Carbohydrates Metabolism

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• Chapter 4
• Carbohydrates Metabolism

The biochemistry and molecular biology department
of CMU
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1 Overview

• Carbohydrates in general are polyhydroxy
  aldehydes or ketones or compounds which yield
  these on hydrolysis.

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Biosignificance of Carbohydrates

• The major source of carbon atoms and energy for
  living organisms.
• Supplying a huge array of metabolic intermediates
  for biosynthetic reactions.
• The structural elements in cell coat or
  connective tissues.

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Glucose transporters (GLUT)

• GLUT15
• GLUT1 RBC
• GLUT4 adipose tissue, muscle

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The metabolism of glucose

• glycolysis
• aerobic oxidation
• pentose phosphate pathway
• glycogen synthesis and catabolism
• gluconeogenesis

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glycogen
Glycogenolysis
Glycogenesis
starch
lactate
Glycolysis
Digestion absorption
glucose
aerobic oxidation
Lactate, amino acids, glycerol
H2OCO2
Gluconeo-genesis
Pentose phosphate pathway
Ribose, NADPH
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2 Glycolysis
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• Glycolysis
• The anaerobic catabolic pathway by which a
  molecule of glucose is broken down into two
  molecules of lactate.
• glucose ?2lactic acid (lack of O2)
• All of the enzymes of glycolysis locate in
  cytosol.

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• 1. The procedure of glycolysis

G
glycolytic pathway
pyruvate
lactic acid
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• 1) Glycolytic pathway
• G ? pyruvate
• including 10 reactions.

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(1) G phosphorylated into glucose 6-phosphate

• Phosphorylated G cannot get out of cell
• Hexokinase , HK (4 isoenzymes) ,
• glucokinase, GK in liver
• Irreversible .

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• hexokinase
  glucokinase
• occurrence in all tissues only in liver
• Km value 0.1mmol/L 10mmol/L
• Substrate G, fructose, glucose
• mannose
• Regulation G-6-P Insulin

Comparison of hexokinase and glucokinase
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(2) G-6-P ? fructose 6-phosphate
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(3) F-6-P ? fructose 1,6-bisphosphate

• The second phosphorylation
• phosphofructokinase-1, PFK-1

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(4) F-1,6-BP ? 2 Triose phosphates

• Reversible

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(5) Triose phosphate isomerization

• G?2 molecule glyceraldehyde-3-phosphate, consume
  2 ATP .

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(6) Glyceraldehyde 3-phosphate ? glycerate
1,3-bisphosphate
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(7) 1,3-BPG ? glycerate 3-phosphate

• Substrate level phosphorylation

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(8) Glycerate 3-phosphate ? glycerate 2-phosphate
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(9) Glycerate 2-phosphate ? phosphoenol pyruvate
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(10) PEP ?pyruvate

• Second substrate level phosphorylation
• irreversible

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• 2) Pyruvate ? lactate

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Summary of Glycolysis
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• Total reaction
• C6H12O6 2ADP 2Pi 2CH3CHOHCOOH
  2ATP 2H2O
• Formation of ATP
• The net yield is 2 P or 2 molecules of ATP per
  glucose.

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2. Regulation of Glycolysis

• Three key enzymes catalyze irreversible
  reactions Hexokinase, Phosphofructokinase
  Pyruvate Kinase.

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1) PFK-1 The reaction catalyzed by PFK-1 is
usually the rate-limiting step of the Glycolysis
pathway. This enzyme is regulated by covalent
modification, allosteric regulation.
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bifunctional enzyme
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2) Pyruvate kinase

• Allosteric regulation
• F-1,6-BP acts as allosteric activator
• ATP and Ala in liver act as allosteric
  inhibitors

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• Covalent modification
• phosphorylated by Glucagon through cAMP and
  PKA and inhibited.

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3) Hexokinase and glucokinase

• This enzyme is regulated by covalent
  modification, allosteric regulation and isoenzyme
  regulation.
• Inhibited by its product G-6-P.
• Insulin induces synthesis of glucokinase.

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• 3. Significance of glycolysis
• 1) Glycolysis is the emergency energy-yielding
  pathway.
• 2) Glycolysis is the main way to produce ATP
  in some tissues, even though the oxygen supply is
  sufficient, such as red blood cells, retina,
  testis, skin, medulla of kidney.
• In glycolysis, 1mol G produces 2mol lactic acid
  and 2mol ATP.

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3 Aerobic Oxidation of Glucose
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• The process of complete oxidation of glucose to
  CO2 and water with liberation of energy as the
  form of ATP is named aerobic oxidation.
• The main pathway of G oxidation.

