Well detailed carbohydrates metabolism with glycolysis and kreby's cycle

1. CARBOHYDRATE METABOLISM
Mr. Allan
BCH Dep’t

2. Pathways involved
The pathways that will be covered will
include:
• Glycolysis
• TCA cycle (Citric acid cycle)
• Gluconeogenesis
• Glycogen metabolism
• Inborn errors of carbohydrate
metabolism

3. Glycolysis
What is glycolysis??
• This is a process by which glucose molecules are
broken down through a series of enzymatic
reactions into two molecules of pyruvate
• Sequence of 10 enzyme-catalyzed reactions
• The reactions of glycolysis take place in the cytosol
of cells
• Glycolysis (embden –Meyerhof- Parnas) involves
the reactions responsible for break down of this
glucose

4. Glycolysis cont’d
The roles of glycolysis are:
1. To produce energy (both directly and by
supplying substrate for the citric acid cycle and
oxidative phosphorylation)
▷ Two ATP molecules are needed for early reactions in
the glycolytic pathway but four ATPs are generated
later, giving a net yield of two ATPs per molecule of
glucose degraded
2. To produce intermediates for biosynthetic
pathways.
▷acetyl CoA, for example, is the precursor for fatty acid
synthesis

5. Glycolysis cont’d
Steps in glycolysis:
• Glucose is phosphorylated by ATP to form glucose 6-
phosphate and ADP (by hexokinase)
• Glucose 6-phosphate is converted to fructose 6-phosphate
(by phosphoglucoisomerase)
• Fructose 6-phosphate is phosphorylated by ATP to form
fructose 1,6 bisphosphate and ADP (by
phosphofructokinase, PFK)
• The fructose 1,6 bisphosphate is split into glyceraldehyde 3-
phosphate and dihydroxyacetone phosphate (by aldolase)
• The above two trioses are inter converted by triose
phosphate isomerase

6. Glycolysis cont’d
• Glyceraldehyde 3-phosphate is converted to 1,3
bisphosphoglycerate (by glyceraldehyde 3-phosphate
dehydrogenase)
• 1,3 bisphosphoglycerate reacts with ADP to give 3-
phosphoglycerate and ATP (catalyzed by
phosphoglycerate kinase)
• The 3-phosphoglycerate is converted to 2-
phosphoglycerate (by phosphoglycerate mutase)
• 2-phosphoglycerate is converted to phosphoenolpyruvate
(PEP) by enolase
• Finally, PEP and ADP react to form pyruvate and ATP
(catalyzed by pyruvate kinase)

7. Glycolysis cont’d

8. Glycolysis cont’d
Summary of the reactions:
• The overall reaction of glycolysis which occurs in
the cytoplasm is represented simply as:
C6H12O6 + 2NAD + + 2ADP + 2P -----> 2 pyruvic acid,
(CH 3(C=O) COOH + 2ATP + 2NADH + 2H+

9. Glycolysis cont’d
Substrate level phosphorylation:
• The two ATP synthetic reactions in glycolysis (catalyzed by
phosphoglycerate kinase and pyruvate kinase) involve the
direct transfer of a phosphate from a sugar–phosphate
intermediate to ADP; these reactions are examples of
substrate-level phosphorylation
• A third example of substrate-level phosphorylation is the
synthesis of GTP by succinate dehydrogenase in the citric
acid cycle (see TCA cycle)
• The GTP can be used to phosphorylate ADP to form ATP

10. Glycolysis cont’d
Fates of pyruvate:
1.Under aerobic conditions, pyruvate can be converted
by pyruvate dehydrogenase to acetyl coenzyme A
(CoA) which can then enter the citric acid cycle.
Pyruvate dehydrogenase
Pyruvate + NAD+ + CoA acetyl CoA + CO2 + NADH

11. Glycolysis cont’d
2. Under anaerobic conditions, pyruvate is
converted to lactate by lactate dehydrogenase
(LDH).
▷The NAD+ regenerated by this reaction allows
glycolysis to continue, despite the lack of oxygen
▷In red blood cells, this fate dominates because they
lack the mitochondria
Lactate dehydrogenase
Pyruvate + NADH + H+ Lactate + NAD+

12. Glycolysis cont’d
3. In anaerobic conditions, yeast and other
organisms carry out alcoholic fermentation that
converts pyruvate to acetaldehyde and then to
ethanol, regenerating NAD+ that allows
glycolysis to continue.
Pyruvate decarboxylase
Pyruvate + H+ acetaldehyde + CO2
Alcohol dehydrogenase
Acetaldehyde + NADH + H+ Ethanol + NAD+

