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CARBOHYDRATE METABOLISM. Mia Kusmiati Departemen Biokimia FK UNISBA. Carbohydrates. Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH 2 O) n.

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carbohydrate metabolism

CARBOHYDRATE METABOLISM

Mia Kusmiati

Departemen Biokimia FK UNISBA

slide3

Carbohydrates

  • Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH2O)n.
  • The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram. No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.
slide9

Glycolysis

What is glycolysis?

 Ten step metabolic pathway to

convert glucose into two molecules

of pyruvate and two molecules

each of NADH and ATP.

 All carbohydrates to be catabolized

must enter the glycolytic pathway.

- Glycolysis is central in generating

both energy and metabolic

intermediaries.

the glycolysis pathway
The Glycolysis Pathway
  • Major anaerobic pathway in all cells
  • NAD+ is the major oxidant
  • Requires PO4
  • Generates 2 ATP’s per glucose oxidized
  • End product is lactate (mammals) or ethanol (yeast)
  • Connects with Krebs cycle via pyruvate
slide11

Phase I. Energy Investment.

1- Glucose is phosphorylated. Glucose enters a cell through a specific glucose transport process. It is quickly phosphorylated at the expense of an ATP. The investment of an ATP here is called “priming.”

Enzymes = hexokinase or

glucokinase

2. Isomerization of glucose 6-phosphate

Enzyme = phosphoglucoisomerase

slide12

glucose 6-phosphate fructose 6-phosphate

aldose to ketose isomerization reversible, G= 1.7 kJ/mole

slide13

3- Second phosphorylation

Enzyme = phosphofructokinase

ATP ADP

fructose 1,6 bisphosphate

  • second ATP investment
  • highly exergonic, essentially
  • irreversible, G°´= -14.2 kJ/mole
  • - highly regulated, modulating carbon
  • flux through glycolysis in response
  • to energy and carbon requirements
slide14

4- Cleavage to two triose phosphates

Enzyme = aldolase

HC=O H2COP

HCOH O=C

HCOP + CH2OH

H

glyceraldehydedihydroxyacetone

3-phosphate phosphate

where P = phosphate

cleaves a 6C sugar to 2 3C sugars

G°´= +23.8 kJ/mole, driven by next Rx.

slide15

5-Isomerization of Dihydroxyacetone

phosphate

Enzyme = triose-phosphate isomerase

H2C-OH

C=O

CH2-O- P

dihydroxyacetone glyceraldehyde

phosphate 3-phosphate

slide16

5-Isomerization of Dihydroxyacetone

phosphate

Enzyme = triose-phosphate isomerase

H2C-OH

C=O

CH2-O- P

dihydroxyacetone glyceraldehyde

phosphate 3-phosphate

slide17

allows interconversion of two triosephosphate products

of aldolase cleavage

only glyceraldehyde 3-phosphate canbe used further

in glycolysis.

 aldose-ketose isomerization similar

to phosphoglucoisomerase rxn

allows dihydroxyacetone phosphateto be metabolized

asglyceraldehyde 3-phosphate

reversible,G°´= +7.5 kJ/mole.

This is important in gluconeogenesis

slide18

****************************************

End of First Phase:

 Production of two glyceraldehyde

3-phosphate molecules from one

glucose molecule with the

expenditure of two ATPs.

 Therefore: the energy yields of the

following steps are multipled by two.

*****************************************

Second Phase:

slide19

6- Oxidation of glyceraldehyde 3-phosphate

Enzyme= glyceraldehyde-3-phosphate

dehydrogenase

O

HOPO

OH NAD NADH O

OPOH C=O

O- HCOH

H2C

O- P

+

glyceraldehyde 3-phosphate 1,3 bisphosphoglycerate

-addition of phosphate, oxidation,

production of NADH, formation of

high energy compound

slide20

7- Transfer of phosphate to make ATP

Enzyme = phosphoglycerate kinase

O=C-O- P O=C-OH P

HC-OH + P HC-OH +P

H2C-O-P P H2C-O-P P

Adenosine Adenosine

1,3PG ADP 3-phosphoglycerate ATP

- first substrate level phosphorylation,

yielding ATP

- 2 1,3 bis PG yield 2 ATPs, thus so far

ATP yield = ATP input

- high free energy yield, G°´=-18.8kJ/mole drives several of the previous steps.

slide21

8- Phosphate shift setup

Enzyme= phosphoglycerate mutase

- shifts phosphate from position 3 to 2

- reversible, ΔG = + 4.6 kJ/mole

slide22

9- Generation of second very highenergy compound

Enzyme = enolase

-- little energy change in this reaction,

ΔG = +1.7 kJ/mole because the

energy is locked into enolphosphate

slide23

10- Final generation of ATP

  • Enzyme = pyruvate kinase
  • P
  • O H ADP ATP O
  • -OOC-C=CH -OOC-C-CH3
  • phosphoenolpyruvate pyruvate
  • second substrate levelphosphorylation
  • yielding ATP
  • - highly exergonic reaction,
  • irreversible, ΔG = -31.4 kJ/mole.
regulation of glycolysis
Regulation of Glycolysis
  • 6-phosphofructokinase-1

Allosteric enzyme

negative allosteric effectors

Citrate , ATP

Positive allosteric effectors

AMP, fructose1,6-bisphosphate, fructose2,6-bisphosphate

Changes in energy state of the cell (ATP and AMP)

regulation of glycolysis1
Pyruvate Kinase

Allosteric enzyme

Inhibited by ATP.

Isoenzyme in liver

activated by fructose 1,6 bisphosphate

inhibited by alanine

Regulated by phosphorylation and dephosphorylation

Hexokinase

Different isoenzymes

Hexokinase IV

glucose 6-phosphate is an allosteric inhibitor

promote biosynthesis

Regulation of Glycolysis
the significance of glycolysis
The Significance of Glycolysis
  • Glycolysis is the emergency energy-yielding pathway
  • Main way to produce ATP in some tissues

red blood cells, retina, testis, skin, medulla of kidney

  • In clinical practice
aerobic oxidation of glucose
Aerobic Oxidation of Glucose
  • Glucose oxidation
  • Oxidation of glucose to pyruvate in cytosol
  • Oxidation of pyruvate to acetylCoA in mitochondria
  • Tricarboxylic acid cycle and oxidative phosphorylation