introduction to carbohydrate metabolism

1. Carbohydrate Metabolism

2. Metabolism
• Metabolism is all the chemical reactions that occur in
an organism
• Cellular metabolism
– Cells break down excess carbohydrates first, then lipids,
finally amino acids if energy needs are not met by
carbohydrates and fat
– Nutrients not used for energy are used to build up structure,
are stored, or they are excreted
– 40% of the energy released in catabolism is captured in ATP,
the rest is released as heat

3. Metabolism
Metabolism involves
• Catabolic reactions
that break down
large, complex
molecules to
provide energy and
smaller molecules.
• Anabolic reactions
that use ATP energy
to build larger
molecules.
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4. Anabolism
• Performance of structural maintenance
and repairs
• Support of growth
• Production of secretions
• Building of nutrient reserves

5. Stages of Metabolism
Catabolic reactions are organized as
Stage 1: Digestion and hydrolysis break down
large molecules to smaller ones that enter the
bloodstream.
Stage 2: Degradation breaks down molecules to
two- and three-carbon compounds.
Stage 3: Oxidation of small molecules in the
citric acid cycle and electron transport provide
ATP energy.
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6. Stages of Metabolism
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7. Metabolism of Carbohydrates
Carbohydrates are the major energy source for
animals and most microorganisms.
Catabolism of carbohydrates:
Polysaccharides  monosacchrides  ATP
Anabolism of carbohydrates:
Monosacchrides    polysacchrides
ATP ADP

8. • All organisms obtain energy from the
oxidation of glucose and other carbohydrates.
• In some cells and organisms, glucose is the
major or sole source of energy:
• brainbrain
• erythrocyteserythrocytes
• many bacteriamany bacteria

9. Metabolism of Glucose
• Glycolysis – degradation of glucoseGlycolysis – degradation of glucose
The main pathway for glucose oxidation.
• Phosphogluconate pathway (pentose phosphatPhosphogluconate pathway (pentose phosphat
pathway or pentose shunt)pathway or pentose shunt)
An auxiliary route for glucose oxidation in animals.
It produces ribose-5-phosphate.
• GluconeogenesisGluconeogenesis
Pathway for the synthesis of glucose from pyruvate.

10. Catabolism of glucose can be aerobic and
anaerobic.
Aerobic metabolism of glucose relies on O2
and is the major way for animals to obtain
energy.
Anaerobic metabolism is an energy producing
process without O2.
- Fermentation is an anaerobic metabolic
process in microorganisms.

11. Carbohydrate Metabolism
• Primarily glucose
– Fructose and galactose enter the pathways at various points
• All cells can utilize glucose for energy production
– Glucose uptake from blood to cells usually mediated by
insulin and transporters
• Liver is central site for carbohydrate metabolism
– Glucose uptake independent of insulin
– The only exporter of glucose

12. Blood Glucose Homeostasis
• Several cell types prefer glucose as energy
source (ex., CNS)
 80-100 mg/dl is normal range of blood
glucose in non-ruminant animals
 45-65 mg/dl is normal range of blood
glucose in ruminant animals
 Uses of glucose:
 Energy source for cells
 Muscle glycogen
 Fat synthesis if in excess of needs

13. Fate of Absorbed Glucose
• 1st
Priority: glycogen storage
– Stored in muscle and liver
• 2nd
Priority: provide energy
– Oxidized to ATP
• 3rd
Priority: stored as fat
– Only excess glucose
– Stored as triglycerides in adipose

14. Glucose Utilization
Glucose
PyruvateRibose-5-phosphate
Glycogen
Energy
Stores
Pentose
Phosphate
Pathway
Glycolysis
Adipose

15. Glycolysis
• Sequence of reactions that converts glucose
into pyruvate
 Relatively small amount of energy produced
 Glycolysis reactions occur in cytoplasm
 Does not require oxygen
Glucose → 2 Pyruvate
Lactate (anaerobic)
Acetyl-CoA (TCA cycle)

