peter pazmany catholic university · pdf filesubdivision: trioses, tetroses, pentoses,...

. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 2011.09.13.. TÁMOP – 4.1.2-08/2/A/KMR-2009-0006 1

Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITYConsortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

PETER PAZMANY

CATHOLIC UNIVERSITYSEMMELWEIS

UNIVERSITY


BIOCHEMISTRY

CATABOLISM OF CARBOHYDRATES

www.se.hu

(BIOKÉMIA )

(A SZÉNHIDRÁTOK LEBONTÁSA )

TRETTER LÁSZLÓ

http://semmelweis-egyetem.hu/


Biochemistry: Catabolism of carbohydrateswww.se.huhttp://semmelweis-egyetem.hu/

CatabolismDEF: Part of intermediary metabolism dealing with the energy-yielding degradation of nutrient molecules

CarbohydratesDEF: Aldehyde or ketone derivatives of polyhydric alcohols

Antonym of catabolism: anabolism

Examples for catabolic processesDecomposition of glucose to pyruvate or lactate called: glycolysisDecomposition of fatty acids to acetyl-CoA called: beta oxidationDecomposition of glycogen to glucose: called glycogenolysisDecomposition of acetyl-CoA to carbon dioxide + water called: citric acid cycle



CLASSIFICATION of CARBOHYDRATES

MonosacharidesDEF: those carbohydrates that cannot be hydrolyzed into a simpler form

subdivision: trioses, tetroses, pentoses, hexoses, heptoses depending upon the number of carbon atom

subdivision: aldoses or ketoses depending upon the presence of of aldehyde or ketone group

DisaccharidesDEF: hydrolysis of disaccharides yields 2 molecules of monosaccharides

OligosaccharidesDEF: Hydrolysis yields 3-6 monosaccharide units

PolysaccharidesDEF: Hydrolysis yields more than 6 monosaccharide units



Examples for monosaccharides

Aldoses Ketoses

Trioses (C3H6O3) GlyceroseSynonym:Glycer-aldehyde

Dihydroxy-acetone

Tetroses (C4H8O4) Erythrose Erythrulose

Pentoses (C5H10O5) Ribose Ribulose

Hexoses (C6H12O6) GlucoseGalactoseMannose

Fructose



Table of contents:

Glycolysis, the anaerobic decomposition of glucose

Catabolism of non-glucose carbohydrates

Regulation of carbohydrate catabolism


Biochemistry: Catabolism of carbohydratesGlycolysis

www.se.huhttp://semmelweis-egyetem.hu/

Glycolysis -reactions

Learning objectives:

Carbohydrates are major energy giving substrates for a living organismGlycolysis an universal pathway to decompose glucose even in the absence of oxygen

At the end of the presentation students will be able:

1. To reproduce the most important steps of glycolysis2. To understand the formation of ATP in the absence of oxygen3. To demonstrate important principles of thermodinamics

using examples taken from glycolysis



GlycolysisDEF

-Anaerobic degradation of glucose to lactate or

-Anaerobic degradation of glucose to pyruvate – a preparatory pathwayfor the aerobic metabolism of glucose

-Can occur in every cell

-Energy yielding pathway (2 ATP/glucose)

In the absence of oxygen every cell would perform glycolysis and the end-product will be lactatethusglycolysis is the most universal metabolic pathway.





Overview of glycolysis GlucoseC6

Hexose phosphates(C6)

Triose phosphate Triose phosphate

Pyruvate(C3)

Lactate(C3)

Lactate

blood

NADH+H+ NAD+ CO2H2OATP

+ O2 Without O2orno mitochondria

ATPformation




Overview of glycolysis

In the absence of oxygen or mitochondria

2 NAD+

2 NADH+H+

In the presence of oxygen and mitochondria






Preparatory phase of glycolysis

2 ATP investedandHexose chain is converted into triose phosphates




ATP requiring reactions of glycolysis

HexokinaseGlucokinase

Phosphofructokinase-1

ΔG’o= -16.7 kJ/molirreversible

ΔG’o= -14.2 kJ/molIrreversibleRate limiting step of glycolysis

Important reactions of the preparatory phase




Important reactions of the preparatory phase

Hexokinase and glucokinase are isoenzymes

IsoenzymesDEF: Enzymes catalyzing the same reactionBut differ:In amino acid sequenceVmax, and/or KMin regulation

Hexokinases are localized in the peripheral tissuesGlucokinase is localized in the liver

Hexokinases show high affinity for glucoseGlucokinase show low affinity for glucoseTheir regulation is different




Important reactions of the payoff phase

ΔG’o= 6.3 kJ/molreversible

ΔG’o= -18.5 kJ/molreversible

ΔG’o= -31.4 kJ/molirreversible

Inorganic phosphate incorporationNADH formationHigh energy acyl-phosphate group formation on the 1st C atom