13. Fate of Pyruvate Summarized

14. Glycolysis cont’d
Uses of glycolysis
1. The main function of glycolysis is energy (ATP)
production
2. Provides pyruvate, a substrate for further oxidation
in the mitochondria
3. Provides dihydroxyacetone-3-phosphate which is
converted into glycerol 3-phosphate, which is a
critical substrate in the pathways of triacylglycerol
and glycerophospholipid (i.e. phospholipid) synthesis;
Dihydroxyacetone 3-phosphate + NADH + H+
Glycerol 3-phosphate + NAD+

15. Glycolysis cont’d
3. Red blood cells use 1,3 bisphosphoglycerate,
another intermediate in glycolysis, to generate 2,3-
bisphosphoglycerate, which is an allosteric
regulator of the interaction of oxygen with
hemoglobin
4. Pyruvate, the end product of glycolysis, can
acquire an amino group by transamination, thus
producing the amino acid alanine
5. Acetyl-CoA, produced by the mitochondrial
oxidation of pyruvate, is a substrate for the synthesis
of both fatty acids and cholesterol.

16. Glycolysis cont’d
6. Although most cells can also utilize fatty acids as
an energy source, some cells, such as
erythrocytes and those in both the lens and
cornea of the eye, contain few or no mitochondria
and rely on glycolysis for essentially all of their
ATP production
7. Even though brain cells do contain mitochondria,
the impermeability of the blood-brain barrier to
most long- chain fatty acids prevents fatty acids
from being an important fuel source for the brain so
they mainly depend on glucose to derive acetyl CoA

17. Glycolysis cont’d
Take home:
1. How are fructose and galactose metabolised?
2. Write short notes on the following:
a)Galactosemia b) hereditary fructose intolerance
c) Warburg effect in cancer cells d) essential fructosuria
e) diabetis mellitus f) mercury poisoning in glycolysis
g) Arsenic poisoning in glycolysis
3. Explain the role of the following in regulation of glycolysis ?
a)phospho-fructokinase-1 b) hexokinase c) pyruvate kinase d)
glut-4 transporters
4.Write short notes on:
a) cori cycle b) glucose transporters
6. Compare glucokinase and hexokinase

18. The citric acid cycle
• It can also be referred to as the Krebs cycle or
the tricarboxylic acid cycle
• In mammals, the reactions of the citric acid
cycle occur in the mitochondria which is also the
location of the electron transport chain
• In presence of oxygen, pyruvate enters the
mitochondrion to be metabolized further
• Pyruvate dehydrogenase catalyzes oxidative
decarboxylation of pyruvate, to form acetyl-CoA

19. TCA cont’d
Pyruvate dehydrogenase complex:
• Pyruvate dehydrogenase (PDH), located in
mitochondria
• Requires NAD+ ,TPP, CoA and Lipoic acid
• Hence, thiamine deficiency will affect action
of this enzyme
Regulation:
• PDH is highly regulated
• This is regulated by high concentrations of
Acetyl CoA

20. TCA cycle cont’d
The Pyruvate dehydrogenase complex

21. TCA cont’d
The citric acid cycle has eight stages:
1. Production of citrate from oxaloacetate and acetyl
CoA (catalyzed by citrate synthase)
2. Isomerization of citrate to isocitrate (catalyzed by
aconitase)
3. Oxidation of isocitrate to α-ketoglutarate (catalyzed
by isocitrate dehydrogenase; the reaction requires
NAD+)
4. Oxidation of α-ketoglutarate to succinyl CoA
(catalyzed by the α-ketoglutarate dehydrogenase
complex; the reaction requires NAD+). Also requires
TPP
, Coenzyme A and Lipoic acid because it
generates a CoA (succinyl CoA) as a product.

22. TCA cont’d
5. Conversion of succinyl CoA to succinate
[catalyzed by succinyl CoA synthetase; the
reaction requires inorganic phosphate and GDP
(or ADP)]. This is a substrate level phosphorylation.
6. Oxidation of succinate to fumarate (catalyzed by
succinate dehydrogenase; the reaction involves
FAD).
7. Hydration of fumarate to malate (catalyzed by
fumarase)
8. Oxidation of malate to oxaloacetate (catalyzed
by malate dehydrogenase; the reaction requires
NAD +)

24. TCA cont’d
Regulation of TCA:
• Regulation is at the level of Isocitrate
dehydrogenase
• It is shut down by high levels of ATP and
NADH
• Isocitrate will be converted to citrate
• Citrate will go out of the mitochondria and
it will go to cytoplasm and inhibit PFK-1,
shutting down glycolysis
• Citrate can also take the route of fatty acid
synthesis

25. Uses of the TCA cycle
• Final pathway for oxidation of fuel molecules
• The GTP generated can also be used as a
phosphoryl donor in protein synthesis and signal
transduction processes
• Most of the carbon atoms in haemoglobin are
synthesized from succinyl CoA
• Alpha keto-glutarate is used in synthesis of amino
acids like glutarate, glutamine, proline and arginine

26. Uses cont’d
• Oxaloacetate is used in synthesis of amino
acids like Iso-leucine, lysine, methionine,
threonine, aspartate and asparagine
• It also provides the substrates for
gluconeogenesis and fatty acid synthesis
• It is also a major pathway for interconversion
of metabolites arising from transamination
and deamination of amino acids

27. TCA take home
1. How is the TCA cycle regulated?
2. Write short notes about the following
glycerol -3-phosphate shuttle and malate
shuttle.

