Carbohydrates Lecture 3

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CarbohydratesLecture 3
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Objectives

• Define and identify disaccharide carbohydrate
  structures
• Explain the role of carbohydrates in energy
  metabolism
• Describe the 3 stages and 10 steps of glycolysis

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4 questions to refresh your memory
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4 questions to refresh your memory
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Lecture Outline

• Lecture 1 Monosaccharides
• Definition
• Nomenclature
• Fischer projections
• Lecture 2 Monosaccharides
• Haworth projections
• Furanose Pyranose rings
• Modified Sugars
• Sugars in DNA

• Lecture 3 Disaccharides Energy storage
• Glycosidic bond
• Glycolysis
• Lecture 4 Complex Carbohydrates Function
• Starch glycogen
• Cellulose
• Blood groups
• Glycoproteins

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Outline

• Disaccharides glycosidic bond
• Sucrose, Lactose, Maltose
• Carbohydrates for energy
• Glycolysis oxidative metabolism of glucose
• 3 stages and 10 steps of glycolysis
• Enzymes involved in glycolysis

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Carbohydrate Ring Structures

• Carbohydrate Ring Structures
• Pyranose Ring
• Furanose Ring
• Drawing sugars
• Haworth projections
• Stereo chemistry

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Modified Sugars Sugars in DNA

• Modified Sugars
• Sugar Alcohols
• Amino Sugars
• Sugar in DNA
• 2-deoxyribose
• Sugar-phosphate backbone

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What is a Carbohydrate?

• Simplest carbohydrate - monosaccharide
• Join two monosaccharides to form a disaccharide
• Complex carbohydrates (polymers) formed by
  dehydration chemistry

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Disaccharides
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Disaccharides

• Maltose 2 D-glucoses joined by O-glycosidic
  linkage
• ?-anomeric carbonyl group (C1) OH group (C4)
• ?-1,4-glycosidic bond
• produced in the malting process during beer
  production

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Disaccharides

• Monosaccharides have multiple OH groups
• Different glycosidic bonds possible

Lactose
Sucrose
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Disaccharides
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Disaccharides
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Disaccharides
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Disaccharides

• Lactose hydrolysed by lactase
• Maltose hydrolysed by maltase
• Sucrose hydrolysed by sucrase
• All these enzymes found on surface of epithelial
  cells lining small intestine

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Disaccharides
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Rasmol Sucrose
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Carbohydrates for Energy

• C02 the most abundant inorganic carbon source
• C02 is converted to glucose during
  photosynthesis
• Glucose is converted to other sugars and
  derivatives in cells as a means of generating
  chemical energy
• Glucose is converted into energy as a part of
  metabolism

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Glucose product of photosynthesis

• Sugars are available in the biosphere through the
  reduction of C02 the most abundant inorganic
  carbon source
• Photosynthesis

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Metabolism

• Chemical transformations of simple organic
  compounds are used by cells to generate energy in
  a usable form
• Chemical inter-conversions of metabolites are the
  basis of metabolism
• Carbohydrates and their simple derivatives play
  central role in metabolism

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Metabolism
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Glycolysis

• Glucose is converted to other sugars and
  derivatives in cells as a means of generating
  chemical energy
• Oxidation of glucose exothermic
• Many cells burn sugars to yield reactive
  chemicals
• Oxidative metabolism of glucose is known as
  glycolysis

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Glycolysis

• Glycolysis comprises 10 step reactions in 3
  stages
• 2 reactions involve an energy investment of ATP
• 3 reactions produce energy in the form of NADH
  and ATP

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Glycolysis Stage 1

• Conversion of glucose into fructose
  1,6-bisphosphate
• Phosphorylation
• Isomerization
• Phosphorylation
• Energy invested to be regained later

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Stage 1 Phosphorylation

• Glucose activated by attaching phosphate group
• Phosphate from ATP

STEP 1
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Hexokinase

• Binding of glucose induces large conformational
  change
• 2 lobes with a cleft between
• On binding one lobe rotates 12 degrees cleft
  closes
• Glucose surrounded by enzymes except for C6 which
  accepts the phosphoryl group from ATP
• Induced fit enzyme action

Rasmol PDB structures 1IG8 1BDG
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Stage 1 Isomerization

• Glucose has an aldehyde Frustose has a keto
  group
• Conversion of an aldose to ketose
• Catalysed by phosphoglucose isomerase
• Open ring Catalyse isomerization Form ring

STEP 2
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Stage 1 Phosphorylation

• 2nd phosphate group attached
• Phosphate from ATP

STEP 3
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Glycolysis Stage 1
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Glycolysis Stage 2

• Conversion of Fructose-1,6-bisphosphate into
• glyceraldehyde 3-phosphate (GAP)
• dihydroxyacetone phosphate (DHAP)
• Catalysed by aldolase
• DHAP GAP interconverted

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Glycolysis Stage 2

• Cleavage step

STEP 4
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Glycolysis Stage 2

• Dihydroxyacetone not on direct glycolysis pathway
• Conversion needed so that a 3 C fragment useful
  for generating ATP is not lost
• Isomers readily interconverted

STEP 5
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Glycolysis Stage 2

• Isomerization catalysed by triose phosphate
  isomerase (TIM)
• Core of ?-strands surrounded by ?-helices
• ??-barrel
• A loop structure closes off active site when
  substrate bound

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Glycolysis Stage 2
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Glycolysis Stage 3

• Energy generation stage
• Oxidation of 3C fragments gives ATP
• 3 reactions produce energy in the form of NADH
  and ATP
• Production of pyruvate

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Glycolysis Stage 3

• Conversion of GAP into 1,3-BPG
• Aldehyde oxidized to carboxylate and a phosphate
  attached
• Energy released in oxidation used to form high
  energy carboxylate-phosphate bond

STEP 6
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Glycolysis Stage 3

• High energy phosphate-carboxylate bond used to
  make first ATP

STEP 7
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Glycolysis Stage 3

• 3-phosphoglycerate converted into pyruvate with
  production of ATP
• Rearrangement shifts position of phosphate C3
  to C2
• Dehydration formation of high energy phosphate
  bond
• Phosphorylation of ADP using high energy
  phosphate bond

STEP 10
STEP 8
STEP 9
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Glycolysis Stage 3
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Glycolysis Energy Production

• Glucose 2Pi 2ADP 2NAD
• 2 Pyruvate 2ATP 2NADH 2H
  2H2O

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Glycolysis

• 10 reactions in 3 stages
• 2 reactions involve an energy investment of ATP
• 3 reactions produce energy in the form of NADH
  and ATP

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10 Steps of Glycolysis
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Importance of Carbohydrates

• Glucose is converted to other sugars and
  derivatives in cells as a means of generating
  chemical energy
• Oxidative metabolism of glucose is known as
  glycolysis
• Enzymes allow cells to convert virtually any
  sugar or derivative into any other sugar or
  related derivative
• Universal currency in the biochemical economy

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Summary Lecture 3

• Joining of monosaccharide units through
  glycosidic bonds to form dissaccharides
• Glucose converted to other sugars derivatives
  in cells to generating chemical energy
• Glycolysis is part of metabolism oxidative
  metabolism of glucose
• 3 stages and 10 steps of glycolysis
• 2 steps use ATP and 2x2 steps produce ATP net
  gain of 2ATP

Glucose 2Pi 2ADP 2NAD 2
Pyruvate 2ATP 2NADH 2H 2H2O