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1. Process of aerobic oxidation
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1) Oxidative decarboxylation of Pyruvate to
Acetyl CoA

• irreversible
• in mitochodria.

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• Pyruvate dehydrogenase complex
• E1 pyruvate dehydrogenase
• Es E2 dihydrolipoyl transacetylase
• E3 dihydrolipoyl dehydrogenase
• thiamine pyrophosphate, TPP
  (VB1)
• HSCoA (pantothenic acid)
  
• cofactors lipoic Acid
• NAD (Vpp)
• FAD (VB2)

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Pyruvate dehydrogenase complex
HSCoA
NAD
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The structure of pyruvate dehydrogenase complex
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HSCoA
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CO2
NADH H
NAD
CoASH
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• 2) Tricarboxylic acid cycle, TCAC
• The cycle comprises the combination of a molecule
  of acetyl-CoA with oxaloacetate, resulting in the
  formation of a six-carbon tricarboxylic acid,
  citrate. There follows a series of reactions in
  the course of which two molecules of CO2 are
  released and oxaloacetate is regenerated.
• Also called citrate cycle or Krebs cycle.

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(1) Process of reactions
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Citrate cycle
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Summary of Krebs Cycle ?
Reducing equivalents
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? The net reaction of the TCAC acetylCoA3NADFA
DGDPPi2H2O ? 2CO23NADH3HFADH2GTP
HSCoA ? Irreversible and aerobic reaction ? The
enzymes are located in the mitochondrial matrix.
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? Anaplerotic reaction of oxaloacetate
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(2) Bio-significance of TCAC

• ? Acts as the final common pathway for the
  oxidation of carbohydrates, lipids, and proteins.
• ? Serves as the crossroad for the
  interconversion among carbohydrates, lipids, and
  non-essential amino acids, and as a source of
  biosynthetic intermediates.

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Krebs Cycle is at the hinge of metabolism.
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• 2. ATP produced in the aerobic oxidation
• acetyl CoA ? TCAC 3 (NADHH) FADH2 1GTP ?
  12 ATP.
• pyruvate ?acetyl CoA NADHH ? 3 ATP
• 1 G ? 2 pyruvate 2(NADHH) ? 6 or 8ATP
• 1mol G 36 or 38mol ATP
• (123 )2 6( 8 )36( 38 )

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3. The regulation of aerobic oxidation

• The Key Enzymes of aerobic oxidation
• The Key Enzymes of glycolysis
• Pyruvate Dehydrogenase Complex
• Citrate synthase
• Isocitrate dehydrogenase (rate-limiting )
• ?-Ketoglutarate dehydrogenase

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(1) Pyruvate dehydrogenase complex
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• (2) Citrate synthase
• Allosteric activator ADP
• Allosteric inhibitor NADH, succinyl CoA,
  citrate, ATP
• (3) Isocitrate dehydrogenase
• Allosteric activator ADP, Ca2
• Allosteric inhibitor ATP
• (4) ?-Ketoglutarate dehydrogenase
• Similar with Pyruvate dehydrogenase complex

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• Oxidative phosphorylation?TCAC?
• ATP/ADP? inhibit TCAC,
• Oxidative phosphorylation ?
• ATP/ADP?,promote TCAC,
• Oxidative phosphorylation ?

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4. Pasteur Effect

• Under aerobic conditions, glycolysis is inhibited
  and this inhibitory effect of oxygen on
  glycolysis is known as Pasteur effect.
• The key point is NADH
• NADH mitochondria
• Pyr TCAC CO2H2O
• Pyr cant produce to lactate.

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4 Pentose Phosphate Pathway
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• 1. The procedure of pentose phosphate
  pathway/shunt
• In cytosol

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1) Oxidative Phase
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2) Non-Oxidative Phase
Fructose 6-p
Glycolysis
Ribose 5-p
Fructose 6-p
Xylulose 5-p
Xylulose 5-p
Glyceraldehyde 3-p

• Transketolase requires TPP
• Transaldolase

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• The net reation
• 3G-6-P 6NADP ?
• 2F-6-P GAP 6NADPH H 3CO2
• 2. Regulation of pentose phosphate pathway
• Glucose-6-phosphate Dehydrogenase is the
  rate-limiting enzyme.
• NADPH/NADP?, inhibit
• NADPH/NADP?, activate.

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3. Significance of pentose Phosphate pathway

• 1) To supply ribose 5-phosphate for bio-synthesis
  of nucleic acid
• 2) To supply NADPH as H-donor in metabolism
• NADPH is very important reducing power for the
  synthesis of fatty acids and cholesterol, and
  amino acids, etc.

