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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Carbohydrates

• Carbohydrates have an Aldehyde or Ketone functional group.

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Carbohydrates: Monosaccharides • Monosaccharides have one of the following structures:

• Carbohydrates can be further classified: • Monosaccharides: 6-C) Glucose, Fructose, Galactose 5-C) Ribose • Simple Carbohydrates: Mono & disaccharides • Disaccharide: Lactose, Sucrose, Maltose • Trisaccharide, etc. • Complex Carbohydrates: Polysaccharides • Oligosaccharides: Smaller than a polysaccharide but larger than a monosaccharide (10-20 monosaccharide residues) • Polysaccharides: Hundreds to thousands of monosaccharide units bonded together. • Starch, cellulose, glycogen, chitin

• D-aldoses: hydroxyl farthest from the carbonyl group points right

• Aldoses: the carbonyl is C1 • Ketoses: the carbonyl is C2.

• D-ketoses: hydroxyl farthest from the carbonyl group points right

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

The Structure and Classification of Some Monosaccharides

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Important Monosaccharides (Hexoses) • Enzymes usually recognize only the D-Enantiomers of sugar molecules (chiral recognition).

• D-Glucose: the most abundant monosaccharide in nature. • Also called Blood Sugar or Dextrose. • Found in combined forms: starch, cellulose, glycogen, chitin, lactose, and sucrose. • D D-Fructose: F t (L (Levulose) l ) is i bonded b d d to t glucose l to t form f sucrose, the th sugar in fruits and table sugar. • D-Galactose: is bonded to glucose to form lactose, the sugar in milk.

Important Monosaccharides (Pentoses)

The Synthesis and Breakdown of Polymers

• D-ribose, D-xylose, and 2-Deoxyribose are important pentoses. • D-Ribose: Component of RNA • 2-Deoxyribose: Component of DNA • In 2-deoxyribose, the –OH at C2 is replaced by –H. • D-Xylose: Component of some plant polysaccharides

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Cyclic Hemiacetal Structure (Glucose)

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Starch (α) vs. Cellulose (β) Structures • Hemiacetal Carbon: Two possible bonding types: -OR (Acetal) -OH (Hemiacetal) • α and β configurations: stereoisomers at the hemiacetal carbon. • α = trans between the –OH at C1 and the –CH2OH at C5. • β = cis between the –OH at C1 and the – CH2OH at C5. • The acyclic form of glucose (open-chain structure) contributes less than 0.2% of the equilibrium mixture and is not stable. • D-glucose consists of an equilibrium mixture of these two hemiacetals: 36% α and 64% β.

Cyclic Hemiacetal Structure (Fructose)

Reducing Sugars: Oxidation of the Aldehyde Group

D-Fructose and other ketohexoses form 5-membered cyclic hemiacetals

• Benedict’s solution: The aldehyde group of an aldose can be oxidized by Cu2+ in alkaline solution to a carboxylic acid. • Oxidation occurs with the acyclic (open chain) form of the aldose and not with the hemiacetal form. • Benedict’s Benedict s reagent is used to test for the presence of glucose in urine urine. Blue Cu2+ color changes to a red Cu2O precipitate. • α-Hydroxyketones are oxidized by Benedict’s because under basic conditions they are converted into aldoses. Most ketones are not oxidized by Benedict’s reagent.

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Disaccharides: Maltose

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Disaccharides: Lactose

• Corn sugar, or malt sugar.

• Milk sugar: constitutes about 4-8% of mammalian milk.

• Because maltose has a free hemiacetal –OH, it is a reducing sugar.

• Lactose is a reducing sugar.

α 1-4 Glycosidic Linkage

β 1-4 Glycosidic Linkage • Lactose Intolerance: Disease involving lactose consumption.

Disaccharides: Sucrose

Disaccharides: Digestion and Absorption

• The most abundant disaccharide in nature. • Formed by acetal formation between the hemiacetal –OH groups of both α-D-glucose and β-D-fructose. The linkage is α,β(1Æ2):

• Maltose (from starch digestion), Lactose, and Sucrose cannot be directly absorbed from the intestinal track. • Each must first be enzymatically hydrolyzed to the monosaccharides: • Maltose Î maltase • Lactose Î lactase • Sucrose Î sucrase

α,β 1-2 Glycosidic Linkage

• Enzymes are often named by adding “–ase” to the name of the compound undergoing the reaction.

