Carbohydrate Metabolism Catabolism 2013

CARBOHYDRATE CARBOHYDRATE MEMETABOLISMTABOLISMCATABOLISMCATABOLISM

EDITED BYEDITED BYLiniyanti D.Oswari,MD.,MNS,MSc.Liniyanti D.Oswari,MD.,MNS,MSc.

For Block For Block 88Medical student, Sriwijaya UniversityMedical student, Sriwijaya University

Carbohydrate MetabolismCarbohydrate Metabolism

GlycolysisGlycolysis2.3. Biphospoglycerate (2.3.BPG)2.3. Biphospoglycerate (2.3.BPG)GlycogenesisGlycogenesisGlycogenolysisGlycogenolysisHMP shuntHMP shuntGluconeogenesisGluconeogenesisREGULATION OF METABOLISM BY HORMONESREGULATION OF METABOLISM BY HORMONES

Carbohydrate Metabolism Carbohydrate Metabolism OverviewOverview glycogen

pentose GLUCOSE other sugars pyruvate

acetyl CoA EtOHlactate

TCA cycle ATP

Glucose UtilizationGlucose Utilization

Glucose

PyruvateRibose-5-phosphate

GlycogenEnergy Stores

Pentose Phosphate Pathway

Glycolysis

Adipose

GLYCOLYSISGLYCOLYSISGlucose can also be available from food intake. Glucose can also be available from food intake. Glucose is also stored as glycogen Glucose is also stored as glycogen

(glycogenesis).(glycogenesis).After After gluconeogenesis,gluconeogenesis, glucose is converted glucose is converted

from glycogen in liver or muscle from glycogen in liver or muscle for glycolysisfor glycolysis. . Glycolysis is the break down of a Glycolysis is the break down of a 6 C glucose 6 C glucose

sugar to two 3C pyruvate.sugar to two 3C pyruvate.

Central role of liver in metabolismCentral role of liver in metabolism

Glucose entering the hepatocyte is phosphorylated by Glucose entering the hepatocyte is phosphorylated by glucokinase to glucose-6-phosphate (G-6-P).glucokinase to glucose-6-phosphate (G-6-P).

Other monosaccharides are also made to G-6-P via Other monosaccharides are also made to G-6-P via gluconeogenesisgluconeogenesis, then glucose can be stored as , then glucose can be stored as glycogen. glycogen.

When we need energy, When we need energy, glycolysis glycolysis converts G-6-P to converts G-6-P to pyruvate and acetyl coA to enter Citric acid cycle to pyruvate and acetyl coA to enter Citric acid cycle to produce ATP energy via oxidative phosporylation produce ATP energy via oxidative phosporylation (aerobic metabolism).(aerobic metabolism).

Glycolysis: Glycolysis: break down of glucose in cytoplasmbreak down of glucose in cytoplasm

Glucose-6-phosphate

Glucose-1-phosphate

UDP-glucose

Glycogen

GlucoseHexokinase

Fructose-6-phosphate

Fructose-1, 6-biphosphate

Glyceraldehyde-3-phosphate

Dihydroxyacetone phosphate (DHAP)

Glycerol

Glyceraldehyde-1, 3-bisphosphate

Glycerate-3-phosphate

Glycerate-2-phosphate

Phospho-enol-pyruvate

NAD + Pi

NADH + H+

ATP

ATP

ADP

ADP H2O

H2O

Pyr

uva

teL

acta

te

Lac

tate

D

ehyd

rog

enas

e

ATPADP

ATPADP

ATPADP

Glycolysis: Phase 1 and 2• Phase 1: Sugar activation

– Two ATP molecules activate glucose into fructose-1,6-diphosphate

• The 1 and 6 indicate which carbon atom to which they are attached.

• Phase 2: Sugar cleavage (splitting) – Fructose-1,6-bisphosphate (6 C’s) is split into

two 3-carbon compounds:• Glyceraldehyde 3-phosphate (GAP)

Glycolysis: Phase 3• Phase 3: Oxidation and ATP formation

– The 3-carbon sugars are oxidized (reducing NAD+); i.e., 2 H’s + NAD NADH2

– Inorganic phosphate groups (Pi) are attached to each oxidized fragment

– The terminal phosphates are cleaved and captured by ADP to form four ATP molecules

– The final products are: • Two pyruvic acid molecules• Two NADH + H+ molecules (reduced NAD+)• A net gain of two ATP molecules

Glycolysis has two stagesGlycolysis has two stages..A. An energy investment phase. Reactions, 1-5. Glucose to two glyceraldehyde -3-phosphate molecules. 2 ATPs are invested.B. An energy payoff phase. Reactions 6-10. two glyceraldehyde 3-phosphate molecules to two pyruvate plus four ATP molecules.-- A net of two ATP molecules overallplus 2 NADH(10 ATP–2 ATP= 8 ATP).