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• NADPH is the coenzyme of glutathione reductase
  to keep the normal level of reduced glutathione

So, NADPH, glutathione and glutathione
reductase together will preserved the integrity
of RBC membrane.
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• Deficiency of glucose 6-phosphate dehydrogenase
  results in hemolytic anemia.
• favism
• NADPH serves as the coenzyme of mixed function
  oxidases (mono-oxygenases). In liver this enzyme
  participates in biotransformation.

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5 Glycogen synthesis and catabolism

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• Glycogen is a polymer of glucose residues linked
  by
• ? (1?4) glycosidic bonds, mainly
• ? (1?6) glycosidic bonds, at branch points.

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1. Glycogen synthesis (Glycogenesis)

• The process of glycogenesis occurs in cytosol of
  liver and skeletal muscle mainly.

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• UDPG G active pattern, G active donor.
• In glycogen anabolism, 1 G consumes 2P.
• Glycogen synthase key E.

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UDPG
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Branching enzyme
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2. Glycogen catabolism (glycogenolysis)
Phosphorylase key E The end products 85 of
G-1-P and 15 of free G There is no the activity
of glucose 6-phosphatase (G-6-Pase) in skeletal
muscle.
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Debranching enzyme glucan transferase
?-1,6-glucosidase
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(?1?6) linkage
Nonreducing ends
Glycogen phosphorylase
Transferase activity of debranching enzyme
(?1?6) glucosidase activity of debranching enzyme
Glucose
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• 3. Regulation of glycogenesis and glycogenolysis
• 1) Allosteric regulation
• In liver
• G phosphorylase glycogenolysis
• In muscle

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• 2) Covalent modification

Adenylyl cyclase
Glucagon epinephrine
receptor
G protein
Phosphorylase
cAMP
PKA
Glycogen synthase
glycogenolysis
Blood sugar
glycogenesis
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6 Gluconeogenesis

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• Concept
• The process of transformation of
  non-carbohydrates to glucose or glycogen is
  termed as gluconeogenesis.
• Materials lactate, glycerol, pyruvate and
  glucogenic amino acid.
• Site mainly liver, kidney.

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1. Gluconeogenic pathway

• The main pathway for gluconeogenesis is
  essentially a reversal of glycolysis, but there
  are three energy barriers obstructing a simple
  reversal of glycolysis.

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1) The shunt of carboxylation of Pyr
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2) F-1, 6-BP ?F-6-P
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3) G-6-P ?G

• 2 lactic acid G consume ATP?

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gluconeogenesis
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2. Regulation of gluconeogenesis

• Substrate cycle
• The interconversion of two substrates
  catalyzed by different enzymes for singly
  direction reactions is called substrate cycle.
• The substrate cycle produces net hydrolysis of
  ATP or GTP.------futile cycle

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• Key enzymes of gluconeogenesis
• PEP carboxykinase
• Pyr carboxylase
• Fructose-bisphosphatase
• Glucose-6-phosphatase

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• 3. Significance of gluconeogenesis
• Replenishment of Glucose by Gluconeogenesis and
  Maintaining Normal Blood Sugar Level.
• Replenishment of Liver Glycogen.
• Regulation of Acid-base Balance.

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First stages (cytosol)
Second stages (Mt.)
Third stages (Mt.)
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Lactic acid (Cori) cycle

• Lactate, formed by the oxidation of glucose in
  skeletal muscle and by blood, is transported to
  the liver where it re-forms glucose, which again
  becomes available via the circulation for
  oxidation in the tissues. This process is known
  as the lactic acid cycle or Cori cycle.
• prevent acidosisreused lactate

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Lactic acid cycle
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• 6 Blood Sugar and Its Regulation

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1. The source and fate of blood sugar
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Blood sugar level must be maintained within a
limited range to ensure the supply of glucose to
brain. The blood glucose concentration is
3.896.11mmol/L normally.
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2. Regulation of blood sugar level

• 1)insulin for decreasing blood sugar levels.
• 2)glucagonfor increasing blood sugar levels.
• 3)glucocorticoid for increasing blood sugar
  levels.
• 4)adrenalinefor increasing blood sugar levels.

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3. Abnormal Blood Sugar Level

• Hyperglycemia gt 7.227.78 mmol/L
• The renal threshold for glucose
  8.8910.00mmol/L
• Hypoglycemia lt 3.333.89mmol/L

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Pyruvate as a junction point
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