• Because both hemiacetal –OH groups are involved in the glycosidic linkage, sucrose is not a reducing sugar.

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Acetal Formation: The Production of Glycosides

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Polysaccharides: Starch and Glycogen • Starch (Plants) & Glycogen (animals): storage for D-glucose.

• Monosaccharides can be converted into glycosides by the acid-catalyzed reaction of the hemiacetal –OH group with an alcohol.

• Amylose: 10-30%, unbranched, α(1Æ4) linkages

• Reaction occurs preferentially at the hemiacetal –OH because it is the most reactive hydroxyl present.

• Amylopectin: 70-90%, branched, α(1Æ4) linkages and α(1Æ6) branches, branches have branches, MW 1,000,000 or more.

Polysaccharides: Starch and Glycogen

Three Glucose Polymers

• Glycogen: similar to amylopectin but more highly branched. • The percentage of free hemiacetal –OH groups is so small that none of these molecules give a positive test with Benedict’s solution. They are all non-reducing sugars. • Removal of D-glucose from glycogen can be very rapid because it can be removed from all of the tips of the glycogen branches simultaneously

• Cellulose: Most abundant organic compound in the biosphere (50% of all organic carbon). • Cellulose is a linear polymer of D-glucose with β(1Æ4) linkages that pack side by side to form fibers. • All of the cellulose molecules are held together in a fiber by H-bonds.

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Microbes Digest Cellulose for Ruminants

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Starch and Glycogen in Metabolism • Starch is the principle carbohydrate in our diet: • Amylase hydrolyzes amylose to maltose in the digestive tract. tract • Maltase cleaves maltose to D-glucose. • Some D-glucose from starch is used immediately for energy (glycolysis). • Excess D-glucose is stored in the liver and skeletal muscles as glycogen ( l (glycogenesis). i ) • Any D-glucose still in excess is converted to fat and deposited in the fat tissues.

Phospholipid Bilayer Only small, nonpolar or hydrophobic molecules can diffuse

Hydrophilic heads

Animal Cell Plasma Membrane, in Cross Section (e.g. Glycoproteins & ABO blood groups)

Water

Hydrophobic tails

Water

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Cell Recognition: Glycolipids and Glycoproteins

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Multiple Alleles: ABO Blood Groups & Antibodies

• Cells interact with and recognize other cells through saccharides attached to the cell surfaces. • Glycolipids y p and Glycoproteins. y p The lipid p or p protein p part of the molecule is integrated into the cell-membrane structure with the saccharide part located on the external membrane surface.

• The ABO blood group types are: A, B, AB, and O. These result from three types of antigens (containing saccharide molecules) A, B, and O. • There are only two types of antibodies: anti-A anti A and anti-B. anti B There is no anti-O.

Cellular Respiration Equation

Redox (Oxidation/Reduction) Reaction

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The Complexity of Metabolism

Eukaryotic Cell

NAD+ as an Electron Shuttle

Structure of ATP & Substrate-Level Phosphorylation

NAD+ → NADH (Redox Reactions) Phosphorylation of ATP: 1) Substrate Level 2) Oxidative 3) Photo

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Glycolysis: Two Stages

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Glycolysis: Energy Investment Phase

• Stage 1: Glucose and other hexoses are converted into glyceraldehyde-3-phosphate.

• Stage 2: Glyceraldehyde-3-phosphate is oxidized and converted into two molecules of pyruvate. • ATP and NADH are generated during stage 2.

• Without O2 pyruvate is converted to lactate, the NADH is used to reduce the pyruvate.

• Step 1: • Step 4:

• Step 2:

• Step 3:

• Dihydroxyacetone phosphate ÍÎ Glyceraldehyde-3-phosphate (G3P) (Interconversion: Triose phosphate isomerase). • G3P is pulled off for Stage-2 of glycolysis: In effect a molecule of fructose-1,6-bisphosphate is converted into two molecules of G3P.

• Phosphofructokinase is the key control point. • Inhibited by high ATP concentrations, no need to oxidize glucose. • Glucose is instead stored as Glycogen.

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Glycolysis: Energy Payoff Phase

• Formation of ATP is called Substrate Level Phosphorylation.