GLYCOLYSIS GLYCOLYSIS Glucose ATP hexokinase ADP Glucose 6-phosphate phosphogluco- isomerase Fructose 6-phosphate ATPphosphofructokinase ADP Fructose 1.6-bisphosphate aldolase

triose phosphate isomerase Dihydroxyacetone Glyceraldehyde phosphate 3-phosphate

Glyceraldehyde 3-phosphateglyceraldehyde NAD+ + Pi

3-phosphate NADH + H+

dehydrogenase 1,3-Bisphosphoglycerate  ADPphosphoglycerate kinase ATP 3-Phosphoglyceratephosphoglyceromutase 2-Phosphoglycerate enolase H2O Phosphoenolpyruvate ADP pyruvate kinase ATP Pyruvate

Three irreversible kinase reactionsprimarily drive glycolysis forward.  hexokinase or glucokinase phosphofructokinase pyruvate kinase

These enzymes will be shown to beregulate glycolysis as well.

Hexokinase Vs Glucokinase

Hexokinase Glucokinase

Site Most tissues Hepatocytes

Islet cells (pancreas)

Kinetics Low Km

Low Vmax

High Km

High Vmax

Regulation G-6-phosphate F-6-phosphate

Insulin: Induction

Function Low glucose conc. High glucose conc.

Glucose sensor

-- REGULATION OF GLYCOLYSISREGULATION OF GLYCOLYSIS

1.1.HEXOKINASE and HEXOKINASE and GLUCOKINASEGLUCOKINASE

HEXOKINASEHEXOKINASE Commiting step in glycolysis:

phosphorylation of glucose. Inhibited by its product, glucose6-phosphate,

as a response to slowing of glycolysis . found in all cells of every organism low

specificity for monosaccharides (simple sugars) i.e., other monosaccharides can be phosphorylated by hexokinase. relatively high affinity for glucose, KM = 0.1 mM

GLUCOKINASEGLUCOKINASE liver enzyme with high KM (10 mM)for glucose so most effective when glucose levels are very high not inhibited by glucose 6-phosphatesensitive to high glucose in circulation from recent meal so it decreases high level of glucose in blood by taking glucose into liver

2. PHOSPHOFRUCTOKINASE PHOSPHOFRUCTOKINASE rate limiting for glycolysis an allosteric multimeric regulatory enzyme. Measures adequacy of energy levels.

Inhibitors: ATP and citrate high energy Activators: ADP, AMP, and fructose 2,6 bisphosphate low energy 

ATP inhibits phosphofructoseactivity by decreasing fructose6-phosphate bindingAMP and ADP reverse ATP inhibition Fructose 2,6 bisphosphate is a very important regulator, controlling the relative flux of carbon through glycolysis versus gluconeogenesis.- It also couples these pathways to hormonal regulation.

3. PYRUVATE KINASEPYRUVATE KINASE PEP + ADP Pyruvate + ATP An allosteric tetramer -inhibitor: ATP & acetyl CoA & fatty acids (alternative fuels for TCA cycle)- activator: fructose 1,6-bisphosphate - (“feed-forward”) Phosphorylation (inactive form) anddephosphorylation (active form)under hormone control.Also highly regulated at the level of gene expression(“carbohydrate loading”)

Glycolysis:Embden-Myerhof

Pathway Oxidation of

glucose Products:

2 Pyruvate 2 ATP 2 NADH

Cytosolic

Aerobic Vs Anaerobic Glycolysis

Aerobic Glycolysis: Total Vs Net ATP Production

Summary of Energy RelationshipsSummary of Energy Relationships for Glycolysis aerobicfor Glycolysis aerobic  

