The sweet side of catabolism: carbohydrates as cellular fuels

Russian National Research Medical University

Maxim A. Abakumov

Moscow, 2014

Carbohydrates metabolism

• Usually comes as polysaccharides

• Two main polysacharides are glicogen and

starch

• Polysacharides can not be used in native form

• Breakdown into monosacharides and transport

from gut to blood stream and periheral tissues

are needed

Digestion of carbohydrates• Digestion – enzyme driven breakdown of large

polysacharide molecules into monosacharides • Usually takes plase in gastrointestinal tract

Glucose polymers

Starch, glycogen

Disaccharides

Maltose Sucrose Lactose

Digestion byamylase

Monosaccharides

2xGlucose Glucose+Fructose Glucose+Galactose

Maltase Sucrase Lactase

Absorption of carbohydrates• Process of monosacharides transport from gut to blood stream or lymph• Involves special transporting proteins located on membrane of intestine cells

DigestionComposition of carbohydrates in your diet: ~ 70% starch (polysaccharide)

~ 20% sucrose (disaccharide) ~ 6% lactose (disaccharide) ~ 2% maltose (disaccharide)

• Polysacharides digestion occurs in mouth and small

intestine

• This process is driven by salivary and pancreatic

amylases

Digestion

Absorption

• Transmembrane transporter proteins are

involved

• First, monosacharides are transported into cell

from intestine

• Second, monosacharides are released into

blood stream

Absorption

Glucose metabolism• Glucose decomposition for energy release (ATP synthesis)

called glycolysis

• Glucose synthesis with energy consume (ATP hydrolysis)

calles gluconeogenesis

• Glycolysis can be diveded into:

a) aerobic

b) anaerobic

• Aerobic products are CO2 and H2O

• Anaerobic product is lactate

• For both intermediate is pyruvate

Glucose metabolism in cell

Glucose

Pyruvate

AcetylCoa

CO2 + H2O + ATP

TCA

Aerobic Glycolisis

ETC+OP

Lactate

Anaerobic Glycolisis

TCA

Glycolisis

1. Glucokinase2. Phosphogluco isomerase3. Phosphofructo kinase-14. Aldolase5. Triosophosphate isomerase

O

OH

HH

H

OH

OH

H OH

H

OH

O

OH

H

OH

OH

H

H

O OH

O-

O-O

P

O

OH

H

OH

OH

H

H

O O

O-

O-O

P O-

O-O

P

OPO3H2

OH

O

OPO3H2

O

OH

O

OH

HH

H

OH

OH

H OH

H

O

O-

O-O

P

OPO3H2

OH

O

OH

CH3O

O

OH

Glucose Glucose-6-P Fructose-6-P Fructose-1,6-diP

ATP ADP ATP ADP

1 2 3

4

5

67

8

9

10

6. Glyceraldehyde phosphate isomerase7. Phosphoglycerate kinase8. Phosphoglycerate mutase9. Enolase10. Pyruvate kinase

OPO3H2

OH

O

H2PO4

OH

H2PO4O

OH

CH2H2PO4

O

OH

1,3-bisphosphoglycerate

3-phosphoglycerate

2-phosphoglycerate

ATP ADP

H2OADP ATP

NAD+NADH

Phosphoenolpyruvate Pyruvate

Sequence of reactions

+Pyruvate

Glucose

Glucose metabolism in cell

Anaerobic Glycolisis

CoA2x + CO2

Aerobic Glycolisis

TCA, ETC, OP Lactate

Glucose phosphorylation

• First step in glucose metabolism –

phoshorylation of OH-group at 6th carbon atom

• Phosporylated glucose (glucose-6-phospate) is

charged and cannot be transported out of the cell

• Glucose-6-P goes to metabolism

• Catalyzed by two types of enzyme (isozymes)

Glucose phosphorylation

Hexokinase Glucokinase

• Low Km value

• High affinity to glucose

• Located in most tissue cells

• Three isoforms (I, II, III)

• High Km value

• Low affinity to glucose

• Located mostly in liver cells

•Actually IV isoform of hexokinase

Glucose-2-18F

• PET tracer

• Indicates glucose cosumption by cells

• Phosphorylates after transport in cell

• OH-group at 2nd carbon atom is substituted by 18F

• Further metabolism is blocked

• Cells with more active metabolism increase glucose

consumption, glucose-2-18F level and consequently

signal on PET scanner

Phase I

• Coversion of glucose (6 carbon) to

dihydroaceton phosphate and gliceraldehyde-

3-phosphate (2x3 carbon)

• 2 ATP are required (will be regenerated later)

• 1st and 3rd reaction are irreversible

Phase I. Preparatory phase.