28. • 1. In which part of the cell does TCA cycle
take place?

29. Gluconeogenesis cont’d
Gluconeogenesis is the process by which the body
synthesizes glucose from endogenous non-
carbohydrate precursors, primarily:
I. lactate
II. glycerol
III. The carbon skeletons of the amino acids
IV. Propionic acid from odd numbered fatty
acids
• Liver and kidney (renal cortex) are the major
gluconeogenic tissues

30. Gluconeogenesis cont’d
• Gluconeogenesis meets the needs of the body for
glucose when carbohydrate is not available in
sufficient amounts from the diet or from glycogen
reserves
• Gluconeogenesis is essential for maintaining the
concentration of blood glucose in the fasting state
and during prolonged exercise
• A regular supply of glucose is necessary especially
for the nervous system and erythrocytes

31. Gluconeogenesis cont’d
• Glucose is also important in maintaining the level
of intermediates of the citric acid cycle even
when fatty acids are the main sources of acetyl-
CoA in the tissues
• Gluconeogenesis clears lactate produced by
muscle and erythrocytes
• Gluconeogenesis also clears glycerol produced
by adipose tissue

32. Gluconeogenesis pathway
Lower section Upper section

33. Gluconeogenesis cont’d
Take home:
1. Read about the alanine cycle and how it is associated
with gluconeogenesis
2. Glycolysis and gluconeogenesis in hepatocytes are
reciprocally regulated. Explain
3. Explain the following as regards to the abnormal
functioning of gluconeogenesis pathway:
▪ Poorly controlled diabetes mellitus type I
▪ Excessive Ethanol consumption

34. Glycogen metabolism cont’d
• Glycogen granules are located in the cytosol.
• They consist of linear chains of glucose with linkages
forming branches after approximately every 8 to 10
glucose residues
• Glycogenolysis (glycogen breakdown) provides a readily
available source of glucose when it is needed
• Glycogen synthesis functions to replenish the glycogen
stores in liver and muscle when dietary carbohydrates
are available

35. Glycogen metabolism cont’d
• Although nearly all cells of the body contain trace
amounts of glycogen, the major glycogen stores
are found in liver and skeletal muscle
• When mobilized, the glucose derived from liver
glycogen is secreted by hepatocytes and used to
maintain the concentration of glucose in the blood
• By contrast, glucose arising from glycogen
breakdown in skeletal and heart muscle remains in
the muscle cells and is used to provide energy for
muscle work

36. Glycogen metabolism cont’d
Reminder about the glycogen structure
• Glycogen is a highly branched-chain made
from α-D-glucose.
• An α(1 4) linkage is the primary
glycosidic bond of glycogen
• There is a branch containing an α(1 6)
linkage present after about eight to ten
glucosyl residues

37. Glycogen metabolism cont’d

38. Glycogen metabolism cont’d

39. Regulation of glycogen metabolism
Glycogenolysis
In a fasting state;
• Glycogen phosphorylase activated by glucose 6-phosphate and ATP
•
• Glycogen synthase is inhibited by glucose 6-phosphate and ATP
• During muscle contraction, calcium binds with calmodulin and
stimulates glycogen phosphorylase
• In muscle under extreme conditions of anoxia and ATP
depletion, increase AMP level in muscle stimulates glycogen
phosphorylase
• Glucagon and epinephrine stimulate glycogenolysis by stimulating
Glycogen phosphorylase enzyme activity
• Insulin inhibits glycogenolysis by inhibiting Glycogen phosphorylase

40. Regulation cont’d
Glycogenesis
In a well fed state;
• Glycogen synthase is activated by glucose 6-phosphate
and ATP
• Glycogen phosphorylase is inhibited by glucose 6-
phosphate and ATP
• Insulin stimulates glycogenesis by stimulating Glycogen
synthase enzyme activity
• Glucagon and epinephrine inhibit glycogenesis by
inhibiting Glycogen synthase enzyme activity

41. Glycogen metabolism cont’d
Take home:
1. Explain the hormonal regulation of
glycogenolysis and glycogen synthesis
2. Write shot notes about the following
abnormalities of glycogen metabolism:
▪ Glycogen storage disease (Von Gierke disease)
▪ Pompe disease
▪ Neonatal hypoglycaemia
▪ Glycogen loading