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Overview of catabolic pathways. Catabolism of glucose via glycolysis and the citric acid cycle . NADH. NADH, FADH 2. Converts: 1 glucose 2 pyruvate. Net reaction of glycolysis. +. Two molecules of ATP are produced Two molecules of NAD + are reduced to NADH.

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net reaction of glycolysis

Converts: 1 glucose2 pyruvate

Net reaction of glycolysis

+

  • Two molecules of ATP are produced
  • Two molecules of NAD+ are reduced to NADH

Glucose + 2 ADP + 2 NAD+ + 2 Pi

2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

glycolysis can be divided into two stages
Glycolysis can be divided into two stages
  • Hexosestage: 2 ATP are consumed per glucose
  • Triosestage: 4 ATP are produced per glucose
  • Net: 2 ATP produced per glucose

x2

fig 11 2
Fig 11.2

Transfer of a phosphorylgroup from ATP to glucose

enzyme: hexokinase

1

Isomerization of glucose 6-phosphateto fructose 6-phosphate

enzyme: glucose 6-phosphate isomerase

2

slide7

Transfer of a secondphosphoryl group from ATP to fructose 6-phosphate

enzyme: phosphofructokinase-1

3

Carbon 3 – Carbon 4bond cleavage, yielding twotriose phosphates

enzyme: aldolase

4

enzyme 1 hexokinase
Enzyme 1. Hexokinase
  • Transfers the g-phosphoryl of ATP to glucose C-6 oxygen to generate glucose 6-phosphate
  • Mechanism: attack of C-6 hydroxyl oxygen of glucose on the g-phosphorous of MgATP2- displacing MgADP-

Fig 11.3

2 glucose 6 phosphate isomerase
2. Glucose 6-Phosphate Isomerase
  • Converts glucose 6-phosphate (an aldose) to fructose 6-phosphate (a ketose)
  • Enzyme converts glucose 6-phosphate to open chain form in the active site

Fig 11.4

3 phosphofructokinase 1 pfk 1
3. Phosphofructokinase-1 (PFK-1)
  • Catalyzes transfer of a phosphoryl group from ATP to the C-1 hydroxyl group of fructose 6-phosphate to form fructose 1,6-bisphosphate (F1,6BP)
  • PFK-1 is metabolically irreversible and a critical regulatorypoint for glycolysis in most cells (PFK-1 is the first committed step of glycolysis)
4 aldolase
4. Aldolase
  • Aldolase cleaves the hexose fructose 1,6-bisphosphate into two triose phosphates: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
5 triose phosphate isomerase
5. Triose Phosphate Isomerase
  • Conversion of dihydroxyacetone phosphate into glyceraldehyde 3-phosphate
6 glyceraldehyde 3 phosphate dehydrogenase gapdh
6. Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH)
  • Conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate (1,3BPG)
  • Molecule of NAD+ is reduced to NADH
box 11 2 arsenate aso 4 3 poisoning
Box 11.2 Arsenate (AsO43-) poisoning
  • Arsenate can replace Pi as a substrate for glyceraldehyde 3-phosphate dehydrogenase
  • Arseno analog which forms is unstable
7 phosphoglycerate kinase
7. Phosphoglycerate Kinase
  • Uses uses the high-energy phosphate of 1,3-bisphosphoglycerate to form ATP from ADP
  • Transfer of phosphoryl group from the energy-rich 1,3-bisphosphoglycerate to ADP yields ATP and 3-phosphoglycerate
8 phosphoglycerate mutase
8. Phosphoglycerate Mutase
  • Catalyzes transfer of a phosphoryl group from one part of a substrate molecule to another
  • Reaction occurs without input of ATP energy
9 enolase 2 phosphoglycerate to phosphoenolpyruvate pep
9. Enolase: 2-phosphoglycerate to phosphoenolpyruvate (PEP)
  • Elimination of water (dehydration) yields PEP
  • PEP has a veryhigh phosphoryl group transfer potential because it exists in its unstable enol form
10 pyruvate kinase
10. Pyruvate Kinase
  • Metabolically irreversible reaction
fates of pyruvate
Fates of pyruvate
  • For centuries, bakeries and breweries have exploited the conversion of glucose to ethanol and CO2 by glycolysis in yeast
metabolism of pyruvate
Metabolism of Pyruvate

