Download PATHWAYS THAT HARVEST CHEMICAL ENERGY CHAPTER 9

PATHWAYS THAT HARVEST CHEMICAL ENERGY
CHAPTER 9
LECTURE OBJECTIVES
How Does Glucose Oxidation Release Chemical Energy?
What Are the Aerobic Pathways of Glucose Metabolism?
How Does Oxidative Phosphorylation Form ATP?
How Is Energy Harvested from Glucose in the Absence of Oxygen?
How Are Metabolic Pathways Interrelated and Regulated?
CELLULAR RESPIRATION
Main fuel of the cells
Complex metabolic pathway
Enzymatic reactions
In organelles (Mitochondrion)
Similar in different organisms
GENERAL REACTION
Glucose oxidation is an exergonic reaction
ΔG is the change in free energy
ΔG from complete combustion of glucose
= –686 kcal/mol
Three metabolic pathways are involved in harvesting the energy of glucose:
Glycolysis
Cellular respiration
Fermentation
ENERGY FOR LIFE
REDOX REACTIONS

OXIDATION STATES AND ENERGY
Coenzyme NAD+ is a key electron carrier in redox reactions
Two forms:


NAD+ (oxidized)
NADH (reduced)
OXYGEN ACCEPTS ELECTRONS FROM NADH
Energy Producing Metabolic Pathways
Where in the cell do these metabolic pathways occur?

GLYCOLYSIS
takes place in the cytosol:
Converts glucose into pyruvate
Produces a small amount of energy
Generates no CO2
GLYCOLYSIS: 10 RXNS
• Energy-investing reactions one–five require ATP.
• Energy-harvesting reactions six–ten yield NADH and ATP.
Results in: 2 molecules of pyruvate
2 molecules of ATP
2 molecules of NADH
GLUCOSE INTO PYRUVATE
PYRUVATE OXIDATION
• Links glycolysis and the citric acid cycle; occurs in the mitochondrial matrix
• Pyruvate is oxidized to acetate and CO2 is released
• NAD+ is reduced to NADH, capturing energy
• Some energy is stored by combining acetate and Coenzyme A (CoA) to form
acetyl CoA
CHANGES IN FREE ENERGY DURING THE PATHWAY
AEROBIC PATHWAY
• Acetyl CoA is the starting point of the eight –reaction citric acid cycle:
• Inputs: acetyl CoA, water and electron carriers NAD+, FAD, and GDP
• Energy released is captured by ADP and electron carriers NAD+, FAD, and GDP
• Outputs: CO2, reduced electron carriers, and GTP, which converts ADP to ATP
Pyruvate Oxidation and Citric Acid Cycle
•
Fermentation—if no O2 is present
•
Oxidative phosphorylation—O2 is present
OXIDATIVE PHOSPHORYLATION
Electron Transport (ETC)
• Electrons from NADH and FADH2 pass through the respiratory chain
• Electron flow results in a proton concentration gradient in mitochondria.
Chemiosmosis
• Protons diffuse back into the mitochondria through ATP synthase, a channel protein.
• Diffusion is coupled to ATP synthesis
ELECTRON TRANSPORT CHAIN
CHEMIOSMOSIS
•
•
ATP synthesis can be uncoupled: If a different H+ diffusion channel is inserted into the
mitochondrial membrane, the energy is lost as heat.
The uncoupling protein thermogenin occurs in human infants and hibernating
animals—H+ is released as heat instead of coupled to ATP synthesis
FERMENTATION (ETHANOL)
FINAL PRODUCT
Cellular respiration yields more energy than fermentation per glucose molecule.
• Glycolysis plus fermentation = 2 ATP
• Glycolysis plus cellular respiration = 32 ATP
• In some cells NADH must be shuttled using ATP and net result = 30 ATP
USING OTHER NUTRIENTS FOR CELLULAR RESPIRATION
CATABOLIC CONVERSIONS
• Polysaccharides → hydrolyzed to glucose, enters glycolysis and cellular respiration
• Lipids → broken down to:
glycerol → DAP
fatty acids → acetyl CoA
• Proteins → hydrolyzed to amino acids—feed into glycolysis or the citric acid cycle
ANABOLIC CONVERSIONS
• Most catabolic reactions are reversible
• Gluconeogenesis: Glucose formed from citric acid cycle and glycolysis intermediates
REGULATING METABOLIC PATHWAYS
ALLOSTERIC REGULATION
CONTROLS OF CELLULAR RESPIRATION
• Glycolysis: phosphofructokinase—allosterically inhibited by ATP.
• Citric acid cycle: isocitrate dehydrogenase—inhibited by NADH + H+ and ATP.
• If ATP levels are high:
1. Accumulation of citrate diverts acetyl CoA to fatty acid synthesis, for storage.
2. Fatty acids may be metabolized later to produce more acetyl CoA.