CHAPTER 6

CHAPTER 6: CARBOHYDRATE METABOLISM

STRUCTURE AND FUNCTIONS OF CHO

Carbohydrates

- The major energy sources, major components of cell walls, structural components of many organisms (the most abundant biomolecules)

-Compounds containing C, H and O

-General formula: C_nH_2nO_n 
-All have carbonyl group (C=O) and hydroxyl group (-OH). 3 types:

• Monosaccharides: the simplest carbohydrate

• Oligosaccharides: a few monosaccharides are linked together

• Polysaccharides: many monosaccharides are bounded together

Monosaccharides

-Monosaccharide: the simplest carbohydrate (cannot be hydrolyzed to a simpler carbohydrate)

•Monosaccharides are the building blocks of all carbohydrates.

•General formula C_nH_2nO_n, where n varies from 3 to 8

•Monosaccharides are classified by their number of carbon atoms.

Aldose vs Ketose

Aldehyde group: a carbonyl bonded to hydrogen and an R group at the end of carbon skeleton 

Ketone group: a carbonyl bonded between two carbon

Enantiomers

Enantiomers: the mirror image stereoisomers. Known as D and L.

•D-monosaccharide: a monosaccharide that has the -OH on its penultimate carbon on the right

•L-monosaccharide: a monosaccharide that has the -OH on its penultimate carbon on the left 

Epimer

•Epimers have identical molecular structure but differ in stereochemical configuration

Monosaccharides: Cyclization of sugars

•Cyclization of sugars occurs due to interaction between functional groups on distant carbons.

•In both cases,the carbonyl carbon is new chiral center and become an anomeric carbon or anomer

Monosaccharides: Pyranose vs furanose

     

Glycosidic Bond Formation

•Glycosidic bond: a sugar hydroxyl group (ROH) bonded to the anomeric carbon reacted with another hydroxyl (R’-OH)

•Glycosidic linkages are responsible for the bonding of monosaccharides to form oligosaccharides and polysaccharides.

Oligosaccharides

•Oligomers of sugars frequently occur as disaccharides, formed by linking two monosaccharide units by glycosidic bonds.

•Three most important oligosaccharides are disaccharides: sucrose, lactose, maltose

Disaccharides: Sucrose

Disaccharides: Lactose

• Made up of D-galactose and one unit of D-glucose joined by a -1,4-glycosidic bond

•Lactose is a reducing sugar (free anomeric carbon)

Disaccharides: Maltose, Cellobiose, Isomaltose

•         Maltose:

•         Two units of D-glucose joined by an -1,4-glycosidic bond

•         Formed from the hydrolysis of starch • Cellobiose:

•         Differs from cellobiose by the conformation of the glycosidic linkage (β-1,4-glycosidic bond)

•         Isomaltose:

•         Linked glucose by  -1,6-glycosidic bond)

•         Maltose, cellobiose and isomaltose are reducing sugars

Polysaccharides

•            Polysaccharides are formed by linking monomeric sugars through glycosidic linkages

•            Starch and glycogen are energy-storage polymers or sugars

•            Cellulose and chitin are structural polymers

•            Polysaccharides are important components of cell walls in bacteria and plants

Polysaccharides: Cellulose

Cellulose: the major structural component of plants, especially wood and plant fibers

•             a linear polymers of -D-glucose units joined by -1,4glycosidic bonds

•             fully extended conformation with alternating 180° flips of glucose units

•             extensive intra- and intermolecular hydrogen bonding between chains

Polysaccharides: Starch

•Starch is used for energy storage in plants

•a polymers of -D-glucose units

Amylose: continuous, unbranched chains of -D-glucose units joined by -1,4-glycosidic bonds

• amylopectin: a highly branched polymer consisting -Dglucose units joined by -1,4-glycosidic bonds and branches created by -1,6-glycosidic bonds

•amylase enzymes catalyze hydrolysis of -1,4-glycosidic bonds

•debranching enzymes catalyze the hydrolysis of -1,6glycosidic bonds

Amylose and Amylopectin

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Polysaccharides: Glycogen

•             Glycogen is used for energy storage in animals

•             A branched-chain polymer of -D-glucose units, similar to amylopectin fraction of starch.

•             Like amylopectin, glycogen consists of -D-glucose units joined by -1,4-glycosidic bonds and branches created by -1,6-glycosidic bonds

•             However, glycogen is more highly branches than amylopectin.