16. Phase I: Cleavage of 1 hexose to
2 triose

17. Phase II:
Generation
of 2 ATPs,
2 NADH
and 2
Pyruvates

18. Glycolysis

19. Hexose Kinase
• 1st step in glycolysis;2 enzymes glucokinase &
hexokinase
• Glucose 6-p phosphatase for reversal of reaction
• ATP is consumed here in order to get more later
• ATP makes the phosphorylation of glucose
spontaneous

21. Rx 2: Phosphoglucoisomerase
• Uses open chain structure as substrate
• Near-equilibrium(reversible)

22. Rx 3: Phosphofructokinase
PFK is the committed step in glycolysis!
• The second priming reaction of glycolysis
• Committed step and large, activated by AMP & fructose
2,6-bisphosphatase
• Reversal needs hexose bisphosphatase

23. Rx 4: Aldolase
• Hexose cleaved to form two trioses
• C1 thru C3 of F1,6-BP -> DHAP
• C4 thru C6 -> G-3-P
• Position of carbonyl group determines which bond
cleaved.
• Only G-3-P directly undergo glycolysis

24. Rx 5: Triose Phosphate Isomerase (TPI)
• Conversion of DHAP to G-3-P by TPI
maintains steady state [G-3-P]

25. Rx 6: Glyceraldehyde-3P-Dehydrogenase
• G3P is oxidized and phosphorylated to 1,3-BPG
• Pi is used as phosphate donor
• C1 phosphoryl group has high group transfer potential,
used to phosphorylate ADP to ATP in next step of
glycolysis
• Arsenate can replace phosphate in reaction (results in
lower ATP)
• NADH generated in this reaction is reoxidized by
respiratory electron transport chain (generates ATP)

26. Rx 7: Phosphoglycerate Kinase (PGK)
• ATP synthesis from a high-energy phosphate
• This is referred to as "substrate-level phosphorylation"

27. Rx 8: Phosphoglycerate Mutase
• Phosphoryl group moves from C-3 to C-2
• Mutases are isomerases that transfer phosphates
from one hydroxyl to another

28. Rx 9: Enolase
• Flouride ions inhibit enolase
• Enolase just rearranges to a form from which more
energy can be released in hydrolysis
• Requires Mg2+
for activity.

29. Rx 10: Pyruvate Kinase
• Substrate level phosphorylation generates
second ATP
• Enzyme requires K or Mg
• Allosterically activated by AMP, F-1,6-bisP
• Allosterically inhibited by ATP, NADH+H and
acetyl-CoA

30. Control Points
in Glycolysis

32. Pyruvate Metabolism
 Conversion to lactate (anaerobic)
 Conversion to alanine (amino acid)
 Entry into the TCA cycle
Formation of oxaloacetic acid
Formation of malic acid
Formation of phosphoenolpyruvate
Gluconeogenesis
Glycogenesis

33. Anaerobic Metabolism of
Pyruvate
COO–
C O
CH3
COO–
HC OH
CH3
LactatePyruvate
Lactate dehydrogenase
NADH+H+
NAD+
(oxidized) (reduced)

34. Anaerobic Metabolism of
Pyruvate
• ATP yield
– Two ATPs (net) are produced during the anaerobic
breakdown of one glucose
• The 2 NADHs are used to reduce 2 pyruvate
to 2 lactate
– Reaction is fast and doesn’t require oxygen

35. Pyruvate Metabolism - Anaerobic
Pyruvate Lactate
NADH NAD+
Lactate Dehydrogenase
 Lactate can be transported by blood to liver and
used in gluconeogenesis

36. Cori Cycle
Conversion of muscle glycogen to
lactic acid & reconversion of this to
muscle glycogen termed cori lactic
acid cycle

37. Pyruvate metabolism
• Convert to alanine and export to blood
COO–
C O
CH3
COO–
HC NH3
+
CH3
Alanineaminotransferase
(AAT)
AlaninePyruvate
Glutamate α-Ketoglutarate
Keto acid Amino acid

38. Pyruvate Dehydrogenase Complex (PDH)
• Prepares pyruvate to enter the TCA cycle
Electron
Transport
Chain
TCA
Cycle
Aerobic Conditions

39. Total ATP from Glucose
• Anaerobic glycolysis – 2 ATP + 2 NADH
• Aerobic metabolism – glycolysis + TCA
31 ATP from 1 glucose molecule