Chapter 8: Outline1

1
Chapter 8 Outline-1

• Glycolysis
• Reactions of the Glycolytic Pathway, Fate of
  Pyruvate
• Energetics of Glycolysis, Regulation of
  Glycolysis
• Gluconeogenesis
• Reactions Substrates Regulation
• Pentose Phosphate Pathway
• Metabolism of Other Important Sugars
• Fructose Galactose Manose
• Glycogen Metabolism
• Glycogenesis, Glycogenolysis, Regulation of
  Glycogen Metabolism

2
Introduction

• Major pathways of carbohydrate metabolism.

Fig 8.1 3rd ed
3
8.1 Glycolysis

• The anerobic oxidation of glucose (G) to give two
  molecules of pyruvate.
• G 2 ADP 2 Pi 2 NAD ?2 pyruvate
• 2 ATP 2 NADH 2 H 2 H2O

4
Glycolysis-overview

• Glucose
• Glucose-6-phosphate
• Fructose-6-phosphate
• Fructose-1,6-bisphosphate
• Dihydroxyacetone phosphate
• Glyceraldehyde 3-phosphate
• Glycerate-1,3-Bisphosphate
• Glycerate-3-Phosphate
• Glycerate-2-Phosphate
• Phosphoenolpyruvate
• Pyruvate

5
Glycolysis Step 1
hexokinase, Mg2
glucose
ADP
ATP
glucose-6-phosphate
6
Glycolysis Step 2
glucose-6-phosphate
fructose-6-phosphate
7
Glycolysis Step 3
fructose-6-phosphate
phosphofructokinase-1 (PFK-1), Mg2
ATP
ADP
fructose-1,6-bisphosphate
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Regulation by PFK-1
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Glycolysis Step 3-b

• The committed step! (irreversible)
• PFK-1 is a tetrameric enzyme
• as M4 in muscle and L4 in liver
• allosteric effectors
• high ATP conc. depresses rate
• fructose-2,6-bisP activates (well see later)

10
Glycolysis Step 4
dihydroxyacetone phosphate
fructose-1,6-bisphosphate
aldolase
D-glyceraldehyde 3-phosphate
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Glycolysis Step 4b
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Glycolysis Step 5
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Glycolysis Step 6
Glycerate-1,3-bisphosphate
glyceraldehyde 3-phosphate dehydrogenase
NAD HPO42-
NADH H
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Glycolysis Step 6b

• A phosphorylation and a two electron oxidation by
  NAD occur.
• G-3-P NAD H2O ?
• 3-P-glycerate - 43.1 kJ
• 3-P-glycerate HPO42- H ?
• glycerate-1,3-bisP 49.3 kJ

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Step 6c1
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Step 6c2
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Glycolysis Step 7
phospho- glycerate kinase Mg2
ADP
ATP
Glycerate-3-phosphate
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Glycolysis Step 8
Phosphoglycerate mutase Mg2
Glycerate-2-phosphate
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Glycolysis Step 9
enolase Mg2
H2O
Phosphoenolpyruvate (PEP)
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Glycolysis Step 10
pyruvate kinase Mg2
H ADP
ATP
pyruvate
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Pyruvate Fermentation
lactate dehydrogenase
NADH H
NAD
lactate
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Pyruvate Fermentation-2

• This reaction produces NAD which is needed for
  further anerobic glycolysis.
• Glyceraldehyde 3-phosphate
• --gt glycerate-1,3-bisphosphate

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Pyruvate Fermentation-3
pyruvate decarboxylase
TPP, Mg2
NADH H
ethanol NAD
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Glycolysis ATP

• Use ATP
• glucose to glucose-6-P
• fructose-6-P to fructose 1,6-bisP
• Produce ATP
• glycerate-1,3-bisP to
• 3-P-glycerate
• PEP to pyruvate

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Energetics of Glycolysis

• Evaluation of free energy changes measured in
  blood cells shows steps 1,3, and 10 have
  significantly negative DG values and are
  irreversible.
• The rest of the steps are close to equilibrium
  and the direction of reaction can be shifted by
  changes in substrate concentrations.