Polysaccharides: Chitin

Chitin: the major structural component of the exoskeletons of invertebrates, such as insects and crustaceans; also occurs in cell walls of algae, fungi, and yeasts

- composed of units of N-acetyl--D-glucosamine joined by -1,4-glycosidic bonds

GLYCOLYSIS

•Glycolysis is activated when energy is required.

•             Glycolysis is the first stage of glucose metabolism

•             One molecule of glucose is converted to 2 molecules of pyruvate

•             It plays a key role in the way organisms extract energy from nutrients

•             Once pyruvate is formed, it has one of several fates

PHASE 1: Preparation Phase (Conversion of Glucose to

Glyceraldehyde-3-Phosphate)

1.         Phosphorylation of glucose to give glucose-6-phosphate

2.         Isomerization of glucose-6-phosphate to give fructose-6phosphate

3.         Phosphorylation of fructose-6-phosphate to yield fructose1,6-bisphosphate

4.         Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3-phosphate and dihyroxyacetone phosphate

5.         Isomerization of dihyroxyacetone phosphate to give glyceraldehyde-3-phosphate

PHASE 2: Payoff Phase (Conversion of Glyceraldehyde-3-

Phosphate to pyruvate)

1.       Oxidation of glyceraldehyde-3-phosphate to give 1,3bisphosphoglycerate

2.       Transfer of a phosphate group from 1,3bisphosphoglycerate to ADP to give 3-phosphoglycerate

3.       Isomerization of 3-phosphoglycerate to give 2phosphoglycerate

4.       Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate

5.       Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate

Final products of glycolysis

•            In the final stages of glycolysis:

•            2 pyruvate are produced for each molecule of glucose

•            In Phase 1 (preparation phase): 2 molecules of ATP is required

•            In Phase 2 (payoff phase): 4 molecules of ATP and 2 molecules of NADH are produced for each molecule of glucose that entered the pathway.

•            Therefore, the net production: 2 ATP and 2 NADH for each glucose

•            Pyruvates produced in glycolysis has one of several fates.

Fates of Pyruvate

Under aerobic condition (O₂ present): pyruvates are converted to acetyl-CoA (aerobic metabolism)

Under anaerobic condition (No O₂ present): pyruvates are converted to lactate or alcohol (anaerobic metabolism)

ANAEROBIC METABOLISM OF GLUCOSE

Anaerobic Metabolism of Pyruvate: lactate formation

• Under anaerobic conditions, the most important pathway for the regeneration of NAD⁺ is reduction of pyruvate to lactate predominantly in skeletal muscle

Anaerobic Metabolism of Pyruvate:

Alcoholic Fermentation

•            Under anaerobic metabolism, 2 reactions lead to the production of ethanol from pyruvate:

•            Decarboxylation of pyruvate to acetaldehyde

•            Reduction of acetaldehyde to ethanol

Net production in anaerobic metabolism

•            In glycolysis, 2 NADH are produced in the payoff phase. However, both anaerobic metabolisms require 2 NADH to produce 2 lactate or 2 ethanol.

-Therefore, the net production: only 2 ATP for each molecule of glucose that entered the pathway.

AEROBIC METABOLISM OF GLUCOSE

Aerobic Metabolism of Pyruvate

• 
Under aerobic respiration, pyruvate will be converted to acetyl-CoA which will enter the Citric Acid Cycle to produce ATP  and more NADH and FADH₂.

•             NADH and FADH₂ will be used as the reducing agents to produced more ATP in Electron transport (oxidative phosphorylation).

Preparatory Reaction

(Pyruvate conversion reaction)

•             Acetyl-CoA (2-carbon unit) is needed to start of the citric acid cycle. Therefore, pyruvate (3-carbon unit) is converted to acetyl-CoA in the preparatory reaction.

•             Pyruvate, generated from glycolysis moves from cytosol into the mitochondrion via a specific transporter.

•             There, pyruvate is converted to acetyl-CoA and CO₂.

•             The overall reaction is the conversion of pyruvate, NAD⁺, and CoA-SH to acetyl-CoA, NADH + H⁺, and CO₂

•             Therefore, in the preparatory reaction, a total of 1 NADH, 1 CO₂ and 1 acetyl-CoA is produced from one molecule of pyruvate, or 2 NADH, 2 CO₂ and 2 acetyl-CoA from one molecule of glucose.