The acyl-phosphate group is transferred to ADPSubstrate level phosphorylation

From the high energy enol-phosphate bond the phosphoryl group is transferred to ADP

Substrate level phosphorylation




Energetic balance of glycolysis

Preparatory phase 2 ATP invested - 2

Payoff phase 2x2 ATP produced(1 hexose 2 triose) +4

Summary Net ATP production +2

Substrate level phosphorylationDEF: Formation of ATP by phosphoryl group transfer froma compound having high energy bound

Examples for substrate level phosphorylation: in glycolysis: phosphoglycerate kinase reaction

pyruvate kinase reactionin citric acid cycle: succinate thiokinase reaction

Antonym of substrate level phosphorylation: oxidative phosphorylation



www.se.huhttp://semmelweis-egyetem.hu/Glycolysis - Summary

Glycolysis – the most important decomposition pathway of the most important carbohydrate

- glycolysis produces energy even in the absence of oxygen

- every higher eukaryotic cells are able to perform glycolysis

- 2 mol of ATP produced from 1 mol of glucose

- in the presence of oxygen and mitochondria glycolysisis continued in the citric acid cycle

- glycolysis has reversible and irreversible steps

- the irreversible reactions of glycolysis are catalyzed byhexokinase (glucokinase in liver), phosphofructokinaseand pyruvate kinase



Biochemistry: Catabolism of carbohydratesCatabolism of non-glucose carbohydrates


Catabolism of non-glucose carbohydrates

Introduction

Carbohydrates are major energy giving substrates for living organism

Besides glucose other carbohydrates (e.g. fructose and galactose) are also taken up by the organism, which sugars can be catabolized or can participate in the synthesis of other molecules

Glycogen is a special storage form of glucose with a function in the maintenance of blood sugar level and in the energy supply of the muscle cells.



Learning objectives

At the end of the presentation students will be able:

To understand the pathways used by individual carbohydrates to join to the mainstream of the metabolism

To understand that consumption of different carbohydrates could change physiological pathways and could have pathological consequences as well.




Fructose metabolism

Availability of fructose: Natural sources: fruit juices, honey, disaccharide sucroseFood industry: High Fructose Corn Syrup

Importance: Mainly changed to glucose in the liver and used in the body

Pathological significance: hereditary fructose intolerance (fructose accumulationplus hypoglycemia), obesity




Entry of fructose into glycolysis1. In liver – major organ of fructose catabolism

Glycolysis, gluconeogenesis

Important: fructose catabolism in theliver bypasses phosphofructokinase-1!




Entry of fructose into glycolysis2. In skeletal muscle – less important in fructose catabolism

Fructose

Fructose 6-phosphate

ATP

ADPHexokinase

Glycolysis

Hexokinase - not entirely specific for glucose

- converts fructose to Fr 6-Pat low [Glucose]at high [Fructose]




Glycolysis, gluconeogenesis

Pathological aspects of fructose metabolism

X

1) Hereditary fructose intoleranceAldolase B deficiency[fr 1-P] increased

Symptom: hypoglycemiaWhy? See:regulation of carbohydrate breakdown

2) High fructose consumption- Susceptibility to obesity,hyperlipidemia

hyperlipidemiaDEF: increased concentrationof lipids in the blood




Galactose metabolism

Availability of galactose: from milk sugar: lactose. Intestinal hydrolysis of lactoseresults in formation of galactose+glucose

Galactose is metabolized mainly in the liver, can be converted to glucose

Importance: needed for synthesis of glycoproteins, glycolipids, lactose (in lactating women)

Pathological significance: galactosemia

Lactose galactose + glucose




Galactose metabolismGalactose

galactokinase

Galactose 1-phosphate

UDP-Glc-Gal 1-Puridyltransferse

Glucose 1-phosphate

phosphoglucomutase

Glucose 6-phosphate

ATP

ADP

Glucose 1-P

UDP-Glcpyrophosphorylase

UDP-glucose

UDP-galactose

UTP

PPi

UDP-gal epimerease




[Galactose]

galactokinase

[Galactose 1-phosphate]

UDP-Glc-Gal 1-Puridyltransferse

Glucose 1-phosphate

phosphoglucomutase

Glucose 6-phosphate

ATP

ADP

Glucose 1-P

UDP-Glcpyrophosphorylase

UDP-glucose

UDP-galactose

UTP

PPi

UDP-gal epimerease

Pathological aspects of galactose metabolism

XGalactosemia:lack of UDP-Glc-Gal 1-P uridyltransferse

Consequence: increased [Galactose] symptom: cataractDEF:opacity in the lens of the eye

increased [Galactose 1-phosphate] symptoms: liver failure, mental retardation



www.se.huhttp://semmelweis-egyetem.hu/Glycogen metabolism

Glycogen: the storage form of glucose in the bodyforms granules in the cytosolmany cells contain glycogenthe most important organs for storage: liver, skeletal muscle

function of liver glycogen: maintenance of blood sugar levelfunction of muscle glycogen: energetic support of contraction