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Glycolysis Free Energy

• 
  kJ/mol
• Glu 2 ADP 2 Pi NAD ?
• 2 pyruvate 2 ATP NADH -73.3 H
• 2 (pyruvate NADH H ?
• lactate NAD ) -50.2
• Net -123.5

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Energy Efficiency

• 2 mol ATP 61 kJ
• Glucose to 2 lactate 184.5 kJ
• (no ATP production)
• Efficiency
• 61/184.5 x 100 33

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Regulation of Glycolysis

• Regulation is controlled primarily by three
  allosteric enzymes hexokinase. PFK-1, and
  pyruvate kinase.

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8.2 Gluconeogenesis

• Synthesis of glucose from noncarbohydrate
  precursors is gluconeogenesis.
• The three irreversible steps of glycolysis are
  bypassed.
• Pyruvate from a variety of sources is converted
  to glucose.

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Gluconeogenesis-overview

• Pyruvate
• Oxaloacetate
• Phosphoenolpyruvate
• Glycerate-2-Phosphate
• Glycerate-3-Phosphate
• Glycerate-1,3-bisphosphate
• Triose phosphates
• (Dihydroxyacetone phosphate)
• (Glyceraldehyde 3-phosphate)
• Fructose-1,6-bisphosphate
• Fructose-6-phosphate
• Glucose-6-phosphate
• Glucose

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Pyruvate to Oxaloacetate
Acetyl-CoA Biotin, Mg2
pyruvate carboxylase
ATP CO2 H2O
ADP Pi H
32
Pyruvate to Oxaloacetate 2
Biotin, which is attached to a lysine residue on
the enzyme, carries carbon dioxide.
Biotin-a one carbon carrier
33
Oxaloacetate to PEP
GTP
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F-1,6-bP ? F-6-P
fructose-1,6-bisphosphate
fructose-1,6-bisphosphatase
fructose-6-phosphate
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G-6-P ? Glucose
glucose-6-phosphatase
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Gluconeogenesis Substrates

• In the Cori cycle, lactate from skeletal muscle
  is transferred to the liver where it is converted
  to pyruvate then glucose.
• Glycerol (fats) enters gluconeogenesis in the
  liver via glycerol-3-P then DHAP.
• In the alanine cycle, pyruvate in muscles is
  converted to alanine which is transported to the
  liver and reconverted to pyruvate. This cycle
  also transports NH4 to the liver!!

37
Substrate Cycling

• Substrate cycling refers to the situation where
  opposing reactions are catalyzed by different
  reactions. The Cori cycle is a good example.
• The opposing reactions
• Glucose?lactate??pyruvate?glucose are separated
  and occur in the muscle or liver and are
  controlled by different enzymes.

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Substrate Cycling 2

• Glycolysis
• Glu 2NAD 2ADP 2Pi?
• 2Pyv 2NADH 4H 2ATP 2H2O
• Gluconeogenesis
• 2Pyv 2NADH 4H 4ATP 2GTP 6H2O ?
• Glu 2NAD 4ADP 2GDP 4Pi
• Net
• 2ATP 2GTP 4H2O ? 2ADP 2GDP 4Pi

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Substrate Cycling 3

• Hydrolysis of 2 ATP and 2 GTP is the energy
  price for the Cori cycle or the simultaneous
  control of the two opposing pathways, glycolysis
  and gluconeogenesis.

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Regulation gluconeogenesis

• The rate for the process is controlled by
  substrate availability, allosteric effectors, and
  hormones.
• Insulin leads to synthesis of glucokinase, PFK-1,
  and PFK-2.
• Glucagon leads to synthesis of PEP carboxykinase,
  fructose-1,6-bisphosphatase, and
  glucose-6-phosphatase.
• The insulin/glucagon ratio exerts the major
  regulatory effects on carbohydrate metabolism.
• Also, control at pyruvate kinase permits maximal
  retention of PEP.

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Regulation gluconeogenesis-3
F-2,6-P activates
F-2,6-P deactivates
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Regulation 4

• Both enzyme activities are on the same dimeric
  enzyme at different sites.

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8.3 Pentose Phosphate Pathway

• Five carbon sugars are produced.
• NADPH is produced for biosynthesis.
• Reshuffling of carbons occurs to give products
  with three, four, six, and seven carbons.

