1

Catabolism of sugars and glycosides

-Glycolysis--Pentose phosphate pathway (PPP)-

-Catabolism of other sugars and glycans-

MAIN CATABOLIC PATHWAYS Energetic catabolism

2

Catabolism of sugars and glycosides

• Structure of sugars• Glucose catabolism (glycolysis)

– Substrate-product balance and localization– Global view of glycolysis– Analysis of glycolysis– Conclusion : Types of reactions in glycolysis– Energetic balance

• Pentose phosphate pathway (PPP)• Other sugars catabolism:

– Catabolism of other hexoses– Catabolism of glycogen– Catabolism of glycerol

Amphibolic character of glycolysis

3

Hexoses• Hexoses are polyalcohols of 6 carbon atoms. The C1 or C2 on hexoses

carries an aldehyde or a ketone (carbonyl).

• Carbon 5 has a D configuration.

• Carbons 2 to 4 can be either D or L, each structure corresponding to aparticular hexose.

Main hexoses in biology

CHO

OHH

HHO

OHH

OHH

CH2OH

CHO

HHO

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

HHO

OHH

CH2OH

Glucose Mannose Galactose

CH2OH

O

HHO

OHH

OHH

CH2OH

Fructose

• These sugars exist mainly in cyclic structures (acetal and hemiacetal) with theanomeric proton in a or b.

• However, catabolism affects essentially the linear (open) form.

• Glucose is the hexose present at the highest concentration in humans (5 mM inblood). Mannose and galactose are glucose epimers in C2 and C4. In fructose thecarbonyl is at C2 (ketone).

Glycolysis

Glucose catabolism

4

Glucose catabolism : GlycolysisSubstrate balance - Localization

Glycolysis transforms one glucose molecules into 2 pyruvates in aerobiosis, and 2 lactates in anaerobiosis.

Glycolysis is located at the cytosol.

CHO

OHH

HHO

OHH

OHH

CH2OH

COO-

CO

CH3

COO-

CHOH

CH3

Glucose

2 ou 2

Pyruvate Lactate

NB : it is possible that masked reactions occurs : reductions can be maskedby oxidations and hydrolysis by condensations and vise versa.

Aerobiosis Anaerobiosis

Oxidation 2 0

Skeleton break 1 1

Condensation 0 0

Expected simple reactions

Global view of the

glycolysis

CHO

OHH

CH2OP

COOP

OHH

CH2OP

NAD+

NADH

ATP

ADP

CH2OP

O

CH2OH

COO-

OHH

CH2OP

DHP

H2O

COO-

OPH

CH2OH

COO-

CH3

OHH

ADP

ATP

COO-

OP

CH2

CHO

OHH

HHO

OHH

OHH

CH2OHATP

NADH NAD+

ADP

COO-

CH3

O

CHO

OHH

HHO

OHH

OHH

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OPATP

ADPCH2OP

O

HHO

OHH

OHH

CH2OP

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

Gluckonaseou Hexokinase

G6P isomérase

Phosphofructokinase

Aldolase

Isomérase

G3PDH

Enolase

Glyc.-3P kinase

Isomérase

Pyruvate kinase

Lactate DH

Glucose

G6P

Fructose-6P (F6P)

F1-6P

Glycéraldéhyde-3P

Glycérate1-3P

Glycérate-3P

Glycérate-2P

Phosphoénolpyruvate(PEP)

Pyruvate Lactate

Irreversible steps are in red

5

Developed structures (some reversible steps are not signaled)

Main sequences in GlycolysisGlycolysis can be decomposed in a series of 4 main successive steps

1) Transformation of glucose in diphosphorylated fructose (F1-6P) :

2) F1-6P is then transformed in glyceraldhyde-3P (G3P), involving skeleton breakdown:

CHO

OHH

HHO

OHH

OHH

CH2OH

CH2OP

O

HHO

OHH

OHH

CH2OP

CH2OP

O

HHO

OHH

OHH

CH2OP

2

CHO

CHOH

CH2OP

3) G3P is oxidized into pyruvate

CHO

CHOH

CH2OP

COO-

C

CH3

O

4) In anaerobiosis, pyruvate is reduced to lactate :

COO-

C

CH3

O

COO-

CHOH

CH3

6

Metabolic step (1) : Glucose G6PReaction type : Condensation (Transfer)

Comments :

Coenzyme : ATPADP

Enzyme : Glucokinase (GK)

or hexokinase (HK) (1)

Energetics : Irreversible

(1) Simple transfer.

