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Chapter 20 Specific Catabolic Pathways: Carbohydrate, Lipid, and Protein Metabolism

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Title: Chapter 20 Specific Catabolic Pathways: Carbohydrate, Lipid, and Protein Metabolism


1
Chapter 20Specific Catabolic PathwaysCarbohydra
te, Lipid, and ProteinMetabolism
2
Convergence of Pathways
  • Figure 20.2 Convergence of the specific pathways
    of carbohydrate, fat, and protein catabolism into
    the common pathway, which is made up of citric
    acid cycle and oxidative phosphorylation.

3
Glycolysis
  • Glycolysis A series of 10 enzyme-catalyzed
    reactions by which glucose is oxidized to two
    molecules of pyruvate.
  • During glycolysis, there is net conversion of
    2ADP to 2ATP.

4
Glycolysis
  • Reaction 1 Phosphorylation of ?-D-glucose.

5
Glycolysis
  • Reaction 2 Isomerization of ?-D-glucose
    6-phosphate to ?-D-fructose 6-phosphate.

6
Glycolysis
  • This isomerization is most easily seen by
    considering the open-chain forms of each
    monosaccharide. It is one keto-enol tautomerism
    followed by another.

7
Glycolysis
  • Reaction 3 Phosphorylation of fructose
    6-phosphate.

8
Glycolysis
  • Reaction 4 Cleavage of fructose 1,6-bisphosphate
    to two triose phosphates.

9
Glycolysis
  • Reaction 5 Isomerization of triose phosphates.
  • Catalyzed by phosphotriose isomerase. The
    mechanism involves two successive keto-enol
    tautomerizations.
  • Only the D enantiomer of glyceraldehyde
    3-phosphate is formed.

10
Glycolysis
  • Reaction 6 Oxidation of the -CHO group of
    D-glyceraldehyde 3-phosphate.
  • The product contains a phosphate ester and a
    high-energy mixed carboxylic-phosphoric
    anhydride.

11
Glycolysis
  • Reaction 7 Transfer of a phosphate group from
    1,3-bisphosphoglycerate to ADP.

12
Glycolysis
  • Reaction 8 Isomerization of 3-phosphoglycerate
    to 2-phosphoglycerate.
  • Reaction 9 Dehydration of 2-phosphoglycerate.

13
Glycolysis
  • Reaction 10 Phosphate transfer to ADP.

14
Glycolysis
  • Summing these 10 reactions gives the net
    equation for glycolysis

15
Reactions of Pyruvate
  • Pyruvate is most commonly metabolized in one of
    three ways, depending on the type of organism and
    the presence or absence of O2.

16
Reactions of Pyruvate
  • A key to understanding the biochemical logic
    behind two of these reactions of pyruvate is to
    recognize that glycolysis needs a continuing
    supply of NAD.
  • if no oxygen is present to reoxidize NADH to
    NAD, then another way must be found to reoxidize.

17
Pyruvate to Lactate
  • In vertebrates under anaerobic conditions, the
    most important pathway for the regeneration of
    NAD is reduction of pyruvate to lactate.
    Pyruvate, the oxidizing agent, is reduced to
    lactate.
  • Lactate dehydrogenase (LDH) is a tetrameric
    isoenzyme consisting of H and M subunits H4
    predominates in heart muscle, M4 in skeletal
    muscle.

18
Pyruvate to Lactate
  • While reduction to lactate allows glycolysis to
    continue, it increases the concentration of
    lactate and also of H in muscle tissue.
  • When blood lactate reaches about 0.4 mg/100 mL,
    muscle tissue becomes almost completely exhausted.

19
Pyruvate to Ethanol
  • Yeasts and several other organisms regenerate
    NAD by this two-step pathway
  • Decarboxylation of pyruvate to acetaldehyde.
  • Acetaldehyde is then reduced to ethanol. NADH is
    the reducing agent. Acetaldehyde is reduced and
    is the oxidizing agent in this redox reaction.

20
Pyruvate to Acetyl-CoA
  • Under aerobic conditions, pyruvate undergoes
    oxidative decarboxylation.
  • The carboxylate group is converted to CO2.
  • The remaining two carbons are converted to the
    acetyl group of acetyl CoA.
  • This reaction provides entrance to the citric
    acid cycle.

21
Pentose Phosphate Pathway
  • Figure 20.5 Simplified schematic representation
    of the pentose phosphate pathway, also called a
    shunt.