1. Glucokinase2. Phosphogluco isomerase3. Phosphofructo kinase-14. Aldolase5. Triosophosphate isomerase

O

OH

HH

H

OH

OH

H OH

H

OH

O

OH

H

OH

OH

H

H

O OH

O-

O-O

P

O

OH

H

OH

OH

H

H

O O

O-

O-O

P O-

O-O

P

OPO3H2

OH

O

OPO3H2

O

OH

O

OH

HH

H

OH

OH

H OH

H

O

O-

O-O

P

Glucose Glucose-6-P Fructose-6-P Fructose-1,6-diP

ATP ADP ATP ADP

1 2 3

4

5

Dyhydroxyacetonephosphate

D-glyceraldehyde -3-phosphate

Glucose to glucose-6-P

1st ATP is hydrolysed

Total ATP count: -1 ATPTotal NADH count: 0 NADH

Glucose-6-P to fructose-6-P

Total ATP count: -1 ATPTotal NADH count: 0 NADH

O

H

H

OH

H

OH

OH

HOH

H

OP

O

O

OOP

O

O

OO

H

OH

H

H

OH

CH2OH

OH

Phosphohexose isomerase

Fructose-6-P to fructose-1,6-diP

2nd ATP is hydrolysed

Total ATP count: -2 ATPTotal NADH count: 0 NADH

PO

OH

OH O

OH

H

OH

OH

H

H

H2C OHO

OH

H

OH

OH

H

H

P P

OH

OH

O OHO

OH

Phosphofructokinase-1

ATP ADP

Fructose-6-P Fructose-1,6-diP

Fructose-2,6-diP to gliceraldehyde-3-phosphate and dihidroxyacetone-phosphate

Total ATP count: -2 ATPTotal NADH count: 0 NADH

O

OH

H

OH

OH

H

H

P P

OH

OH

O OHO

OH

OPO3H2

O

OH

OPO3H2

OH

O

Aldolase

Fructose-2,6-diP

Dyhydroxyacetonephosphate

D-glyceraldehyde-3-phosphate

Phase II. Payoff phase

• Coversion of dihydroaceton phosphate and

gliceraldehyde-3-phosphate (2x3 carbon) to

pyruvate (2x3 carbon)

• 4 ATP are restored

• Last reaction is irreversible

Phase II. Payoff phase.

OPO3H2

OH

O

OPO3H2

OH

O

OH

CH3O

O

OH

67

8

9

10

OPO3H2

OH

O

H2PO4

OH

H2PO4O

OH

CH2H2PO4

O

OH

1,3-bisphosphoglycerate

3-phosphoglycerate

2-phosphoglycerate

ATP ADP

H2O

ADP ATP

NAD+NADH

6. Glyceraldehyde phosphate isomerase7. Phosphoglycerate kinase8. Phosphoglycerate mutase9. Enolase10. Pyruvate kinase

Phosphoenolpyruvate Pyruvate

Gliceraldehyde-3-phosphate to 1,3-bisphosphoglycerate

2x

Total ATP count: -2 ATPTotal NADH count: 2 NADH

OPO3H2

OH

O

NAD+ NADH

Pi H+

Glyceraldehyde-3-phosphate dehydrogenase

OPO3H2

OH

O

H2PO3D-glyceraldehyde-3-phosphate

1,3-bisphosphoglycerate

1,3-bisphosphoglycerate to 3-phosphoglycerate

2x

Total ATP count: 0 ATPTotal NADH count: 2 NADH

2 ATP are synthesized

OPO3H2

OH

O

H2PO3

OPO3H2

OH

O

OH

1.3-bisphosphoglycerate

ADP ATP

Phosphoglycerate kinase

3-Phosphoglycerate

3-phosphoglycerate to 2-phosphoglycerate

2x

Total ATP count: 0 ATPTotal NADH count: 2 NADH

OPO3H2

OH

O

OH

3-Phosphoglycerate

OH

H2PO3O

OH

Phosphoglycerate mutase

2-Phosphoglycerate

2-phosphoglycerate to phosphoenolpyruvate

2x

Total ATP count: 0 ATPTotal NADH count: 2 NADH

OH

H2PO3O

OH

2-Phosphoglycerate

H2O CH2

H2PO3

O

OH

Phosphoenolpyruvate

Enolase

Phosphoenolpyruvate to pyruvate

2x

Total ATP count: 2 ATPTotal NADH count: 2 NADH

2 ATP are synthesized

CH2

H2PO3

O

OH

Phosphoenolpyruvate

CH3

O

O

OH

ADP ATP

Pyruvate kinase

Pyruvate

Glucose→PyruvateTotal energy output

• 2 ATP are consumed

• 4 ATP are synthesized

• Total 2 ATP from 1 glucose

• 2 NADH are synthesized

• All ATP is synthesized without O2 (substrate-

level phosphorylation)

• Anaerobic glycolysis

Glucose→AcCoA→CO2+ H2OTotal energy output

• Total 2 ATP + 2 NADH from anaerobic glycolisis.