• Kinases (K) are transferases that tranfer a phosphate group.

• GK is the isoenzyme in liver, HK is the isoenzyme in muscle.Their kinetic properties are different and adapted to the organ.

CHO

OHH

HHO

OHH

OHH

CH2OH

CHO

OHH

HHO

OHH

OHH

CH2OP

Glucose Glucose-6P (G6P)

ATP ADP

NB : Phosphorylated glucose can not be transported through the cellmembrane. Phosphorylation of glucose, and of other glycolysisintermediates, avoids transport to the extracellular space. This enhances theefficacy of glycolysis.

Step 2 : G6PF6P(Transposition)

Double Keto–enol tautomerism (1)

Comments :

non

G6P,F6P isomerase (1)

Reversible

1) New type of simple reaction : Keto–enol tautomerism .(see next slide)

CHO

OHH

HHO

OHH

OHH

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OP

G6P Fructose-6P(F6P)

Reaction type :

Coenzyme :

Enzyme :

Energetics :

7

Reaction type: Keto–enol tautomerism • This reaction involves the isomerization of an aldehyde or ketone into a enol

group.

This reaction is endergonic (DG’°>>0), aldehyde and ketone are preferential.

These reactions are very rapid. They are always coupled with a reaction in a previousstep.

Mechanisme of transposition of sugars or P-sugars (ex ; G6P F6P)

The fixation of G6P on the isomerasestabilize the enediol allowing thetransposition.

HC C

HO C C

H

OH

Aldéhyde Enol

HC C

O

C C

OH

Cétone EnolKeton

CHO

OHH

HHO

OHH

OHH

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OP

CHOH

OH

HHO

OHH

OHH

CH2OP

F6PG6PEne diol

Fixé sur l'enzyme

Step 3 : F6P F1-6P

Condensation (transfer)

Comments :

ATPADP

Phosphofructokinase (transferase)

Irreversible

1. Reaction analogue to step 1 (GG6P).

CH2OH

O

HHO

OHH

OHH

CH2OP

CH2OP

O

HHO

OHH

OHH

CH2OP

Fructose-6(F6P)

Fructose-1-6P(F16P)

ATP ADP Reaction type :

Coenzyme :

Enzyme :

Energetics :

8

Step 4 : F16P G3P + DHP

C-C skeleton breakdown (1)

subtype: retroaldolisation

Comments :

non

Aldolase

Reversible (2)

(1) Non red-ox

The reaction sub-type is retro-aldolisation (see next slide)

(2) Retro-aldolisation are impossible under standard chemicalconditions but they are reversible in vivo

CH2OP

O

HHO

OHH

OHH

CH2OP

CH2OP

C O

CH2OH

CHO

CHOH

CH2OPF16P

Dihdroxyacétone-P(DHP)

Glycéraldéhyde-3P(G3P)

Reaction type :

Coenzyme :

Enzyme :

Energetics :

Reaction type: Skeleton synthesis / breakdown subtype: aldolisation - retroaldolisation

• In chemistry, aldolisation is a reaction in where new C-C bond is formed(skeleton synthesis) between two aldehydes carrying a mobile a hydrogen :

• In biochemistry aldolisation is a skeleton synthesis reaction in which analdehyde or a ketone form a C-C bond on a C having a mobile hydrogen in apolarized molecule (the opposite reaction is called retroaldolisation)

C OH

+ C

H

OH

C C OH

CH R

H mobile Fonction polarisante

• Aldolisation / retroaldolisation are reversible in vivo.

C C

HO

H

C C

HO

H

+ C C

H H

OH

C C OH

Faux

C OH

+ C

H

OH

C RCH R

H mobile Fonction polarisante

a a

Polarizing function Polarizing function

Enzyme: aldolaseCoenzyme: non

9

Step 5 : DHP G3P (1)

Double tautomery

Comment :

non

Isomerase

Reversible

1. This reaction is an analogue of step 2 (G6P F6P)

CH2OP

C O

CH2OH

CHO

CHOH

CH2OP

DHP G3P

Reaction type :

Coenzyme :

Enzyme :

Energetics :

Step 6 : G3P Glycerate-1-3POxidation + Condensation (1)

Comments :

NAD+ NADH

Dehydrogenase

Reversible (2)

(1) Coupled reaction: oxidation affords energy for condensation :

(2) The enzyme catalyze the reaction in the two directions.