Cori Cycle

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Pyruvate: A Key Juncture in Catabolism (Fermentation)

In extremely active skeletal muscle, pyruvate is reduced to lactate by NADH (produced earlier in glycolysis).

Pentose Phosphate Pathway • Pentose Phosphate Pathway: oxidizes glucose to produce pentoses (for DNA, RNA) and NADPH (for reductive biosynthesis).

• This pathway includes enzymes that allow the interconversion of pentoses and hexoses. • Allows four alternatives: • Produce both pentoses and NADPH. • Produce only NADPH when pentoses are not required. • Produce only pentoses when NADPH is not required. • Produce ATP (glycolysis) and NADPH when both are required. • Lactic acid produced in muscle cells diffuses into the blood and is converted back into glucose in the liver by a process called Gluconeogenesis.

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Conversion of Pyruvate to Acetyl-CoA

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Conversion of Pyruvate to Acetyl-CoA • The five steps in the conversion of pyruvate to acetyl-S-CoA are: Decarboxylation (CO2)

• This reaction is carried out by a multienzyme complex consisting of three different enzymes and five cofactors (4 are vitamins).

Pyruvate dehydrogenase kinase (PDK): Inactivates Enz1 by phosphorylating it. Pyruvate dehydrogenase phosphatase (PDP): Activates Enz1 by removing the phosphate. Phosphorylation is a common control mechanism.

Coenzymes

The Citric Acid (TCA) Cycle

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Cycle Intermediates

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Energy Transfers (Redox) During Electron Transport & ATP Synthase

• Several of the citric acid cycle intermediates are used in the biosynthesis of other biomolecules. • α-Ketoglutarate, succinate, and oxaloacetate are all precursors for amino acids. • The cell can replenish its supply of lost citric acid cycle intermediates by the synthesis of oxaloacetate from pyruvate and CO2: Pyruvate + CO2 + ATP + H2O Î Oxaloacetate + 2 H+

Chemiosmosis: Coupling the Electron Transport Chain to ATP Synthesis H+

Intermembrane space

.

H

H+

H+

Protein complex

H+

+

H+ H+ H

H+

Electron carrier

+

• The electron-transport chain is a series of electron carriers along which the electrons from NADH and FADH2 are passed, eventually reaching O2 with the formation of water.

FADH2

Mitochondrial matrix

FAD

1 NAD+

NADH

• Oxidative Phosphorylation: Most ATP is generated in the electronTransport Chain (ETC) by the oxidation of NADH and FADH2 using oxygen, O2. (Oxygen is the final electron acceptor)

ATP synthase

Inner mitochondrial membrane Electron flow

Electron-Transport Chain

H+

2

• During electron transport down this chain, protons are pumped across the inner mitochondrial membrane forming a pH gradient.

O 2 + 2 H+

H+ H+

H2 O

Electron Transport Chain

ADP +

ATP

P H+ Chemiosmosis

• As these protons re-enter the mitochondria, ATP is synthesized.

OXIDATIVE PHOSPHORYLATION

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Electron-Transport Chain Electron-Transport Chain • UQ or Ubiquinone (Complex II) is a hydrophobic small molecule that carries electrons within the inner mitochondrial membrane to Complex III.

• The electron-transport chain is organized into four distinct complexes:

• UQ contains a long hydrocarbon chain (“R” in the diagram) which firmly anchors it in the hydrophobic interior of the inner mitochondrial membrane.

• Complex I accepts electrons from NADH.

• Complex II accepts electrons from FADH2.

• As electrons pass through Complexes I, III, and IV, protons are pumped out of the mitochondria.

Electron-Transport Chain

• Carbon monoxide or Cyanide poisoning shuts down the electron-transport system by combining with Complex IV, preventing the transfer of electrons to oxygen. • DNP (Dinitrophenol)

• Reducing power is collected from many sources during catabolism in the form of NADH or FADH2.

Rotenone

H+

H+

• NADH introduces its electrons into the electron transport chain at Complex I (3 ATP).

Oligomycin

Cyanide, carbon monoxide

H+

H+ H+ H+

H+

H+

H+

ATP Synthase

DNP

• FADH2 introduces its electrons at ubiquinone, bypassing the proton pumping p p g mechanism of Complex p I.

FADH2

FAD 1

NAD+

NADH

• Because of this, reoxidation of FADH2 generates less ATP than reoxidation of NADH (2 ATP).

