CATABOLISM OF CARBOHYDRATES

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

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium 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 ***

PETER PAZMANY CATHOLIC UNIVERSITY SEMMELWEIS

UNIVERSITY

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BIOCHEMISTRY

CATABOLISM OF CARBOHYDRATES

www.se.hu

(BIOKÉMIA )

(A SZÉNHIDRÁTOK LEBONTÁSA )

TRETTER LÁSZLÓ

http://semmelweis-egyetem.hu/

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Biochemistry: Catabolism of carbohydrates

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

Catabolism^DEF: Part of intermediary metabolism dealing with the energy-yielding degradation of nutrient molecules

Antonym of catabolism: anabolism Examples for catabolic processes

Decomposition of glucose to pyruvate or lactate called:

glycolysis

Decomposition of fatty acids to acetyl-CoA called: beta oxidation Decomposition of glycogen to glucose: called glycogenolysis Decomposition of acetyl-CoA to carbon dioxide + water called:

citric acid cycle

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Biochemistry: Catabolism of carbohydrates

CLASSIFICATION of CARBOHYDRATES

Monosacharides^DEF: 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

Disaccharides^DEF: hydrolysis of disaccharides yields 2 molecules of monosaccharides

Oligosaccharides^DEF: Hydrolysis yields 3-6 monosaccharide units Polysaccharides^DEF: Hydrolysis yields more than 6 monosaccharide

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Biochemistry: Catabolism of carbohydrates

Examples for monosaccharides

Aldoses Ketoses Trioses (C₃H₆O₃) ^Glycerose

Synonym:

Glycer- aldehyde

Dihydroxy- acetone

Tetroses (C₄H₈O₄) Erythrose Erythrulose Pentoses (C₅H₁₀O₅) Ribose Ribulose Hexoses (C₆H₁₂O₆) Glucose Fructose

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Biochemistry: Catabolism of carbohydrates

Table of contents:

Glycolysis, the anaerobic decomposition of glucose Catabolism of non-glucose carbohydrates

Regulation of carbohydrate catabolism

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Biochemistry: Catabolism of carbohydrates

Glycolysis

Glycolysis -reactions Learning objectives:

Carbohydrates are major energy giving substrates for a living organism

Glycolysis 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 glycolysis

2. To understand the formation of ATP in the absence of oxygen 3. To demonstrate important principles of thermodinamics

using examples taken from glycolysis

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Glycolysis^DEF

-Anaerobic degradation of glucose to lactate or

-Anaerobic degradation of glucose to pyruvate – a preparatory pathway for 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 lactate

thus

glycolysis is the most universal metabolic pathway.

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Glycolysis -reactions Overview of glycolysis Glucose

C6

Hexose phosphates (C6)

Triose phosphate Triose phosphate NADH+H⁺NAD⁺

CO₂ H₂O ATP

ATP formation

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Overview of glycolysis

In the absence of oxygen or mitochondria

2 NAD⁺ 2 NADH+H⁺

In the presence of oxygen and mitochondria

Glycolysis -reactions

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Glycolysis -reactions Preparatory phase of glycolysis

2 ATP invested and

Hexose chain is converted into triose phosphates

Biochemistry: Catabolism of carbohydrates

Glycolysis

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ATP requiring reactions of glycolysis

Hexokinase Glucokinase

Phosphofructokinase-1

ΔG^’o= -16.7 kJ/mol irreversible

ΔG^’o= -14.2 kJ/mol Irreversible

Rate limiting step of glycolysis

Important reactions of the preparatory phase

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Important reactions of the preparatory phase

Hexokinase and glucokinase are isoenzymes

Isoenzymes^DEF: Enzymes catalyzing the same reaction But differ:

In amino acid sequence V_max, and/or K_M

in regulation

Hexokinases are localized in the peripheral tissues Glucokinase is localized in the liver

Hexokinases show high affinity for glucose Glucokinase show low affinity for glucose Their regulation is different

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Important reactions of the payoff phase