• 2 NADH from PDH

• 6 NADH+ 2 FADH2 from TCA

• 2 GTP from TCA

• Total 10 NADH+4 ATP + 2FADH2= 32 ATP

• All ATP is synthesized with O2 (oxidative

phosphorylation)

• Aerobic glycolysis

Sequence of reactions

+ Pyruvate

Glucose

Glucose metabolism in cell

Anaerobic Glycolisis

CoA2x + CO2

Aerobic Glycolisis

TCA, ETC, OP

Lactate

32 ATP 2 ATP

Glycolysis regulationGlucose

Glucose-6-P

Fructose-6-P

Fructose-1,6-diP

Phosphoenolpyruvate

Pyruvate

HexokinaseGlucose 6-phosphatase

Fructose-1,6-bisphosphatase

Phosphofructokinase1 (PFK1)

Pyruvatecarboxylase

Pyruvatekinase

AMP

ATP

Citrate

ATP

Acetyl-CoA

AMP

Acetyl-CoA

Inhibition

Activation

Glycolysis regulation

• 3 enzymes catalyzing irreversible steps are

regulated:

1) Hexokinase

2) Phosphofructokinase-1

3) Pyruvate kinase

• Feedback or hormonal control

Hexokinase regulationFeedback mechanism

PFK1 and PFK2.Distinguish them.

• Only kinase activity

• Phosporylates F-6-P

• Produces F-1,6-BP for further

glycolysis

• Insulin activated

• Glucagon inhibited

• Both kinase and phospatase

activity

• Regulates F-6-P and F-2,6-BP

amount

• F-2,6-BP doesn’t go to

glycolisis

PFK1 PFK2

PFK1regulation. Feedback mechanism.

Pyruvate kinase regulationFeedback mechanism

Hormonal control• Insulin and glucagon are two main hormones

controlling glucose methabolism

• Insulin – fed state hormone

• Insuline provides glycolysis, glicogen and fatty

acid synthesis

• Glucagon – fasting state hormone

• Glucagon provides gluconeogenesis, glicogen

and fatty acids decomposition

Hormonal control over PFK1 and pyruvate kinase

CH3

POH

OH

O

O

O

OHH

OH

OH

H

CH2

H

OHO

O

H

OH

OH

H

H

O

O-

O-O

P

OH

O-

O

P

PFK2FBPase-2

FBPase-2 PFK2 FBPase-2

CH3

POH

OH

O

O

O

OHH

OH

OH

H

CH2

H

OHO

O

H

OH

OH

H

H

O

O-

O-O

P

OH

O-

O

P

ATP

ADP

ATP ADP

H2O

Pi

Active Active

P

H2OPi

Protein kinase-1

Protein phosphatase-1

Glucagon

Insulin

Activation of gluconeogenesisActivation of glycolysis

Fructose-1-P

Fructose-2,6-diP Fructose-2,6-diP

Fructose-1-P

Sequence of reactions

+ Pyruvate

Glucose

Aerobic and anaerobic glycolysis ATP production

Anaerobic Glycolisis

CoA2x + CO2

Aerobic Glycolisis

TCA, ETC, OP

Lactate

32 ATP 2 ATP

In mammals

Anaerobic(lactic acid fermentation

Aerobic Oxidation Anaerobic(alcoholic fermentation)

Lactate Pyruvate Ethanol

Pyruvate fate

Pyruvate dehydrogenase

NAD+ NADH

COOH

CH3

O HS-CoA

S-CoA

CH3

O CO2+C C +

Pyruvate to AcCoAPDH

PDH regulation

Pyruvate to lactate

Pyruvate to oxaloacetate

• Pyruvate kinase reaction is irreversible

• In cytosol glucose and oxaloacetate can not be

synthesized from pyruvate

• Oxaloacetate is TCA intermediate

• If unsufficient can be synthesized from

pyruvate in mytochondria

• Catalyzed by pyruvatecarboxylase

Pyruvate carboxylase

O

OH

COOH

O

O

CH3

COOH

HCO3ATP ADP+Pi

Aspartate (transamination)

Citrate (TCA cycle)

Phosphoenolpyruvate(gluconeogenesis)