2

O2

+ 2 H+

H+ H+

H2O

ADP +

P

ATP

H+

Electron Transport Chain

Chemiosmosis

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Chem-131 Lec-18-23 10-1 Carohydrate & Metabolism

Review: Yield of ATP During Cellular Respiration

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Catabolism of Various Food Molecules (The reverse occurs when excess energy is available to the body) Triglycerides Starch & Glycogen

Hydrolyzed to

Hydrolyzed to

Glycerol & Fatty Acids

Glucose Fatty Acids Hydrolyzed to Acetyl CoA

Proteins Hydrolyzed to Amino Acids (No Storage)

Carbohydrates: Quick Energy Fats: Energy Storage

NADH Transport into Mitochondria • Outer mitochondrial membrane is permeable to almost all low molecular weight solutes.

NADH Transport into Mitochondria • Malate-aspartate shuttle (Heart, Liver, & Kidney):

• Inner mitochondrial membrane is impermeable to almost all solutes unless a specific transport system for the solute is embedded in the membrane. • e.g. the inner membrane is impermeable to H+, OH-, K+, and Cl• Cytoplasmic NADH (Glycolysis) must enter the mitochondria in order to be oxidized by molecular oxygen, however, the inner mitochondrial membrane is impermeable to NADH. • Two shuttle systems accomplish this task: • Malate-asparate shuttle: Heart, liver, and kidney. ~3 ATP • Glycerol-phosphate shuttle: Brain, muscle. ~ 2 ATP • NADH (cytosol) Æ NADH (mitochondria), 3 ATP

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NADH Transport into Mitochondria • Glycerol-phosphate shuttle (Brain, muscle): FAD is reduced to FADH2 in this reaction • Th The FADH2 electrons l t are passed d directly di tl to t UQ, UQ bypassing b i the th proton t pump Complex I. ~ 2 ATP • NADH (cytosol) Æ FADH2 (mitochondria), 2 ATP

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Glycogenesis & Glycogenolysis • High Concentration of ATP: Glycogen synthesized, stored in liver and muscle cells. • Glycogen Synthase is activated by the presence of insulin in the g blood sugar). g ) blood ((high • When a growing α1→4 glycogen strand reaches a certain length (≈ 11), enzymes break the α1→4 chain and remakes an α1→6 linkage. • Results in frequent branching, which can each be extended by further α1→4 linkages.

• The total energy yield from the oxidation of glucose to carbon dioxide and water is: • 36 ATP in cells using the glycerol-phosphate shuttle. • 38 ATP in cells using the malate-aspartate shuttle. • This amount of energy represents about 40% of the total energy released by the oxidative process. The rest is liberated as heat.

• Low blood glucose concentration (< 5 mM), glycogen in the liver is degraded and released into the bloodstream. • Glucose, as glucose-1-phosphate, is released from glycogen. • Glycogen Phosphorylase activated by Glucagon (low blood sugar) and Epinephrine (stress) in the bloodstream.

Glycogenolysis • Glycogenolysis is similar to a hydrolysis reaction except a molecule of phosphoric acid takes the place of water:

Glycogenolysis • Epinephrine on the outer cell membrane results in the release of, cAMP, within the cell. • cAMP activates a cascade of phosphorylation reactions which ends with the activation of glycogen phosphorylase.

• Debranching enzymes are required when α1→6 branches are encountered.

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Glycogenolysis • The activation of glycogen phosphorylase is an example of a cascade amplification of enzyme activity:

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Conversion of Pyruvate to Acetyl-CoA •

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Conversion of Pyruvate to Acetyl-CoA

The five cofactors are: 1. Thiamine (vitamin B1) in thiamine pyrophosphate (TPP). The combination of this cofactor with its apoenzyme is abbreviated as Enz1-TPP. 2. Lipoic acid, a growth factor that can be synthesized by vertebrates. The combination of lipoic p acid with its apoenzyme p y is abbreviated as shown below.

3. Pantothenic acid in coenzyme A. 4. Riboflavin in flavin adenine dinucleotide (FAD). The combination of FAD with its apoenzyme is abbreviated in this case as Enz3 Enz3-FAD. FAD 5. Nicotinic acid in nicotinamide adenine dinucleotide (NAD).

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