Structure: highly branched structurechains: alpha [1-4] glucosidic linkagebranches: alpha [1-6] glucosidic linkage

protein glycogenin is localized in the core of glycogenglycogenin is required for the synthesis

molecular mass: in the order of millions



www.se.huhttp://semmelweis-egyetem.hu/The structure of glycogen

1

6

6

144 1 4 1

141

Glycogenin




GlycogenolysisDEF: Intracellular decomposition of glycogenResulting glucose in the liver and kidney cortexResulting glucose 6-P in the muscle

Synonym: glycogen breakdownAntonym: glycogenesis or glycogen synthesis

The purpose of glycogenolysis in liver (and to a smaller extent in kidney cortex): maintenance of blood sugar level. Blood sugar level should be kept constant, because there are cells and tissues which gain energy exclusively from glucose

The purpose of glycogenolysis in muscle cells: to support the energy requirement of contraction.




The fate of glycogen-derived glucoseafter breakdown

In liver: release to the bloodstream

In muscle: glycolysis then citric acid cycleIn muscle in shortage of oxygen:

glycolysis ending with lactateproduction

phosphoglucomutase

Lactatedehydrogenase

Pi

Glycogen

glycogen phosphorylase

Glucose 1-P

Glucose 6-P

PyruvatePDH complex

Acetyl-CoACitric acid cycle

CO2 + H2O

Glycolysisglucose 6-Pase

Glucose

Lactate



www.se.huhttp://semmelweis-egyetem.hu/Steps of glycogen breakdown 1

Glycogen

Glycogenphosphorylase

Debranching enzymealpha (1→4) → alpha (1→4)

transferase activity

Debranching enzymeAmylo (1→6)-glucosidase

activity

PPi

gl 1-PglH2O

Debranching enzyme has two catalytic activitiesProducts of catabolism: shorter glycogen

glucose 1-Pglucose (captured by hexo- or glucokinase)



www.se.huhttp://semmelweis-egyetem.hu/Steps of glycogen breakdown 2

phosphoglucomutase glucose 6-phosphatase

H2O Pi

fructose 6-phosphate

phosphohexoseisomerase

ER inliver




Pathological aspects of glycogen breakdown

Glycogen storage diseasesDEF:inherited disorders characterized byabnormal quantity or type of glycogen in tissues

Examples: Name Deficiency Consequences

Von Gierke’s disease Lack of glucose 6-phosphatase in liver and kidney

Hypoglycemia, hyperlipemia

Cori’s disease Lack of debranching enzyme

Accumulation of abnormally branched glycogen

Mc Ardle’s disesase Lack of glycogen phosphorylase in the skeletal muscle

Diminished tolerance to exercise




Summary:

Catabolism of non-glucose carbohydrates were discussed

Glycogen, fructose and galactose catabolism follows individual pathways

All of the individual decomposition pathways will join to glycolysis, so the complete breakdown of these saccharides will be similar to that of glucose.




Biochemistry: Catabolism of carbohydratesRegulation of carbohydrate catabolism

Learning objectivesAt the end of the presentation students will be able:

To understand the adaptation of carbohydrate catabolic pathways to the current requirement of the organism and the cell.

To understand the concept that metabolic pathway can be regulated by different ways: the most important ones are:

- Regulation by changing the gene expression- Regulation by reversible covalent modification (e.g.

phosphorylation/dephosphorylation of the key enzymes- Regulation by allosteric effectors

To understand that only a few of the enzymes are regulated in the metabolic pathways, usually the rate-limiting ones and those which catalyze irreversible reactions




Multilevel regulation of glycolytic enzymes

Gene expression Covalent modification Allosteric

Enzyme Inducer repressor phosphorylation

dephosphorylation activator inhibitor

Hexokinase Gluc 6-P

Glucokinase insulinGlucagon (cAMP)

Starvation

Fructose 1-P(through

glucokinase regulatory protein)

Fructose 6-Pthrough

glucokinase regulatory protein)

Phosphofructokinase-1 insulin starvation

Fructose 2,6-P2

AMPATP, citrate, fatty acids

Pyruvate kinase insulin

Glucagon (cAMP)

Starvation

cAMP,Ca-CaM

inactivation

insulinActivation

Fructose 1,6-P2 ATP, alanine




Regulation of glucokinase (liver)