Input = 2 ATP 1. glucose + ATP glucose-6-P 2. fructose-6-P + ATP fructose 1,6 bisphosphateOutput = 4 ATP + 2 NADH1. 2 glyceraldehyde-3-P + 2 Pi + 2 NAD+ 2 (1,3 bisphosphoglycerate) + 2 NADH2. 2 (1,3 bisphosphoglycerate) + 2 ADP 2 (3-P-glycerate) + 2 ATP3. 2 PEP + 2 ADP 2 pyruvate + 2 ATPNet = 2 ATP and 2 NADH( 8 ATP)

Energy Yield From GlycolysisEnergy Yield From Glycolysis

glucose 6 CO2 = -2840 kJ/mole

2 ATPs produced = 2 x 30.5 = 61 kJ/mole glucose Energy yield = 61/2840 = 2% recovered as ATP- subsequent oxidation of pyruvate and NADH can recover more of the free energy from glucose.

Carbohydrate Metabolism Primarily glucose

Fructose and galactose enter the pathways at various points

All cells can utilize glucose for energy production Glucose uptake from blood to cells usually mediated by insulin

and transporters

Liver is central site for carbohydrate metabolism Glucose uptake independent of insulin The only exporter of glucose

Blood Glucose Homeostasis Several cell types prefer glucose as energy

source (ex., CNS) 70-110 mg/dl is normal range of fasting blood glucose Uses of glucose:

Energy source for cells Muscle glycogen Fat synthesis if in excess of needs

Fates of Glucose

Fed state Storage as glycogen

Liver Skeletal muscle

Storage as lipids Adipose tissue

Fasted state Metabolized for energy New glucose synthesized

Synthesis and breakdown occur

at all times regardless of

state...

The relative rates of synthesis and

breakdown change

Synthesis and breakdown occur

at all times regardless of

state...

The relative rates of synthesis and

breakdown change

High Blood Glucose

Glucose absorbed

Insulin

Pancreas

Muscle

Adipose Cells

Glycogen

Glucose absorbed

Glucose absorbed

immediately after eating a meal…

Glucose Metabolism Four major metabolic pathways:

Energy status (ATP) of body regulates which pathway gets energy

Same in ruminants and non-ruminants

Immediate source of energy Pentophosphate pathway Glycogen synthesis in liver/muscle Precursor for triacylglycerol synthesis

Fate of Absorbed Glucose 1st Priority: glycogen storage

Stored in muscle and liver 2nd Priority: provide energy

Oxidized to ATP 3rd Priority: stored as fat

Only excess glucose Stored as triglycerides in adipose

Pyruvate Metabolism Three fates of pyruvate:

Conversion to lactate (anaerobic) Conversion to alanine (amino acid) Entry into the TCA cycle via pyruvate dehydrogenase pathway (create ATP)

Fate of Product of Glycolysis- Fate of Product of Glycolysis- PyruvatePyruvate- Pyruvate is at a central branch point in metabolism. Recall: Aerobic pathway - through citric acid cycle and respiration; this pathway yields far more energy and will be discussed later.

NADH + O2 NAD+ + energyPyruvate + O2 3CO2 + energy

Cori Cycle

Lactate is converted to pyruvate in the liver

Two Two anerobicanerobic pathways: pathways:

 - to lactate via lactate dehydrogenase - to ethanol via ethanol dehydrogenase

- Note: both use up NADH produced so only 2 ATP per glucose consumed

Pyruvate metabolism

Convert to alanine and export to blood

COO–

C O

CH3

COO–

HC NH3+

CH3Alanine amino transferase

(AAT)

AlaninePyruvate

Glutamate -Ketoglutarate

Keto acid Amino acid

Pyruvate Dehydrogenase Complex (PDH) Prepares pyruvate to enter the TCA cycle

Electron Transport Chain

TCA Cycle

Aerobic Conditions

1. Lactate FermentationLactate Fermentation Enzyme = Lactate Dehydrogenase COO- COO-

C=O + NADH + H+ H-C-OH + NAD+

CH3 CH3

pyruvate lactate

-Note: uses up all the NADH(reducing equivalents) produced in glycolysis.

Helps drive glycolysis by using up NADH reversible so pyruvate can beregenerated in alternative metabolism lactate fermentation important in red blood cells, parts of the retina, and in skeletal muscle cells during strenuous exercise.-Also important in plants and in microbes growing in absence of O2.

-- Lactate Dehydrogenase (LDH) hasmultiple forms. It is an isozyme.Two polypeptides M and H cometogether to form LDH. It is a tetramerso a mixture is formed:M4, M3H, M2H2, MH3 and H4

M M M H H H H H H H M M M M M M M H H H

Skeletal muscle and liver containpredominantly the “MM” forms;heart the “HH” forms. During andafter myocardialinfarction (heartattack), heartcells die releasingLDH into thecirculation.