ΔG^’o= 6.3 kJ/mol reversible

ΔG^’o= -18.5 kJ/mol reversible

ΔG^’o= -31.4 kJ/mol irreversible

Inorganic phosphate incorporation NADH formation

High energy acyl-phosphate group formation on the 1^stC atom

The acyl-phosphate group is transferred to ADP Substrate level phosphorylation

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

Substrate level phosphorylation

Biochemistry: Catabolism of carbohydrates

Glycolysis

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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 phosphorylation^DEF: Formation of ATP by phosphoryl group transfer from a compound having high energy bound

Examples for substrate level phosphorylation:

in glycolysis: phosphoglycerate kinase reaction pyruvate kinase reaction

Biochemistry: Catabolism of carbohydrates

Glycolysis

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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 glycolysis is continued in the citric acid cycle

- glycolysis has reversible and irreversible steps

- the irreversible reactions of glycolysis are catalyzed by hexokinase (glucokinase in liver), phosphofructokinase

Biochemistry: Catabolism of carbohydrates

Glycolysis

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Biochemistry: Catabolism of carbohydrates C

atabolism 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.

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Learning objectives

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.

Biochemistry: Catabolism of carbohydrates

Catabolism of non-glucose carbohydrates

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Fructose metabolism

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

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

Pathological significance: hereditary fructose intolerance (fructose accumulation plus hypoglycemia), obesity

Biochemistry: Catabolism of carbohydrates

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Entry of fructose into glycolysis

1. In liver – major organ of fructose catabolism

Glycolysis, gluconeogenesis Important: fructose catabolism in the liver bypasses phosphofructokinase-1!

Biochemistry: Catabolism of carbohydrates

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Entry of fructose into glycolysis

2. In skeletal muscle – less important in fructose catabolism Fructose

Fructose 6-phosphate ATP

ADP

Hexokinase

Glycolysis

Hexokinase - not entirely specific for glucose - converts fructose to Fr 6-P

at low [Glucose]

at high [Fructose]

Biochemistry: Catabolism of carbohydrates

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Glycolysis, gluconeogenesis

Pathological aspects of fructose metabolism

X

1) Hereditary fructose intolerance Aldolase B deficiency

[fr 1-P] increased

Symptom: hypoglycemia

Why? See:regulation of carbohydrate breakdown

2) High fructose consumption

- Susceptibility to obesity,hyperlipidemia hyperlipidemia^DEF: increased concentration of lipids in the blood

Biochemistry: Catabolism of carbohydrates

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Galactose metabolism

Availability of galactose: from milk sugar: lactose. Intestinal hydrolysis of lactose results 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

Biochemistry: Catabolism of carbohydrates

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Galactose metabolism

Galactose

galactokinase Galactose 1-phosphate

UDP-Glc-Gal 1-P uridyltransferse

Glucose 1-phosphate

phosphoglucomutase Glucose 6-phosphate

ATP ADP

Glucose 1-P

UDP-Glc

pyrophosphorylase UDP-glucose

UDP-galactose

UTP PP_i

UDP-gal epimerease

Biochemistry: Catabolism of carbohydrates

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[Galactose]

galactokinase [Galactose 1-phosphate]

UDP-Glc-Gal 1-P uridyltransferse

Glucose 1-phosphate

phosphoglucomutase

ATP ADP

Glucose 1-P

UDP-Glc

pyrophosphorylase UDP-glucose

UDP-galactose

UTP PP_i

UDP-gal epimerease

Pathological aspects of galactose metabolism

X

Galactosemia:

lack of UDP-Glc-Gal 1-P uridyltransferse

Biochemistry: Catabolism of carbohydrates

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Glycogen metabolism

Glycogen: the storage form of glucose in the body forms granules in the cytosol

many cells contain glycogen

the most important organs for storage: liver, skeletal muscle function of liver glycogen: maintenance of blood sugar level function of muscle glycogen: energetic support of contraction Structure: highly branched structure

chains: alpha [1-4] glucosidic linkage branches: alpha [1-6] glucosidic linkage

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

molecular mass: in the order of millions

Biochemistry: Catabolism of carbohydrates

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The structure of glycogen

1

6

6 4 1

4 1 4 1

1 4 1

Glycogenin

Biochemistry: Catabolism of carbohydrates

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Glycogenolysis^DEF: Intracellular decomposition of glycogen Resulting glucose in the liver and kidney cortex

Resulting glucose 6-P in the muscle Synonym: glycogen breakdown

Antonym: 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.

