a
phosphate group into an organic molecule.
 Oxidative phosphorylation: The formation of high-energy
phosphate bonds by phosphorylation of ADP to ATP
coupled to the transfer of electrons from reduced
coenzymes to molecular oxygen via the electron transport
chain (ETC); it occurs in the mitochondria.
 Substrate-level phosphorylation: The formation of high-
energy phosphate bonds by phosphorylation of ADP to ATP
(or GDP to GTP) coupled to cleavage of a high-energy
metabolic intermediate (substrate). It may occur in cytosol
or mitochondria
Summary: Regulation of Glycolysis
Regulatory Enzymes (Irreversible reactions):
Glucokinase/hexokinase
PFK-1
Pyruvate kinase
Regulatory Mechanisms:
Rapid, short-term:
Allosteric
Covalent modifications
Slow, long-term:
Induction/repression
Apply the above mechanisms for each enzyme where applicable
Aerobic Glycolysis: ATP Production
ATP Consumed:
2
ATP
ATP Produced:
Substrate-level
Oxidative-level
Total
2X2=
2X3=
4
6
10
ATP
ATP
ATP
Net:
10 – 2 =
8
ATP
Take Home Message
 Glycolysis is the major oxidative pathway for
glucose
 Glycolysis is employed by all tissues
 Glycolysis is a tightly-regulated pathway
 PFK-1 is the rate-limiting regulatory enzyme
Take Home Message
 Glycolysis is mainly a catabolic pathway
for ATP production, But it has some
anabolic features (amphibolic)
 Pyruvate kinase deficiency in RBCs
results in hemolytic anemia
THANK YOU