Diagnostic.

LACTIC ACID (CORI) CYCLELACTIC ACID (CORI) CYCLE  glucose glucose glucose glucose-6-P glucose-6-P glycogen glycogen  ATP ATP  NADH BloodBlood NADH pyruvate pyruvate  lactate lactate lactate LiverLiver MuscleMuscle

The liver uses most of this lactate tomake glycogen. Only small amountsof free glucose released.

Glycogen can be broken down intoglucose when needed.

2.2.Alcoholic FermentationAlcoholic Fermentation

COO- CO2 CH2OH H O

C=O C + NADH CH3 +CH3 CH3 NAD+

 pyruvate acetaldehyde ethanol pyruvate decarboxylase- irreversible alcohol dehydrogenase- reversibleNote: NADH used up

- pathway is active in yeast.- second step helps drive glycolysis-second step is reversible- reverse is ethanol oxidation, eventially yields acetate, which ultimately goes into fat synthesis.- ethanol acetaldehyde acetate - humans have alcohol dehydrogenase in liver which mainly disposes of ethanol.- acetaldehyde is reactive and toxic.

SummarySummary GlucoseGlucoseoof Reactionsf Reactions 2 ATP 2 NADH 2 pyruvate2 NADH 2 NADHaanaerobic naerobic anaerobicanaerobic 2 ethanol + CO2 2 lactate

2 acetyl CoA + 2 CO2

  O2 aerobicaerobic 4 CO2 + 4 H2O

Siklus 2,3 Biphosphoglicerat

2.3 Biphosphoglycerate(BPG)

Human Hb and binding site for 2,3 BPG

The rate of Glycolysis will influent the affinity oxygen and Hemoglobine,with the intermediate 2,3 BPG pathway

Disorder in glycolysis will influent the affinity hemoglobine and oxygen.

Defficiency Piruvat kinase, so the level of 2.3 BPG will increase.

The affinity of oxygen and hemoglobine loose, and hypoxia in the tissue

Anemia hemolytic.

Deficiency Hexokinase - Genetic disease

- 2.3 BPG in RBC low - Affinity Hb and Oxygen is very strong

(abnormal) - Hypoxia in the tissue

Defficiency Piruvate kinase(Anemia Hemolitik)

- Blockade The end of glycolytic pathway, The affinity of oxygen and Hb decrease. turun.

- The production of ATP is not enough, so it decrease the activity of Na+ & K+, stimulate ion ATP ase pump.

It will keep the membran cell of RBC. Defficiency Piruvate Kinase will make RBC

Lysis.

The important pathways of glucose metabolism. Note that the glycogen degradations pathways end in -lysis, while the glycogen synthesis pathways end with -genesis.

Glycogenesis

Glycogen synthesis Occurs in cytosol of liver,muscle& kidney Occurs when blood glucose levels are high Excess glucose is stored (limited capacity)

liver and muscle are major glycogen storage sites liver glycogen used to regulate blood glucose levels brain cells cannot live for > 5 minutes without glucose muscle glycogen used to fuel an active muscle

Glycogen Synthesis Glucose units are activated for transfer by formation

of sugar nucleotides What are other examples of "activation"?

acetyl-CoA, biotin, THF, Leloir showed in the 1950s that glycogen synthesis

depends on sugar nucleotides UDP-glucose pyrophosphorylase - Fig. 23.18

a phosphoanhydride exchange driven by pyrophosphate hydrolysis

Glycogen Synthase Forms alfa-(1 4) glycosidic bonds in glycogen

Glycogenin (a protein!) forms the core of a glycogen particle

First glucose is linked to a tyrosine -OH Glycogen synthase transfers glucosyl units from

UDP-glucose to C-4 hydroxyl at a nonreducing end of a glycogen strand.

Note another oxonium ion intermediate

Control of Glycogen Metabolism

A highly regulated process, involving reciprocal control of glycogen phosphorylase and glycogen

synthase GP allosterically activated by AMP and inhibited

by ATP, glucose-6-P and caffeine GS is stimulated by glucose-6-P Both enzymes are regulated by covalent

modification - phosphorylation

Phosphorylation of GP and GS Covalent control

Edwin Krebs and Edmond Fisher showed in 1956 that a "converting enzyme" converted phosphorylase b to phosphorylase a(P)

Phosphorylation causes the amino terminus of the protein (res 10-22) to swing through 120 degrees, moving into the subunit interface and moving Ser-14 by more than 3.6 nm

Nine Ser residues on GS are phosphorylated!

