1. Glycolysis is the pathway that breaks down glucose to pyruvate, generating a small amount of ATP. 2. Gluconeogenesis is the formation of glucose from non-carbohydrate precursors like amino acids or glycerol, mainly occurring in the liver and kidneys. 3. The hexose monophosphate shunt generates reducing power in the form of NADPH and produces ribose-5-phosphate from glucose-6-phosphate.

1. 1

2. A Map of The Major Metabolic
Pathways in A Typical Cell

3. 3

4. A-Anabolic pathways:
Transforming small molecules into big molecules
constituting the body structures and machinery. It is
energy requiring, e.g., glycogenesis and lactose synthesis
B-Catabolic pathways:
Breakdown of large molecules into smaller
molecules to produce energy or smaller molecules or
reducing equivalents, e.g., HMP-shunt and uronic acid
pathway.
C-Amphibolic pathways:
They are utilized for both anabolic and catabolic
purposes, e.g., glycolysis and Krebs' cycle.

5. Learning Objectives• Outline the three stages of glycolysis.
• Describe the steps of glycolysis between glucose and
pyruvate and recognize all the intermediates and
enzymes and the cofactors that participate in the
reactions.
• Mention ATP-generating reactions.
• Illustrate the regulation of glycolysis.
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6. March 17, 2018 Dr. Mohamed Z Gad 6
CHO metabolism in mammalian cells can be
classified into:
1. Glycolysis: Oxidation of glucose to
pyruvate (aerobic state) or lactate
(anaerobic state)
2. Krebs cycle: After oxidation of pyruvate
to acetyl CoA, acetyl CoA enters the
Krebs cycle for the aim of production of
ATP.
3. Hexose monophosphate shunt: Enables
cells to produce ribose-5-phosphate and
NADPH.
4. Glycogenesis: Synthesis of glycogen from
glucose, when glucose levels are high
5. Glycogenolysis: Degradation of glycogen
to glucose when glucose in short supply.
6. Gluconeogenesis: Formation of glucose
from noncarbohydrate sources.
Glucose is the major fuel of mostGlucose is the major fuel of most
organisms. The major pathways of CHOorganisms. The major pathways of CHO
metabolism either begin or end withmetabolism either begin or end with
glucose.glucose.

7. March 17, 2018 Dr. Mohamed Z Gad 7
Glycolysis occurs in all human cells. Glycolysis is believed to beGlycolysis occurs in all human cells. Glycolysis is believed to be
among the oldest of all the biochemical pathways.among the oldest of all the biochemical pathways.
Aerobic: Glucose  Pyruvate
Anaerobic: Glucose  Lactate (or ethanol & acetic
acid)Glycolysis (10 reactions in 3 stages, all in cytoplasm)
1) Priming stage: D-Glucose + 2ATP  D-fructose 1,6-biphosphate + 2ADP + 2H+
2) Splitting stage : D-Fructose 1,6-biphosphate  2 D-Glyceraldehyde 3-phosphate
3) Oxidoreduction – Phosphorylation stage:
2 D-Glyceraldehyde 3-phosphate + 4ADP + 2Pi + 2H+  2Lactate + 4ATP
-----------------------------------------------------------------------------
Sum:
Glucose + 2ADP + 2Pi ----- 2 Lactate + 2ATP + 2H2O (Anaerobic)
Glucose + 2ADP + 2Pi + 2NAD+
---- 2 pyruvate + 2ATP + 2NADH + 2H+
+ 2H2O
(Aerobic)

8. March 17, 2018 Dr. Mohamed Z Gad 8
1) Priming Stage
• Glucose (and other hexoses) are
phosphorylated immediately upon
entry into the cell. Phosphorylation
prevents transport of glucose out of
the cell and increases the reactivity
of oxygen in the resulting phosphate
ester.
• Several isoenzymes of hexokinase
with different Km values for
glucose are located in different
tissues. Brain hexokinase has a
particularly low Km for glucose.
• The major enzyme for
phosphorylating glucose in liver is
glucokinase.
• Steps catalyzed by hexokinase &
PFK-1 are irreversible.