22
Energy Yield in Glycolysis
23
Catabolism of Glycerol
  • Glycerol enters glycolysis via dihydroxyacetone
    phosphate.

24
Fatty Acids and Energy
  • Fatty acids in triglycerides are the principal
    storage form of energy for most organisms.
  • Hydrocarbon chains are a highly reduced form of
    carbon.
  • The energy yield per gram of fatty acid oxidized
    is greater than that per gram of carbohydrate
    oxidized.

25
?-Oxidation
  • ?-Oxidation A series of five enzyme-catalyzed
    reactions that cleaves carbon atoms two at a time
    from the carboxyl end of a fatty acid.
  • Reaction 1 The fatty acid is activated by
    conversion to an acyl CoA. Activation is
    equivalent to the hydrolysis of two high-energy
    phosphate anhydrides.

26
?-Oxidation
  • Reaction 2 Oxidation by FAD of the ?,?
    carbon-carbon single bond to a carbon-carbon
    double bond.

27
?-Oxidation
  • Reaction 3 Hydration of the CC double bond to
    give a 2 alcohol.
  • Reaction 4 Oxidation of the 2?alcohol to a
    ketone.

28
?-Oxidation
  • Reaction 5 Cleavage of the carbon chain by a
    molecule of CoA-SH.

29
?-Oxidation
  • This cycle of reactions is then repeated on the
    shortened fatty acyl chain and continues until
    the entire fatty acid chain is degraded to acetyl
    CoA.
  • b-Oxidation of unsaturated fatty acids proceeds
    in the same way, with an extra step that
    isomerizes the cis double bond to a trans double
    bond.

30
Energy Yield on ?-Oxidation
  • Yield of ATP per mole of stearic acid (C18).

31
Ketone Bodies
  • Ketone bodies Acetone, ?-hydroxybutyrate, and
    acetoacetate
  • Are formed principally in liver mitochondria.
  • Can be used as a fuel in most tissues and organs.
  • Formation occurs when the amount of acetyl CoA
    produced is excessive compared to the amount of
    oxaloacetate available to react with it and take
    it into the TCA for example
  • Dietary intake is high in lipids and low in
    carbohydrates.
  • Diabetes is not suitably controlled.
  • Starvation.

32
Ketone Bodies
33
Protein Catabolism
  • Figure 20.7 Overview of pathways in protein
    catabolism.

34
Nitrogen of Amino Acids
  • -NH2 groups move freely by transamination
  • Pyridoxal phosphate forms an imine (a CN group)
    with the ?-amino group of an amino acid.
  • Rearrangement of the imine gives an isomeric
    imine.
  • Hydrolysis of the isomeric imine gives an
    ?-ketoacid and pyridoxamine. Pyridoxamine then
    transfers the -NH2 group to another
    a-ketoacid.

35
Nitrogen of Amino Acids
  • Nitrogens to be excreted are collected in
    glutamate, which is oxidized to a-ketoglutarate
    and NH4.
  • The conversion of glutamate to ?-ketoglutarate is
    an example of oxidative deamination.
  • NH4 then enters the urea cycle.

36
Urea CycleAn Overview
  • Urea cycle A cyclic pathway that produces urea
    from CO2 and NH4.

37
Amino Acid Catabolism
  • The breakdown of amino acid carbon skeletons
    follows two pathways.
  • Glucogenic amino acids Those whose carbon
    skeletons are degraded to pyruvate or
    oxaloacetate, both of which may then be converted
    to glucose by gluconeogenesis.
  • Ketogenic amino acids Those whose carbon
    skeletons are degraded to acetyl CoA or
    acetoacetyl CoA, both of which may then be
    converted to ketone bodies.

38
Amino Acid Catabolism
  • Figure 20.9 Catabolism of the carbon skeletons
    of amino acids.

39
Amino Acid Catabolism
40
Heme Catabolism
  • When red blood cells are destroyed
  • Globin is hydrolyzed to amino acids to be reused.
  • Iron is preserved in ferritin, an iron-carrying
    protein, and reused.
  • Heme is converted to bilirubin.
  • Bilirubin enters the liver via the bloodstream
    and is then transferred to the gallbladder where
    it is stored in the bile and finally excreted in
    the feces.

41
Heme Catabolism
  • Figure 20.10 Heme degradation from heme to
    biliverdin to bilirubin.

42
Heme Catabolism
  • Figure 20.11 A summary of catabolism showing
    the role of the common metabolic pathway.
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