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Oxidation-1, PP Path
6-phospho-D-glucono-d-lactone
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Oxidation-2, PP PAth
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Oxidation-3, PP Path
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PP Path-2
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PP Shunt-3
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PP Path -4
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PP Path -5
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Control, PP Path

• Glycolysis PP Pathway
• Glucose-6-phosphate ribulose-5-phosphate
• 2 NADP CO2 2 NADPH
• fructose-6-phosphate ribose-5-phosphate
• fruct-1,6-bisP
• glyceraldehyde-3-phosphate
• dihydroxyacetone phosphate

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Control, PP Path-2

• If NADPH is needed, oxidative steps must occur.
• If ribose is needed, fructose and glyceraldehyde
  phosphates react via the PP Pathway to make
  ribose.
• Overall Oxidation
• 6 G-6-P 12 NADP 6 H2O ?
• 6 R-5-P 6 CO2 12 NADPH 12 H

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Hemolytic Anemia

• The PP Pathway supplies NADPH in red blood cells.
  
• Glucose-6-phosphate deficiency leads to less
  NADPH, less reduced glutathione (a peptide needed
  to maintain sulfhydryl groups and to keep iron as
  iron(II) in hemoglobin) thus anemia develops.
• Over 100 million people have the deficiency which
  conveys some resistance to malaria.

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Hemolytic Anemia-2
Glutathione
Reduced form provides H to reduce protein
disulfide links.
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8.4 Metabolism of Other Sugars

• In the liver, fructose is converted to
  fructose-1-phosphate and then split into DHAP and
  glyceraldehyde.
• Glyceraldehyde is then converted to
  glyceraldehyde-3-phosphate by glyceraldehyde
  kinase.
• In muscle and fat tissue fructose is converted to
  F-6-P.

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Other Sugars-2

• In fetus and childhood

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Other Sugars-3

• Galactosemia is the result of a lack of
  galactose-1-pyrophosphate uridyltransferase.
• Beginning in adolescence, UDP-galactose is
  produced in a reaction cartalyzed by
  UDP-galactose pyrophorylase.
• UDP-galactose is isomerized to UDP-glucose.
• Other Sugars-4
• Mannose is phosphorylated to M-6-P and then
  isomerized to F-6-P.

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8.5 Glycogen Metabolism

• Glycogen metabolism is carefully regulated so
  that sufficient glucose is available for the
  bodys energy needs.
• Insulin, glucagon, and epinephrine control
  glycogenesis and glycogenolysis.

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Glycogenesis-1

• Chain priming begins with attachment of the C-1
  OH of a glucose molecule to a specific tyrosine
  residue on the enzyme glycogenin.
• Two glucose molecules cannot spontaneously link
  to start the glycogen polymer.
• Chain elongation requires an activated glucose
  molecule, UDP-glucose

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Glycogenesis-2
hexokinase
ATP, Mg2

• Final product via 1,6-bisphosphate

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Glycogenesis-3
UDP glucose- phosphorylase

• Pyrophosphate hydrolyzes

UDPG
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Glycogenesis-4

• Glycogen synthase

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Glycogenesis-5

• Branching is accomplished by a branching enzyme.
  This enzyme removes about seven glucose units
  from a chain at least 11 units long and transfers
  them to make an a-1,6 branch at least four units
  away from the nearest existing branch.

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Glycogen Breakdown

• Glycogen phosphorylase removes glucose units
  until four are left approaching a branch point.
  The glucose is produced as glucose-1-P.
• G-1-P isomerizes to G-6-P.
• Note this saves one ATP molecule when glucose
  from glycogen goes through glycolysis! A net of
  three ATP are produced.
• When the four glucose units next to a branch
  remain, another enzyme is needed.
• When four glucose units remain, a second enzyme
  (debranching enzyme) goes into action.
• Debranching enzyme removes three (limit branch)
  of the four units and transfers them to the end
  of another chain.
• The glucose 1,6 bond is cleaved and glucose is
  the product, not the phosphorylated glucose.

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Regulation

• Glycogen metabolism is regulated primarily by
  three hormones insulin, glucagon, and
  epinephrine. The process depends on second
  messengers and a cascade reaction which will
  be discussed later.
• Covalent modification of enzymes (primarily
  phosphorylation) is central to control of
  glycogen metabolism and is overall an important
  means of enzyme regulation.

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Regulation-2
Cont. next slide
cAMP
67
Regulation-2b