9. March 17, 2018 Dr. Mohamed Z Gad 9
Differences between Glucokinase & Hexokinase
HexokinaseHexokinase GlucokinaseGlucokinase
 Present in all tissues  Liver only
 Low Km for glucose (<0.1
mM)
 Higher Km for glucose
 Strongly inhibited by G6P  Not inhibited by G6P
 Non-inducible enzyme, not
affected by diabetes or
insulin
 Inducible, synthesis induced by
insulin & repressed in diabetes
 Level of enzyme is not
affected by fasting or high
CHO diet
 Depends on glucose concentration
 Act on glucose, fructose and
galactose
 Glucose only

10. March 17, 2018
2) Splitting Stage
• The reaction catalysed by aldolase is
the reverse of aldol condensation.
• Although the cleavage of F1,6BP is
energetically unfavourable, rapid
removal of the product drives the
reaction forward.
• Of the two products of the aldolase
reaction, only GAP (or G3P) serves as a
substrate for the next reaction in
glycolysis.
• To prevent the loss of the other three-
carbon unit, triose phosphate isomerase
catalyses the interconversion of DHAP
& G3P. Because of this reaction, the
original molecule of glucose has now
been converted to two molecules of
G3P.

11. March 17, 2018 Dr. Mohamed Z Gad 11
3) Oxidoreduction –
Phosphorylation
Stage• G-3-P dehydrogenase is a tetramer,
each subunit contains 1 binding site
for G3P & another for NAD+
(NAD+
is permanently bound to the enzyme).
• G3P  1,3-BPG  3PG and PEP
 Pyruvate are examples of
“substrate–level” phosphorylation.
• 3-PG  2-PG is mediated by an
intermediate [2,3-BPG]. Most cells
have low amounts of 2,3-BPG except
in RBCs, which act as allosteric
modifier of Hb-O2 binding.
• PEP  Pyruvate is an “irreversible
reaction” due to free energy loss
associated with tautomerization of
the enol to the more stable keto form.

12. ADP is converted to ATP by the direct
transfer of a phosphoryl group from a high
energy compound.
What is meant by “substrate–
level phosphorylation” ?

13. Why glycolysis under anaerobic
conditions proceed to lactate and
not just stop at pyruvate
formation ?
The reaction of lactate dehydrogenase is essential in
anaerobic glycolysis , as it is the mean for reoxidizing
NADH formed in the G-3-P dehydrogenase step to re-
enter into the glycolysis cycle. In aerobic glycolysis
reoxidation takes place in mitochondria by the
respiratory chain.

14. Overall pathway of Glycolysis & ATP
Formation
Number of ATP generated from
glycolysis
Enzyme Aerobic Anaerobic
Hexokinase -1 ATP -1 ATP
PFK-1 -1 ATP -1 ATP
G-3-P
dehydrog.
+6 ATP ----
Phospho-
glycerate
kinase
+2 ATP +2 ATP
Pyruvate
kinase
+2 ATP +2 ATP
Sum +8 ATP +2 ATP
Dr. Mohamed Z Gad 14

15. March 17, 2018 Dr. Mohamed Z Gad 15
Louis Pasteur, the great 19th century French chemist
and microbiologist, was the first scientist to observe the
following phenomenon. “Cells that can oxidize glucose
completely to CO2 and H2O utilize glucose more rapidly
in the absence of O2 than in its presence”. It would
appear that O2 inhibits glucose consumption. Thus,
another definition for Pasteur effect is: inhibition of
glucose utilization and lactate accumulation by the
initiation of respiration (O2 consumption)… Can you
explain why ?
The reason behind this phenomenon is that complete oxidation of glucose
under aerobic conditions yield much more ATP (~38 ATP) than anaerobic
glycolysis (~2ATP). Thus it is anticipated that the rate of glucose consumption
will be 19-20 times faster under anaerobic condition to meet the metabolic
demand in a way equivalent to aerobic conditions.