G3P + NAD+Glyc.-3P +NADH DG'°<0Glyc.-3P + P Glyc.-13P DG'°>0

Reaction type :

Coenzyme :

Enzyme :

Energetics :

C

CHOH

CH2OP

O H NAD+

PO43-

C

CHOH

CH2OP

OO

PO

O-

O-

G3PGlycérate-1-3P

NADH + H+

10

Step 7 : Glyc-1-3P Glyc-3PH/C : Simple transfer (1)

Comments :

ADP ATP

Transferase: Kinase

Reversible

1. Steps 6 and 7 allow production of ATP.

Reaction type :

Coenzyme :

Enzyme :

Energetics :

COOP

CHOH

CH2OP

COO-

CHOH

CH2OP

Glycérate-1-3P Glycérate-3P

ADP ATP

Step 8 : Glyc-3P Glyc-2P

H/C : Simple transfer (1)

Comments :

non

Isomerase (Mutase) (2)

Reversible (3)

(1) ST: hydrolysis of P in carbon 3 and condensation of P on carbon 2. It is asimple transfer with transfer of matter, but the reaction is carried by the enzymeas an isomerization .

(2) Isomerization is not a reaction type. Isomerizations are linked to tautomery,to hydrolyses/condensations and to oxido/reductions.

(3) Because of the isomerization.

COO-

CHOH

CH2OP

COO-

CHOP

CH2OH

Glycérate-3P Glycérate-2P

Reaction type :

Coenzyme :

Enzyme :

Energetics :

11

Step 9 : Glyc-2P PEPDehydration (1)

Comments :

non

Dehydratase

Reversible

(1) This dehydration affords an enol function stabilized by a phosphatewhich inhibits transformation into a ketone.

COO-

C

CH3

O

Pyruvate

Reaction type :

Coenzyme :

Enzyme :

Energetics :

COO-

CHOP

CH2OH

COO-

COP

CH2

H2O

Glycérate-2P Phosphoénolpyruvate(PEP)

Step 10 : PEP PyruvateSimple transfer + Tautomery

ADP ATP

Transferase: Kinase

Irreversible (ΔG’°=-30 kJ) (1)

(1) PEP is instable and so rich in energy : 60 kJ (2 ATP). It contains the condensation andtautomery energies.

COO-

COP

CH2

ADP ATP COO-

C

CH3

O

PEP Pyruvate

Equivalent reactions:

Reaction type :

Coenzyme :

Enzyme :

Energetics :

COO-

COP

CH2

COO-

COH

CH2

COO-

COH

CH2

ATP

COO-

C

CH3

O

PEP

Pyruvate

PEP

+ P

PEP

ADP + P

DG'°< 0

DG'°< 0

DG'°= + 30 kJ

- 60 kJ

Hydrolyse

Tautomérie

Condensation

12

Step 11 : Pyruvate Lactate (in anaerobiosis)

Reduction (1)

NADH NAD+

Dehydrogenase

Reversible (2)

(1) Reduction reactions use in general NADPH, this reduction is NADHdependent since this allows recycling of NADH in NAD+ in the absence ofoxygen (anaerobiosis).

COO-

C

CH3

O

COO -

CHOH

CH3

Pyruvate Lactate

NADH NAD+

Comments :

(2) The reaction is also involved in the catabolism of lactate in aerobiosis.

Reaction type :

Coenzyme :

Enzyme :

Energetics :

Substrate and energetic balances in glycolysis

In anaerobiosis

Glucose + 2 ADP + 2 P -----> 2 lactate + 2 ATP

4 ATP are formed and 2 are consumed

(Remember: 1 glucose produces two pyruvates)

Glucose + 2 NAD+ + 2 ADP + 2 P ----> 2 pyruvate + 2 NADH + 2 ATP

In aerobiosis

ATP balance in aerobiosis (considering respiratory chain)

Number of coenzymes produced

Number of ATP formed after coenzyme

regeneration

NADH 2 6

FADH2 0 0

ATP 2 2

Total 8

13

Pentose phosphate pathway (PPP)

14

CHO

OHH

HHO

OHH

OHH

CH2OP

CO2

CH2OH

O

OHH

OHH

CH2OP

COO-

OHH

HHO

OHH

OHH

CH2OP

NADP+ NADPH NADP+ NADPH

Gluconate 6P Ribulose 5PGlucose 6P

CH2OH

O

HHO

OHH

CH2OP

CHO

OHH

OHH

OHH

CH2OP

CHO

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OP

OHH

OHH

CH2OH

O

HHO

OHH

OHH

CH2OP

CHO

OHH

OHH

CH2OP

Xylulose 5PRibose 5P

F6P

Erythrose 4P

Sédoheptulose 7P

CHO

CH2OP

OHH

• It is not compulsory to memorize this metabolism.