Enzyme Cascades and GP/GS Hormonal regulation

Hormones (glucagon, epinephrine) activate adenylyl cyclase

cAMP activates kinases and phosphatases that control the phosphorylation of GP and GS

GTP-binding proteins (G proteins) mediate the communication between hormone receptor and adenylyl cyclase

Hormonal Regulation of Glycogen Synthesis and Degradation

Insulin is secreted from the pancreas (to liver) in response to an increase in blood glucose

Note that the portal vein is the only vein in the body that feeds an organ!

Insulin stimulates glycogen synthesis and inhibits glycogen breakdown

Note other effects of insulin

Hormonal Regulation II Glucagon and epinephrine

Glucagon and epinephrine stimulate glycogen breakdown - opposite effect of insulin!

Glucagon (29 res) is also secreted by pancreas Glucagon acts in liver and adipose tissue only! Epinephrine (adrenaline) is released from adrenal

glands Epinephrine acts on liver and muscles The phosphorylase cascade amplifies the signal!

O

O

OO

-[1- 4] linkages

O

O

O

O

O

O

O -[1-6] linkage

O ........

CH2OH CH2OH

CH2OH CH2OH CH2CH2OH

. The glycogen structure showing the glycosidic bonds

O

Liver 7–10% of wet weight Use glycogen to export glucose to the bloodstream when

blood sugar is low Glycogen stores are depleted after proximately 24 hrs of

fasting (in humans) De novo synthesis of glucose for glycogen

Skeletal muscle 1% of wet weight

More muscle than liver, therefore more glycogen in muscle, overall Use glycogen (i.e., glucose) for energy only (no export of

glucose to blood) Use already-made glucose for synthesis of glycogen

Glycogenesis

Pathway of glycogen synthesis (glycogenesis).

Glucose

Glucose-6-phosphate

Hexokinase(muscle)Glucokinase(liver)

ADP

UTP PPi

UDP-glucose

Glucose-1-PUridyltransferase

Glucose-1-phosphate

Phospho-glucomutase

ATP

UDP

(Glucose)n

(Glucose)n+

1

Glycogen Synthase

Control of enzyme activity

Rate limiting step

Glycogen synthesisGlucose 6-P→ glucose 1-P.glucose 1-P + UTP→UDP-glucose + PPi.PPi + H2O→ 2 Pi.UDP-glucose + glycogenn → glycogenn+1.

UDP + ATP → UTP + ADP.

Glucose 6-P + ATP + glycogenn + H2O →glycogenn+1 + ADP + 2Pi.

(nucleoside diphosphokinase)

Only one ATP is used to store one glucose residue in glycogen.

Glycogen synthesis and breakdown are reciprocally regulated

Red=inactive forms, green = active forms.

Protein phosphatase 1 (PP1) regulates glycogen metabolism.

InactiveActive

Glycogenolysis Glycogen degradation Occurs in cytosol Signal that glucose is needed is given by

hormones epinephrine stimulates glycogen breakdown in

muscle glucagon which stimulates glycogen breakdown

in liver in response to low BG used to sustain blood glucose level between meals

and to provide energy during an emergency/exercise

Glycogen Catabolism Getting glucose from storage (or diet)

-Amylase is an endoglycosidase It cleaves amylopectin or glycogen to maltose,

maltotriose and other small oligosaccharides It is active on either side of a branch point, but

activity is reduced near the branch points Debranching enzyme cleaves "limit dextrins" Note the 2 activities of the debranching enzyme

Glycogen

X glycolysis

LIVER PATHWAY

Glycogenolysis and the fate of glycogen in liver and kidney

Pi

glycogenphosphorylase

Glucose-1-phosphate

phosphoglucomutase

Glucose-6-phosphate

(inhibited by lack of fructose-2,6-bisP

glucose-6-phosphataseGlucose

Pi

. Glycogenolysis and the fate of glycogen in muscle.