16. Regulation of
Glycolysis
March 17, 2018 Dr. Mohamed Z Gad 16
Rate of glycolysis is controlled primarily by
allosteric regulation of the 3 key enzymes
(irreversible steps), hexokinase, PFK-1, and
pyruvate kinase.
Enzyme Activator Inhibitor
Hexokinase AMP, ADP, Pi G-6-P
PFK-1 F-6-P, AMP, F-
2,6-DP (liver
only)
NADH, H+
, citrate,
ATP
Pyruvate
kinase
AMP, F-1,6-DP ATP, acetyl CoA,
phosphorylation
• PFK-1 is the major regulatory
enzyme of glycolysis. In the liver
only, PFK-1 is activated by fructose-
2,6-diphosphate (F-2,6-DP).
• PFK-2, the enzyme that synthesize
the activator F-2,6-DP, is itself a
regulatory enzyme. It is inhibited by
citrate & ATP and by
phosphorylation. The reverse
reaction is catalyzed by fructose-2,6-
diphosphatase(F-2,6-DPase).
• Hormones also regulate glycolysis
e.g., glucagon inhibits glycolysis by
repressing the synthesis of F-2,6-DP.
Insulin promotes glycolysis by
stimulating the synthesis of F-2,6-DP.

17. What effects do fluoride and
magnesium have on glycolysis ?

18. March 17, 2018 Dr. Mohamed Z Gad 18
•
Comments on Glycolysis
 Glycolysis is the only pathway that produce ATP in absence of O2.
 The best known inhibitors of the glycolytic pathway include:
 2-Deoxyglucose:2-Deoxyglucose: causes inhibition of hexokinase.
 Sulfhydryl reagentsSulfhydryl reagents (e.g. Hg-compounds and alkylating agents as
iodoacetate); inhibit glyceraldehydes-3-phosphate dehydrogenase
which has cysteine residue in the active site.
 Fluoride: a potent inhibitor of enolase. Thus, fluoride is usually
added to blood samples to inhibit glycolysis before estimation of
blood glucose.
 MagnesiumMagnesium:: required for kinase reactions by forming Mg-ATP
complex.
 Accumulation of lactate is responsible for muscle fatigue and cramps
observed under heavy exercise (anaerobic glycolysis).
 In RBCs, glycolysis is the major source of ATP since RBCs lack
mitochondrial oxidation.

19. Pyruvate dehydrogenase complex (PDC) is a complex of
three enzymes that converts pyruvate into acetyl-CoA by a
process called pyruvate decarboxylation. Acetyl-CoA may
then be used in the citric acid cycle to carry out
cellular respiration, and this complex links the glycolysis
metabolic pathway to the citric acid cycle

20. Pyruvate dehydrogenase (E1)
Dihydrolipoyl transacetylase (E2)
Dihydrolipoyl dehydrogenase (E3)

22. This is Why
OAA  D, N, I, K,
T, M

23. Formation of glucose from noncarbohydrate
sources
23

24. The source of pyruvate and oxaloacetate for
gluconeogenesis during fasting or carbohydrate
starvation is mainly amino acid catabolism.
Some amino acids are catabolized to pyruvate,
oxaloacetate, or precursors of these.
Muscle proteins may break down to supply amino
acids. These are transported to liver where they are
deaminated and converted to gluconeogenesis
inputs.
Glycerol, derived from hydrolysis of triacylglycerols
in fat cells, is also a significant input to
gluconeogenesis.
24

25. Dietary & muscle
proteins
Amino acids
Noncarbohydrate precursors of
glucose
25

26. Main sites of gluconeogenesis:
• Major site: Liver.
• Minor site: Kidney.
• Very little:
– Brain.
– Muscle (skeletal and heart).
In liver and kidney it helps to maintain the glucose level
in the blood so that brain and muscle can extract
sufficient glucose from it to meet their metabolic
demands.
26

27. Gluconeogenesis Versus Glycolysis:
7 steps are shared between glycolysis and
gluconeogenesis.
3 essentially irreversible steps shift the equilibrium
far on the side of glycolysis.
Most of the decrease in free energy (consuming
energy) in glycolysis takes place during these 3 steps.
27

28. 28

29. 29
1. Phosphoenolpyruvate is formed from
pyruvate:
2. Fructose 6-phosphate is formed from fructose 1,6-
bisphosphate:
3. Glucose is formed by hydrolysis of glucose 6-phosphate:

31. HMP Shunt
Hexose Mono Phosphate Shunt = Pentose Phosphate
Pathway = Complete Glucose Oxidation
Function : Production of
For NADPH
Ribose 5P
Site :
In the cytoplasm of all cells except muscle, and
nonlactating mammary gland (low activity)

32. NADPH + H+
is formed from
two separate reactions.
The glucose 6-phosphate DH
(G6PD) reaction is the rate
limiting step and is essentially
irreversible.
Cells have a greater need for
NADPH than ribose 5-
phosphate.