Carbohydrate Metabolism

What is glycolysis?

A molecule of glucose is degraded in a series of enzyme-catalysed reactions to yield two molecules of the three-carbon compound pyruvate (some of the free energy is released in the form of ATP and NADH)

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What is fermentation?

A general term for anaerobic degradation of glucose or other organic nutrients to obtain energy conserved as ATP

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How many steps are involved in glycolysis?

10

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Describe what happens in the first five steps of glycolysis

In the preparatory phase, ATP is invested, raising the free energy content of the intermediates and carbon chains are converted into glyceraldehyde 3-phosphate

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In which phase is energy gained?

The payoff phase

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For each molecule of glucose, how many molecules of ATP are generated?

Net gain of 2 molecules of ATP

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Is the conversion of glucose to pyruvate exergonic or endergonic?

Exergonic

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Is the formation of ATP exergonic or endergonic?

Endergonic

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Under standard conditions, is glycolysis a reversible process?

Glycolysis is an irreversible process, driven to completion by a large net decrease in free energy

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Describe the first step in glycolysis

Glucose is activated by phosphorylation at C6 to produce glucose-6-phosphate (ATP is the phosphoryl donor). Irreversible reaction. Catalysed by hexokinase. Exergonic

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Describe the function of hexokinase

Requires Mg2+ (true substrate is MgATP2- complex). Mg2+ shields negative charges of phosphoryl groups in ATP, making terminal P atom an easier target for nucleophilic attack by OH of glucose. Induced fit/blocks water. Soluble, cytosolic protein

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What are isozymes?

Enzymes which catalyse the same reaction but are encoded in different genes

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Describe the second step in glycolysis

The enzyme phosphohexose isomerase (phosphoglucose isomerase) catalyses the reversible isomerisation of glucose-6-phosphate (aldose) to fructose-6-phosphate (ketose). Endergonic

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Describe the third step in glycolysis

Phosphofructokinase (PFK-1) catalyses the transfer of phosphoryl group from ATP from fructose-6-phosphate to yield fructose 1,6-bisphosphate. Irreversible reaction. Exergonic

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Describe the features of PFK-1

It is a regulatory enzyme (bacteria/protoctists/plants have PFK that use pyrophosphate as phosphoryl group donor in synthesis of fructose 1,6-bisphosphate

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What is the function of PFK-2?

Catalyses the formation of fructose 2,6-bisphosphate from fructose-6-phosphate in a separate pathway

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Describe the fourth step in glycolysis

Fructose 1,6-bisphosphate aldose (aldolase) is used to catalyse a reversible aldol condensation. Fructose 1,6-bisphosphate is cleaved to form glyceraldehyde 3-phosphate (aldose) and dihydroxyacetone phosphate (ketose). Endergonic

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Describe the classes of enzymes

Class I (in animals and plants), Class II in fungi/bacteria (use of Zn2+). Aldose is a positive standard free energy change in direction of forward/cleavage reaction (small energy at lower concentrations)

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Describe the fifth step in glycolysis

Only glyceraldehyde 3-phosphate can be directly degraded. Dihydroxyacetone phosphate is reversibly converted to glyceraldehyde 3-phosphate by triose phosphate isomerase. Endergonic reaction

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In the conversion of two molecules of glyceraldehyde to two molecules of pyruvate leads to the formation of how many molecules of ATP?

4 (net gain of 2 due to 2 ATP molecules invested in preparatory phase to phosphorylate two ends of hexose molecule)

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Describe the sixth step of glycolysis

Glyceraldehyde 2-phosphate is oxidised to 1,3-biphosphoglycerate using glyceraldehyde 3-phosphate dehydrogenase. Endergonic. Aldehyde oxidised to acid anhydride with phosphoric acid (acyl phosphate/high free energy)

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Describe the seventh step in glycolysis

Phosphoglycerate kinase transfers pi group to carboxyl group of 1,3-bisphosphoglycerate to ADP to form ATP and 3-phosphoglycerate. Reversible. Exergonic (coupled with step 6/overall reaction as exergonic). Substrate level phosphorylation

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Describe the eight step in glycolysis

Phosphoglycerate mutase catalyses the reversible shift of phosphoryl group between C2 and C3 of glycerate. Mg2+. Endergonic.

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Describe the ninth step in glycolysis

Enolase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate (PEP). Endergonic. High free energy of hydrolysis of phosphoryl group. Redistribution of energy (due to same energy in molecules)

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Describe the tenth step in glycolysis

Phosphoryl group is transferred from phosphoenolpyruvate to ADP. Catalysed by pyruvate kinase (requires K+ and either Mg2+ or Mn2+). Tautomerization. Negative free energy. Spontaneous. Driving force/ATP synthesis. Exergonic. Irreversible

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Describe the process of lactic acid fermentation

Pyruvate is converted to L-lactate. NADH is oxidised. Lactate dehydrogenase used. Exergonic

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Describe the process of ethanol fermentation

Pyruvate is converted into acetaldehyde using pyruvate dehydrogenase (CO2 produced). Acetaldehyde is converted to ethanol using alcohol dehydrogenase (NAD is oxidised)

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Where does glycolysis occur?

Cytosol in cell and in the mitochondrion

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How many molecules of ATP are produced in anaerobic respiration?

2 ATP molecules per glucose molecule

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How many molecules of ATP are produced in aerobic respiration?

38 ATP molecules per glucose molecule

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Carbohydrate Metabolism

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