Biochemistry: Catabolism of carbohydrates

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The fate of glycogen-derived glucose after breakdown

In liver: release to the bloodstream

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

glycolysis ending with lactate

production phosphoglucomutase

Lactate dehydrogenase

P_i

Glycogen

glycogen phosphorylase Glucose 1-P

Glucose 6-P

Pyruvate

PDH complex Glycolysis glucose 6-Pase

Glucose Lactate

Biochemistry: Catabolism of carbohydrates

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Steps of glycogen breakdown 1

Glycogen

Glycogen phosphorylase

Debranching enzyme alpha (1→4) →alpha (1→4)

transferase activity

Debranching enzyme Amylo (1→6)-glucosidase

activity P_i P

gl 1-P

gl H₂O

Debranching enzyme has two catalytic activities Products of catabolism: shorter glycogen

glucose 1-P

glucose (captured by hexo- or glucokinase)

Biochemistry: Catabolism of carbohydrates

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Steps of glycogen breakdown 2

phosphoglucomutase glucose 6-phosphatase H₂O P_i

phosphohexose isomerase

ER in liver

Biochemistry: Catabolism of carbohydrates

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Pathological aspects of glycogen breakdown

Glycogen storage diseases^DEF:inherited disorders characterized by abnormal 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

Biochemistry: Catabolism of carbohydrates

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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 carbohydrates

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Learning objectives

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

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Multilevel regulation of glycolytic enzymes

Gene expression Covalent modification Allosteric Enzyme Inducer repressor phosphorylat

ion

dephosphory

lation activator inhibitor

Hexokinase Gluc 6-P

Glucokinase insulin

Glucagon (cAMP) Starvation

Fructose 1-P

(through glucokinase

regulatory protein)

Fructose 6-P

through glucokinase regulatory protein)

Phosphofruct

okinase-1 insulin starvation Fructose 2,6-P₂

AMP

ATP, citrate, fatty acids Pyruvate

insulin

Glucagon (cAMP)

cAMP,

Ca-CaM insulin

Fructose 1,6-P ATP, alanine

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Regulation of glucokinase

(liver)

Inative in the 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

Accumulation In fructose intolerance

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Regulation of phosphofructokinase-1 in liver Insulin stimulates glycolysis

Insulin +

Phosphorylated: inactive Dephosphorylated:

active

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Phosphorylated: inactive

Glucagon +

X X

Regulation of phosphofructokinase-1 in liver Glucagon inhibits glycolysis

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

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 activity

Kinase activity will be dominant Glycolysis activated

Liver specific enzyme: Phosphorylation (PKA) near the kinase domain inactivates kinase activity Phosphates activity will be dominant.

Glycolysis is inactivated

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

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-P₂the most important regulator of the rate limiting step of glycolysis. The level of Fr 2,6-P₂reflects hormonal changes.

In liver insulin elevates [Fr 2,6-P₂], glycolysis is stimulated Glucagon decreases [Fr 2,6-P₂], glycolysis is inhibited Adrenalin decreases [Fr 2,6-P₂], glycolysis is inhibited BUT!

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Regulation of glycogen breakdown

cAMP 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

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

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).

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Biochemistry: Catabolism of carbohydrates

Regulation of carbohydrate catabolism

Recommended literature

Orvosi Biokémia (Ed. Ádám Veronika)

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Biochemistry: Catabolism of carbohydrates

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?

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Biochemistry: Catabolism of carbohydrates

Questions:

Which of the following statements are true for the PFK1?

1. The reaction catalized by the enzyme is irreversible in vivo 2. The activity of the enzyme can be inhibited by ATP

3. Its function is influenced by the ATP/ADP ratio 4. It is the fastest enzyme of the glycolysis 5. 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 liver

2. Fructose metabolism does require a specific aldolase for Fr 1-P 3. Fructose can be converted to either pyruvate or glucose

4. Fructose consumption can not elevate the blood sugar level 5. Fructose catabolism in the liver bypasses phosphofructokinase