lactate dehydrogenaseLactate

anaerobic

pyruvatedehydrogenase

Acetyl CoA

MUSCLE PATHWAYGlycogen

Pi

glycogenphosphorylase

phosphoglucomutase

Glucose-1-phosphate

Glucose-6-phosphate

glycolysis

Pyruvate

CO2

citric acid cycle

aerobic

Glikogenesis & Glikogenolisis Glucose anabolism

Glucose storage: glycogenesis glycogen formation is

stimulated by insulin glucose not needed

immediately is stored in the liver (25%) and in skeletal muscle (75%)

Glucose release: glycogenolysis converts glycogen to

glucose occurs between meals,

stimulated by glucagon and epinephrine

SIMPLISTIC SUMMARY:SIMPLISTIC SUMMARY:-- Epinephrine and glucagon stimulate glycogenolysis & inhibit glycogenesis via a cAMP and a phosphorylation cascade. release glucose-- Glycogenesis is stimulated by insulin in a pathway ending in the dephosphorylation of glycogen synthase.-- Glycogenolysis is also inhibited via dephosphorylation. take up glucose

Glycogen Storage Diseases:Glycogen Storage Diseases:  A family of serious, although notnecessarily fatal, diseases caused bymutations in the enzymes involvingin glycogen storage and breakdown.

Glycogen Storage Diseases

Type I: Von Gierke Disease; Glucose-6-phosphatase Defect

Hypoglycemia occurs due to defect of the final step of gluconeogenesis. This disease, affects only liver and renal tubule cells Decreased mobilization of glycogen produces hepatomegaly. Decreased gluconeogenesis causes increased lactate leading to lactic acidemia.

Type V: McArdle Disease; Skeletal Muscle Glycogen Phosphorylase Defect

Skeletal muscle is affected, whereas the liver enzyme is normal. Temporary weakness and cramping of skeletal muscle occurs after exercise. There is no rise in blood lactate during strenuous exercise. Muscle contains a high level of glycogen with normal structure

Type VI: Hers Disease; Liver Glycogen Phosphorylase Defect

Liver is affected, whereas the skeletal muscle enzyme is normal. Marked hepatomegaly occurs due to a high level of glycogen with normal structure.. Following administration of glucagon, there is no increase in blood glucose.

Pentose Phosphate Pathway=Hexose Monophosphat Shunt

Generation of NADPH and Pentoses

Has 2 functions1.Generate reducing equivalents NADPH (reduced cosubstrate/ coenzyme) needed for fatty acid synthesis, folate reduction2. Produce ribose 5-phosphate needed for DNA and RNA synthesis

Occurs in cytosol of cells particularly important in anabolic tissues,liver, adrenal cortex, mammary glands and fat tissues

muscle cells do NOT have HMS enzymes

Pentose Phosphate Pathway

Glucose-6-phosphate

6-Phospho- glucono-lactone

6-Phospho- gluconate

D-Ribulose-5-phosphate

D-Ribose- 5-phosphate

RNA or DNA

A scenario in which the cell requires NADPH but does not require ribose-5-P

NADPH is used for biosynthetic reactions and glutathione metabolism

Glucose-6-P-dehydrogenase

Glucose Glucose 6-P

ATP ADP

6-Phosphogluconate

NADP NADPH

Ribulose 5-PCO2

NADPH

NADP6-Pgluconate dehydrogenase

Oxidative branch

Xylulose 5-P Ribose 5-P (5 carbons)

Sedoheptulose 7-P (7 carbons)

Erythrose 4-P

Transketolase

Transaldolase

Glyceraldehyde 3-P

Fructose 6-P

Fructose 6-P

TDP

TDP

Tra

nsk

eto

lase

Non

-oxi

dati

ve b

ranc

h

Glyceraldehyde 3-P

Glyceraldehyde-3-P and fructose-6-P return to the glycolytic pathway

Ribulose 5-P

Xylulose 5-P Ribose 5-P (5 carbons)

Sedoheptulose 7-P (7 carbons)

Erythrose 4-P

Transketolase

Transaldolase

Glyceraldehyde 3-P

Fructose 6-PFructose 6-P

Glyceraldehyde 3-P

TDP

TDPT

ran

sket

ola

se

A scenario in which the cell requires ribose-5-P but does not require NADPH

Ribose-5-P is the sugar required for the synthesis of nucleic acids

Oxidative branch is feedback inhibited by excess NADPH at glucose-6-P dehydrogenase

Nucleic acids

Glucose Glucose 6-P

Ribulose 5-P

6-Phosphogluconate

Ribose 5-P (5 carbons)

