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CARBOHYDRATE METABOLISM
Mia Kusmiati Departemen Biokimia FK UNISBA
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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.
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Carbohydrate Metabolism
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Glucose transport
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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.
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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
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Phase I. Energy Investment. Enzymes = hexokinase or glucokinase
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
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glucose 6-phosphate fructose 6-phosphate
aldose to ketose isomerization reversible, G= 1.7 kJ/mole
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Enzyme = phosphofructokinase
3- Second phosphorylation Enzyme = phosphofructokinase ATP ADP fructose 1,6 bisphosphate second ATP investment highly exergonic, essentially irreversible, G°´= kJ/mole - highly regulated, modulating carbon flux through glycolysis in response to energy and carbon requirements
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4- Cleavage to two triose phosphates Enzyme = aldolase
HC=O H2COP HCOH O=C HCOP CH2OH H glyceraldehyde dihydroxyacetone 3-phosphate phosphate where P = phosphate cleaves a 6C sugar to 2 3C sugars G°´= kJ/mole, driven by next Rx.
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5-Isomerization of Dihydroxyacetone phosphate
Enzyme = triose-phosphate isomerase H2C-OH C=O CH2-O- P dihydroxyacetone glyceraldehyde phosphate phosphate
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5-Isomerization of Dihydroxyacetone phosphate
Enzyme = triose-phosphate isomerase H2C-OH C=O CH2-O- P dihydroxyacetone glyceraldehyde phosphate phosphate
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only glyceraldehyde 3-phosphate can be used further in glycolysis.
allows interconversion of two triose phosphate products of aldolase cleavage only glyceraldehyde 3-phosphate can be used further in glycolysis. aldose-ketose isomerization similar to phosphoglucoisomerase rxn allows dihydroxyacetone phosphate to be metabolized as glyceraldehyde 3-phosphate reversible,G°´= +7.5 kJ/mole. This is important in gluconeogenesis
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Second Phase: End of First Phase: Production of two glyceraldehyde
**************************************** 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:
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+ 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 ,3 bisphosphoglycerate -addition of phosphate, oxidation, production of NADH, formation of high energy compound
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- first substrate level phosphorylation, yielding ATP
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 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.
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8- Phosphate shift setup Enzyme= phosphoglycerate mutase
- shifts phosphate from position 3 to 2 - reversible, ΔG = kJ/mole
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9- Generation of second very high energy compound Enzyme = enolase
-- little energy change in this reaction, ΔG = +1.7 kJ/mole because the energy is locked into enolphosphate
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phosphoenolpyruvate pyruvate second substrate level phosphorylation
10- Final generation of ATP Enzyme = pyruvate kinase P O H ADP ATP O -OOC-C=CH OOC-C-CH3 phosphoenolpyruvate pyruvate second substrate level phosphorylation yielding ATP - highly exergonic reaction, irreversible, ΔG = kJ/mole.
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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)
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Regulation of Glycolysis fig.6-4
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Regulation of Glycolysis
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
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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
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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
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