Pentose phosphate pathway (PPP)

The intermediates in glycolysis are indicated

• This is a glycolysis variant in which glucose-6P is transformed in glyceraldehyde-3Pand 3 CO2

Pentose phosphate pathway (PPP)

• As the glycolysis, PPP is located in the cytosol

• This metabolism involves 6 dependent NADP+

oxidations producing 6 NADPH (source of NADPHused in anabolic reactions)

CHO

OHH

HHO

OHH

OHH

CH2OP

CHO

CHOH

CH2OP

+ 3 CO2

6 NADP+ 6 NADPH

CHO

OHH

CH2OP

COOP

OHH

CH2OP

NAD+

NADH

ATP

ADP

CH2OP

O

CH2OH

COO-

OHH

CH2OP

DHP

H2O

COO-

OPH

CH2OH

COO-

CH3

OHH

ADP

ATP

COO-

OP

CH2

CHO

OHH

HHO

OHH

OHH

CH2OHATP

NADH NAD+

ADP

COO-

CH3

O

CHO

OHH

HHO

OHH

OHH

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OPATP

ADPCH2OP

O

HHO

OHH

OHH

CH2OP

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

Gluckonaseou Hexokinase

G6P isomérase

Phosphofructokinase

Aldolase

Isomérase

G3PDH

Enolase

Glyc.-3P kinase

Isomérase

Pyruvate kinase

Lactate DH

Glucose

G6P

Fructose-6P (F6P)

F1-6P

Glycéraldéhyde-3P

Glycérate1-3P

Glycérate-3P

Glycérate-2P

Phosphoénolpyruvate(PEP)

Pyruvate Lactate

Glycolysis

15

Substrate balance

CHOOHHHHOOHHOHH

CH2OP

3 CO2 +CHOCHOHCH2OP

+ 2

CH2OHOHHOOHHOHH

CH2OP

3

Glucose-6P Glyceraldehyde-3P Fructose-6P

Pentose phosphate pathway (PPP)

• This metabolism is decomposed in two parts :

– First part (oxidative phase) : irreversible oxidation/skeleton breakdown of 3 molesof glucose-6P into 3 moles of pentose-5P and 3 CO2

– Second part (non-oxidative phase) : reversible combination of several skeletonbrake/skeleton synthesis allowing transformation of the 3 pentoses-5P intoglyceraldehyde-3P and 2 fructose-6P that will regenerate 2 glucose-6P.

• Role of PPP pathway– Allows pentoses anabolism and catabolism (nucleotides synthesis and

degradation… ADN, ARN…).

– Allows regeneration of NADPH consumed in anabolic reactions (about 30% ofglucose is used by the PPP pathway in anabolic organs including liver, andmuscles).

Pentose phosphate pathway (PPP)

Glucose-6Px3 Pentose 5P

CO2x2

Glycéraldéhyde-3P

Suite de la glycolyse

x3

x1

To glycolysis

16

PHASE 1Oxydative

PHASE 2Non oxydative

CHOOHHHHOOHHOHH

CH2OP

CH2OHOOHHOHH

CH2OP

+ CO2

4 e

Pentoses-P formation through the oxidation and skeleton breakdown

PHASE 1: Synthesis of Ribulose-5P

Irreversible (decarboxylations are irreversible)

Pentose phosphate pathway (PPP)Fist part: oxidative phase

17

CHO

OHH

HHO

OHH

OHH

CH2OP

CO2

CH2OH

O

OHH

OHH

CH2OP

COO-

OHH

HHO

OHH

OHH

CH2OP

NADP+

NADPH

COO-

O

OHH

OHH

CH2OP

NADP+

NADPH

G6P déshydrogénase

Gluconate 6P

OHH

Ribulose 5P

Glucose 6P

Gluconate 6P désydrogénase

Synthesis of ribulose 5P

Pentose phosphate pathway (PPP)Oxidative phase

PHASE 1Oxydative

PHASE 2Non oxydative

18

Synthesis of ribose 5P and xylose 5P

Transposition of ribulose 5P into ribose 5P (isomerisations)