1. Aerobicconditions: pyruvate is oxidized to acetyl CoA, which enters the citric acid cycle for further oxidation

2. Anaerobicconditions (microorganisms): pyruvate is converted to ethanol

3. Anaerobicconditions (muscles, redbloodcells): pyruvate is converted to lactate

fig 11 10
Fig 11.10
  • Fates of pyruvate
fig 11 11
Fig 11.11
  • Two enzymes required:
  • (1) Pyruvate decarboxylase
  • (2) Alcohol dehydrogenase
  • Anaerobic conversion of pyruvate to ethanol (yeast - anaerobic)
reduction of pyruvate to lactate muscles anaerobic
Reduction of Pyruvate to Lactate (muscles - anaerobic)
  • Muscles lack pyruvate dehydrogenase and cannot produce ethanol from pyruvate
  • Muscle lactate dehydrogenase converts pyruvate to lactate
  • This reaction regenerates NAD+ for use by glyceraldehyde 3-phosphate dehydrogenase in glycolysis
  • Lactate formed in skeletal muscles during exercise is transported to the liver
  • Liver lactate dehydrogenase can reconvert lactate to pyruvate
  • Lactic acidosis can result from insufficient oxygen (an increase in lactic acid and decrease in blood pH)
metabolically irreversible steps of glycolysis
Metabolically Irreversible Steps of Glycolysis
  • Enzymes not reversible: Reaction 1 - hexokinase Reaction 3 - phosphofructokinase Reaction 10 - pyruvate kinase
  • These steps are metabolicallyirreversible, and these enzymes are regulated
  • All other steps of glycolysis are near equilibrium in cells and not regulated
regulation of glycolysis
Regulation of Glycolysis
  • 1. When ATP is needed, glycolysis is activated
  • Inhibition of phosphofructokinase-1 is relieved.
  • Pyruvate kinase is activated.
  • 2. When ATP levels are sufficient, glycolysis activity decreases
  • Phosphofructokinase-1 is inhibited.
  • Hexokinase is inhibited.
fig 11 14 regulation of glucose transport
Fig 11.14 Regulation of glucose transport
  • Glucose uptake into skeletal and heart muscle and adipocytes by GLUT 4 is stimulatedbyinsulin
regulation of hexokinase
Regulation of Hexokinase
  • Hexokinase reaction is metabolically irreversible
  • Glucose 6-phosphate (product) levels increase when glycolysis is inhibited at sites further along in the pathway
  • Glucose 6-phosphate inhibits hexokinase.
regulation of phosphofructokinase 1 pfk 1
Regulation of Phosphofructokinase-1 (PFK-1)
  • ATP is a substrate and an allostericinhibitor of PFK-1
  • High concentrations of ADP and AMP allosterically activate PFK-1 by relieving the ATP inhibition.
  • Elevated levels of citrate (indicate ample substrates for citric acid cycle) also inhibit Phospofructokinase-1
regulation of pfk 1 by fructose 2 6 bis phosphate f2 6bp
Regulation of PFK-1 by Fructose 2,6-bisphosphate (F2,6BP)
  • Fructose 2,6-bisphosphate is formed from Fructose 6-phosphate by the enzyme phosphofructokinase-2 (PFK-2)
  • Fig 11.17 Fructose 2,6-bisphosphate
fig 11 18
Fig. 11.18
  • Effect of glucagon on liver glycolysis

Chapter 11

regulation of pyruvate kinase pk
Regulation of Pyruvate Kinase (PK)
  • Pyruvate Kinase is allosterically activated by Fructose 1,6-bisphosphate, and inhibited by ATP
  • The hormone glucagon stimulates protein kinase A, which phosphorylates Pyruvate Kinase, converting it to a lessactive form.

Chapter 11

other sugars can enter glycolysis
Other Sugars Can Enter Glycolysis
  • Glucose is the mainmetabolicfuel in most organisms
  • Other sugars convert to glycolytic intermediates
  • Fructose and sucrose (contains fructose) are major sweeteners in many foods and beverages
  • Galactose from milk lactose (a disaccharide)
  • Mannose from dietary polysaccharides, glycoproteins

Chapter 11

formation of 2 3 bis phosphoglycerate in red blood cells
Formation of 2,3-Bisphosphoglycerate in Red Blood Cells
  • 2,3-Bisphosphoglycerate (2,3BPG) allosterically regulates hemoglobin oxygenation (red blood cells)
  • Erythrocytes contain bisphosphoglycerate mutase which forms 2,3BPG from 1,3BPG
  • In red blood cells about 20% of the glycolytic flux is diverted for the production of the important oxygen regulator 2,3BPG

Chapter 11

fig 11 24
Fig 11.24
  • Formation of 2,3BPG in red blood cells
feedback inhibition
Feedback inhibition
  • Product of a pathway controls the rate of its own synthesis by inhibiting an enzyme catalyzing an early step

Feed-forward activation

  • Metabolite early in the pathway activates an enzyme further down the pathway

Chapter 11

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