Inative inthe nucleus

The regulation of glucokinase explains hypoglycemia detected in fructose intolerance. Accumulation of fructose 1-P suspends the regulatory protein-mediated inhibition of glucokinase, thus glycolysis will be accelerated

Fructose 6-P mediated inhibition of glucokinase represents a negative fee back mechanism

AccumulationIn fructose intolerance




Regulation of phosphofructokinase-1 in liverInsulin stimulates glycolysis

Insulin

+

Phosphorylated: inactiveDephosphorylated: active




Phosphorylated: inactive

Glucagon

+

XX

Regulation of phosphofructokinase-1 in liverGlucagon inhibits glycolysis




Structure of 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase enzyme

Tandem enzyme: one polypeptide chain – two catalytic activities

Heart specific enzyme: Phosphorylation (PKA) of the phosphatase domain inactivates phosphatase activityKinase activity will be dominantGlycolysis activated

Liver specific enzyme: Phosphorylation (PKA) near the kinase domain inactivates kinase activityPhosphates activity will be dominant. Glycolysis is inactivated




Allosteric regulation of glycolysis

Meaning of regulators

High [gluc 6-P] indicates that hexokinase activity is too high

High [AMP] – indicates low energy charge of the cell (local regulator)High [ATP] – indicates high energy charge of the cell

High [citrate] indicates the overflow of fatty acid synthesis precursors from the mitochondria to the cytosol

Fructose 2,6-P2 the most important regulator of the rate limiting step of glycolysis. The level of Fr 2,6-P2 reflects hormonal changes.

In liver insulin elevates [Fr 2,6-P2], glycolysis is stimulatedGlucagon decreases [Fr 2,6-P2], glycolysis is inhibitedAdrenalin decreases [Fr 2,6-P2], glycolysis is inhibitedBUT!In the heart adrenaline elevates [Fr 2,6-P2], glycolysis is stimulated




Regulation of glycogen breakdowncAMP level is elevated by some hormones

cAMP activated protein kinase A phosphorylates and activates phosphorylase kinase

Activated phosphorylase kinase phosphorylates and activates glycogen phosphorylase

Activated glycogen phosphorylase catalyzes glycogen breakdown

Those hormones which decrease cAMP level has opposite effect on glycogen breakdown

Calcium activates phosphorylase kinase.

Activated phosphorylase kinase phosphorylates and activates glycogen phosphorylase, which catalyzes glycogen breakdown

Glucose (in liver) inhibits glycogen phosphorylase, i.e. inhibits glycogenolysis

AMP in muscle stimulates glycogen phosphorylase, i.e. activates glycogenolysis




Summary:

Carbohydrates are important sources of energy for the organisms.

Glycolysis is a fundamental energy yielding metabolic pathway in every cell.

The rate of glycolysis is strictly regulated by various mechanisms

Changes of the gene expression of the most important enzymes are regulated by hormones and by the nutrition.

Reversible chemical modification of the enzymes (phosphorylation/dephosphorylation) usually reflects hormonal influence.

The level of allosteric modificators might reflect the actual changes in the local intracellular environment, but could be changed by hormonal effects as well (e.g. [fruc 2,6-P2] is dependent upon hormonal status).




Recommended literature

Orvosi Biokémia (Ed. Ádám Veronika)



Questions:

Describe the regulation of fructose catabolism, compare it with the regulation of glucose catabolism

Which are the irreversible steps of glycolysis?

Which enzyme reaction is the rate-limiting enzyme in the glycolysis?

How many ATP can be produced, if glycolysis starts from previously synthesized glycogen and ends up with lactate formation?

What is the consequence of having different PFK2 isoenzymes in the heart and liver considering the effect of adrenaline on glycolysis?



Questions:

Which of the following statements are true for the PFK1?

1. The reaction catalized by the enzyme is irreversible in vivo2. The activity of the enzyme can be inhibited by ATP3. Its function is influenced by the ATP/ADP ratio4. It is the fastest enzyme of the glycolysis5. It works even in the absence of ATP

A:2,3,5 B:1,2,3 C:1,2,3,4 D:2,3,4,5 E:1,3,4,5

Which of the following statements are true for the fructose metabolism?

1. Fructose is phosphorylated by hexokinase in the liver2. Fructose metabolism does require a specific aldolase for Fr 1-P3. Fructose can be converted to either pyruvate or glucose4. Fructose consumption can not elevate the blood sugar level5. Fructose catabolism in the liver bypasses phosphofructokinase

A:2,3,5 B:1,2,3 C:1,2,3,4 D:2,3,4,5 E:1,3,4,5

peter pazmany catholic university · pdf filesubdivision: trioses, tetroses, pentoses,...

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