ATP ADP NADP NADPH

CO2

NADPH

NADP

Glucose-6-P-dehydrogenase

6-Pgluconate dehydrogenase

A scenario in which the cell requires both NADPH and ribose-5-P

Nucleic acids

Overview Function

NADPH production Reducing power

carrier Synthetic pathways

Role as cellular antioxidants

Ribose synthesis Nucleic acids and

nucleotides

Characteristics: Tissue Distribution Demand for NADPH

Biosynthetic pathways FA synthesis (liver, adipose, mammary) Cholesterol synthesis (liver) Steroid hormone synthesis (adrenal, ovaries, testes)

Detoxification (Cytochrome P-450 System) – liver Reduced glutathione as an antioxidant (RBC) Generation of superoxide (neutrophils)

Characteristics:Oxidative and Non-oxidative Phases

Oxidative phases Reactions producing

NADPH Irreversible

Non-oxidative phases Produces ribose-5-P Reversible reactions feed

to glycolysis

NADPH producing reactions Glucose-6-P dehydrogenase 6-P-gluconate dehydrogenase

The Pentose Phosphate Pathway:Non-oxidative phases

Regulation Glucose-6-P dehydrogenase

First step Rate limiting

Allosteric Regulation Feedback inhibited by NADPH

Inducible enzyme Induced by insulin

HMS ( Hexose Monophospat Shunt) Nicotinamide adenine dinucleotide phosphate

phosphorylated form of reduced nicotinamide adenine dinucleotide (NADH)

generated in a series of reactions comprising the oxidation-reduction phase of HMS

Ribose 5-phosphate sugar used as the backbone of DNA and RNA

Cell’s requirement for ATP (glycolysis) or NADPH and ribose 5-P (HMS) determines which path it will take.

Stages of HMS Reactions occur in 3 main stages

oxidation-reduction generation of NADPH

isomerization stage generation of ribose 5-phosphate

carbon bond cleavage-rearrangement stage conversion of three 5-carbon sugars to two 6-carbon sugars

(Fructose 6-phosphate) and one 3-carbon sugar (Glyceraldehyde 3-phosphate)

these series of reactions occur in cells where demand for NADPH is high F 6 P can be converted back to G 6 P which can re-enter HMS

Reactions of Stages 1 and 2

G6P is oxidized to 6-phosphoglucono-lactone by G6P dehydrogenase that uses NADP as coenzyme produces NADPH and 6-phosphoglucono

6-phosphoglucono is hydrolyzed (addition of water) to 6-phosphogluconate

6-phosphogluconate is oxidized by 6 phosphogluconate dehydrogenase produces NADPH and ribulose 5 phosphate

Ribulose 5-phosphate is isomerized to ribose 5 phosphate

Regulation of Metabolism Revisited

Allosteric Enzyme Modulation enzymes can be stimulated or inhibited by certain

compounds modulators act by altering conformational

structure of their allosteric enzymes causes shifts between relaxed and tight conformations

relaxed is most active

ratio of ADP (or AMP) to ATP is important in regulation of energy metabolism

Allosteric Enzyme Modulation low ADP:ATP ratio signals less need to

produce ATP inhibition of key enzymes in glycolysis and the

TCA cycle PFK, PDH, CS, and isocitrate dehydrogenase

high ADP:ATP ratio signals need for ATP activation of the above enzymes

ATP is end product in oxidative catabolism and its accumulation would signal to decrease catabolic pathway activity

Allosteric Enzyme Modulation

ratio of NADH to NAD+ is also important in regulation NADH is end product of catabolic pathway accumulation would signal to decrease activity diminution would signal to increase activity key enzymes are affected in glycolytic and TCA

cycle PK, PDH, CS, isocitrate dehydrogenase and alpha KG

dehydrogenase

Role of NADPH in the RBC Production of superoxide

Hb-Fe2+-O2 -> Hb-Fe3+ + O2-.

Spontaneous rxn, 1% per hour

O2-. + 2H2O -> 2H2O2

Both O2-. & H2O2 can produce reactive free

radical species, damage cell membranes, and cause hemolysis

Pentose Phosphate PathwayGlucose 6-phosphate dehydrogenase deficiency

Detoxification of Superoxide Anion and Hydrogen Peroxide Antioxidant enzymes

Superoxide dismutase Glutathione peroxidase Glutathione reductase