CHO

OHH

OHH

OHH

CH2OP

Ribose 5PRibulose 5P

CH2OH

O

OHH

OHH

CH2OP

CHOH

OH

OHH

OHH

CH2OP

Epimerization du ribulose 5P into xylulose 5P

Xylulose 5P

CH2OH

O

OHH

OHH

CH2OP

CH2OH

OH

OH

OHH

CH2OP

CH2OH

O

HHO

OHH

CH2OP

Ribulose 5P

CHO

OHH

HHO

OHH

OHH

CH2OP

CO2

CH2OH

O

OHH

OHH

CH2OP

COO-

OHH

HHO

OHH

OHH

CH2OP

NADP+ NADPH NADP+ NADPH

Gluconate 6P Ribulose 5PGlucose 6P

CH2OH

O

HHO

OHH

CH2OP

CHO

OHH

OHH

OHH

CH2OP

CHO

CH2OP

CH2OH

O

HHO

OHH

OHH

CH2OP

OHH

OHH

CH2OH

O

HHO

OHH

OHH

CH2OP

CHO

OHH

OHH

CH2OP

Xylulose 5PRibose 5P

F6P

Erythrose 4P

Sédoheptulose 7P

CHO

CH2OP

OHH

19

Sugars interconverstion

It involves :

Reversible reactionc

Transformation of 3 moles of sugar-5P (15C) in glyceraldehyde-3P(3C) and 2 fructose-P (12C).

Transaldolisation: transfert of 3 carbons

Transketolisation : transfert of 2 carbons

Pentose phosphate pathway (PPP)Second part : non oxidative phase

Xylulose 5P Glycéraldéhyde 3P Fructose 6P

Ribose 5P Sédoheptulose 7P Erythrose 4P

Xylulose 5P Glycéraldéhyde 3P

Fructose 6P

C5

C5 C7

C3 C6

C4C6

C3C5

Global description

Sequence of transketolisation, transaldolisation and transketolisation

C5 + C5 C3 + C7 (Transketolisation)

C3 + C7 C6 + C4 (Transaldolisation)

C5 + C4 C6 + C3 (Transketolisation)

C balance3 C5 2 C6 + 1 C3

20

The phase 2 conects PPP pathway to glycolysis (through glyceraldehyde 3P)

Other sugars catabolism:

Catabolism of other hexoses

Catabolism of glycogen

Catabolism of glycerol

21

Catabolism of other hexoses (Mannose, Galactose)• After phosphorylation, mannose and galactose are isomerized in G6P byinversion of configuration on C2 and C4 respectively.

• Inversion of configuration in a sp3 carbon occurs always by subtraction a H, this gives sp2

carbon; Hydrogen is then fixed at the other side of the molecule resulting in inversion ofconfiguration:

X

H YZ

X

Y HZ

X

Y Z

Plan H

Carbone sp3 Carbone sp3Carbone sp2

• In the case of mannose, this mechanism involves a double ketoenolic tautomerie.

CHO

OHH

HHO

OHH

OHH

CH2OP

G6P

CHO

H

HHO

OHH

OHH

CH2OP

CHOH

OH

HHO

OHH

OHH

CH2OP

Mannose-6PEnediol

Fixé sur l'enzyme

HO

Catabolism of glycogenGlycogen is a poly a glucose that represents the main reserve of glucose in animals (cellulose in plants)

• Another enzyme hydrolysis 1-6 bonds.

OCH2OH

O

OCH2OH

O Gluc Gluc

P

OCH2OHP

OCH2OH

O Gluc Gluc

G6PGlycolyse

Glycogène phosphorylase(Transférase)

G6P,G1P isomérase

O

CH2OHO

CH2OH

O

O

CH2OH

O

O

CH2OH

O

CH2

O

O

O

CH2OH

.....O

Chaine linéaire

Ramification

CATABOLISMa1-4 chains in glycogen are depolymerized by the enzyme glycogene phosphorylase that liberates glucose 1P (ST reaction). Glucose 1P is then transformed into glucose 6P by an isomerase.

22

Other sugars related molecules

Starch is degardated by hydrolysis and phosphorylation (unknownmechanism).

Glycerol produced by lipid metabolism is transformed into dihydroxyketone P (intermediate of glycolysis).

Glycerol catabolism

CH2OH

CHOH

CH2OH

CH2OH

CHOH

CH2OP

ATP ADP

Kinase

CH2OH

CO

CH2OPDH

NAD+ NADH

Amphibolic role of glycolysis

• Glucose is the main source of carbon in animal organisms and plants.

• Every molecule made by un organism can be made from glycose.

• Glycolysis afford not only substrates for catabolism but also precursors foranabolism.

For instance alanine is lade from pyruvate and glycerol (triglycerides) fromdihydroxyketone-P,

• Involvement of a catabolism in an anabolic pathway is called « amphibolic catabolism »

Glycolysis is amphibolic