•             The acetyl-CoA is needed at the start of the citric acid cycle.

The Citric Acid Cycle (Kreb Cycle)

-The citric acid cycle is amphibolic (plays a role in both catabolism and anabolism). It is the central metabolic pathway

•In eukaryotes, the cycle takes place in the mitochondrial matrix

The Reactions of the Citric Acid Cycle

•             Step 1: condensation of acetyl-CoA with oxaloacetate to form citrate

•             Step 2: citrate is isomerized to isocitrate.

•             Step 3: oxidation of isocitrate followed by decarboxylation to form -ketoglutarate and CO₂

•             Step 4: oxidative decarboxylation of -ketoglutarate to form succinyl-CoA

•             Step 5: hydrolysis of succinyl-CoA to form succinate

•             Step 6: oxidation of succinate to fumarate

•             Step 7: hydration of fumarate to L-malate

•             Step 8: oxidation of malate to form Oxaloacetate

Products of citric acid cycle

•            In the citric acid cycle:

•            One molecule of acetyl-coA (2C) is oxidized to 2 molecules of CO₂ as a result of oxidative decarboxylation

•            The oxidations are accompanied by reductions involving NAD⁺ to NADH, FAD to FADH₂

•            GDP is phosphorylated to GTP

•            Therefore, in the citric acid cycle, a total of 3 NADH, 1 FADH₂ and 1 GTP (~ATP) is produced from one molecule of acetyl-CoA, or 6 NADH, 2 FADH₂ and

2 GTP (ATP) from one molecule of glucose.

Electron Transport Chain in Aerobic Metabolism

•             The NADH and FADH2 generated from glycolysis, preparatory reaction and citric acid cycle transfer electrons to oxygen in the series of reactions known as the electron transport chain.

•             As the result, ATP is produced as the result of oxidative phosphorylation (requires oxygen), in which ADP is phosphorylated to ATP.

•             Electron transport chain occurs in the inner mitochondrial membrane.

Electron Transport Chain (ETC)

•             Electron transport is carried out by 4 complexes (Complex I, II, III and IV) and 2 electron carriers (coenzyme Q and cytochrome c)

•             In a series of oxidation-reduction reactions, electrons from FADH₂ and NADH are transferred from one complex to the next until they reach O₂

•             O₂ is the final acceptor of electron

•             As a result of electron transport, protons (H+) are pumped across the inner membrane to the intermembrane space, creating a pH gradient

•             The proton gradient is coupled to the production of ATP in aerobic metabolism

Electron Transport Complexes

Electron Transport Chain

Establishment of the Proton Gradient

Chemiosmotic Coupling

ATP Synthase

•             The coupling of electron transport to oxidative phosphorylation requires a multisubunit membrane-bound enzyme, ATP synthase. This enzyme has a channel for protons to flow from the intermembrane space into the mitochondrial matrix.

•             The proton flow is coupled to ATP production in a process that appears to involve a conformational change of the enzyme.

P/O Ratio

•            A term called the P/O ratio is used to indicate the coupling of ATP production to electron transport.

•            The number of moles of P_i consumed in phosphorylation of ADP + P_i to ATP to the number of moles of oxygen atoms consumed in oxidation

•            Phosphorylation: ADP + P_i ----> ATP + H₂O

•            Oxidation: 1/2O₂ + 2H⁺ + 2e^- ---> H₂O

•            P/O = 2.5 ATP is produced when NADH is oxidized

•            P/O = 1.5 ATP is produced when FADH₂ is oxidized

Shuttle Mechanisms for oxidative phosphorylation of NADH from glycolysis

•             NADH is produced in glycolysis, which occurs in the cytosol, but NADH in the cytosol cannot cross the inner mitochondrial membrane to enter the electron transport chain.

•             However, the electrons can be transferred to a carrier that can cross the membrane. The number of ATP generated depends on the nature of the carrier.

•             2 shuttles:

•             Malate-aspartate shuttle

•             Glycerol-phosphate shuttle

The Glycerol phosphate shuttle

•          In the glycerol phosphate shuttle,  the transfer of electrons from NADH in the cytosol produces FADH₂ in the mitochondria

•          Therefore, 1.5 mitochondrial ATP are produced in the mitochondria for each cytosolic NADH

The Malate-Aspartate Shuttle

•             This shuttle is found in mammalian kidney, liver, and heart

•             The transfer of electrons from NADH in the cytosol (from glycolysis) produces NADH in the mitochondria

•             Therefore, 2.5 mitochondrial  ATP are produced for each cytosolic NADH

The ATP Yield from Complete Oxidation of

The ATP yield from complete oxidation of glucose

Pathway	ATP	NADH		FADH2
Glycolysis	2		2	-
Preparatory reaction (Pyruvate conversion reaction)	-		2	-
Citric Acid Cycle	2		6	2
ETC (glycerolphosphate shuttle)	8 x 2.5 =20 4 x 1.5 = 6		-	-
ETC (Malate Aspartate shuttle)	10 x 2.5 =25 2 x 1.5 = 3		-	-
TOTAL ATP	GPS=30 MAS=32

The ATP Yield from Complete Oxidation of Glucose

•            In the complete oxidation of glucose, a total of 30 or 32 molecules of ATP are produced for each

molecule of glucose, depending on the shuttle mechanism

•            In glycerol-phosphate shuttle: 30 ATP are produced

•            In malate-aspartate shuttle: 32 ATP are produced

GLYCOGENOLYSIS AND GLYCOGENESIS

Glycogen

•             Glucose polymerizes to form glycogen when the organism has no immediate need for the energy derived from glucose breakdown via glycogenesis, for example after having a carbohydrate-containing meal

•             Glycogen is the storage form of glucose in animals, including humans. Glycogen releases glucose when energy demands are high via glycogenolysis

•             Glucose derived from glycogen will be the primary source of blood glucose to be used for fuel up to 12 hours after meal.

Glycogenolysis

•            Glycogenolysis- glycogen breakdown.

•            STEP 1: Glycogen is cleaved by phosphate to glucose-1-phosphate

•            STEP 2: glucose-1-phosphate is isomerized to glucose-6phosphate

•            STEP 3: debranching enzymes to degrade the (1>6) linkages to complete breakdown

Glycogenesis

•             Glycogenesis- formation of glycogen.

•             Not exact reversal of glycogen breakdown to glucose

•             Glycogen synthesis requires energy

•             Energy supplied by hydrolysis of UTP

GLUCONEOGENESIS

Gluconeogenesis

•             Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids.

•             Gluconeogenesis enables the maintenance of blood glucose levels long after all dietary glucose has been absorbed & completely oxidized, eg in starvation, fasting, lowcarbohydrate diets, exercise, etc

Gluconeogenesis

•             The conversion of pyruvate to glucose occurs by a process called Gluconeogenesis (anabolism of glucose)

•             Gluconeogenesis is not the exact reversal of glycolysis.

•             Net result of gluconeogenesis is reversal of the following three steps in glycolysis, but by different reactions and using different enzymes:

-   Phosphoenolpyruvate to pyruvate + ATP

-   Fructose-6-phosphate to fructose-1,6-bisphosphate

-   Glucose to glucose-6-phosphate

The Cori Cycle link glycolysis and gluconeogenesis in anaerobic metabolism

•             The Cori cycle or lactic acid cycle: cycling of glucose from glycolysis in muscle and gluconeogensis in liver.

•             This cycle is important in producing ATP during vigorous (energetics) anaerobic activities (eg: sprinting).

•             In anaerobic activities, glycolysis in muscle tissue converts glucose to pyruvate; NAD⁺ is regenerated by reduction of pyruvate to lactate (lactic acid)

•             Lactate from muscle is transported to the liver where it is reoxidized to pyruvate and converted to glucose via gluconeogenesis.

•             The glucose produced in the liver is then transported back to muscle via blood where it becomes an energy store for the next burst of exercises/activities.

PENTOSE PHOSPHATE PATHWAY (PPP)

Pentose Phosphate Pathway (PPP)

•The Pentose Phosphate Pathway (PPP) is an alternative to glycolysis, and differs in several ways:

• ATP and NADH are not produced.

• Ribose-5-phosphate (five-carbon sugar) are produced for nucleic acid biosynthesis

• Oxidizing agent is NADP⁺; it is reduced to NADPH.

NADPH is a reducing agent in biosynthesis of lipids