Download - Chapter 14 Slides 2017 - Calvin University
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CHAPTER 14: Metabolism and Bioenergetics
SUMMARY OF CATABOLISM• Your body is capable of
“burning” many sorts of fuel to produce ATPØ Carbohydrates
Ø Lipids
Ø Proteins
• All three “fuels” feed into the citric acid cycle to complete catabolism:Ø Begins with acetyl-CoAØ Requires O2
Ø Occurs in the mitochondria
• 14.1 Acetyl CoA and the Citric Acid Cycle
• 14.2 Oxidative Phosphorylation
• 14.3 Entropy and Bioenergetics (not covered)
CHAPTER 14: Metabolism and Bioenergetics
OUTLINE
• Acetyl CoA is produced between the second & third stages of carbohydrate catabolism:
Ø Both proteins & fat can also can be converted into acetyl-CoA to produce ATP
• Further catabolism of acetyl-CoA forms NADH & FADH2 during the citric acid cycle.Ø NADH & FADH2 are re-oxidized to make ATP
CHAPTER 14: Metabolism and Bioenergetics
ACETYL-CoA PRODUCTION FROM PYRUVATE
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• Acetyl CoA is a complex molecule consisting of pantothenic acid (vitamin B5), a thiol amine, and ADP, and the acetyl group
• Coenzyme A can carry other groups besides the acetyl in other metabolic pathways (ie. acyl chains)
CHAPTER 14: Metabolism and Bioenergetics
ACETYL CoA
• The key to Coenzyme-A chemistry is the thiolgroup located at the very end of the molecule:
CHAPTER 14: Metabolism and Bioenergetics
THE CHEMISTRY OF ACETYL CoA
The thiol can react with a variety of carbonyl-containingmolecules to form a high-energy thioester bond
• If oxygen is present, pyruvate is oxidized to acetyl-CoA in preparation for the citric acid cycle:Ø Requires both coenzyme A (SH-CoA) and NAD+
Ø Reaction involves the loss of a CO2 (decarboxylation)
• Reaction is catalyzed by the mitochondrial enzyme pyruvate dehydrogenase
CHAPTER 12: Carbohydrates: Structure and Function
OXIDATION OF PYRUVATE TO ACETYL-CoA
Oxidation + CO2 loss
Oxidative Decarboxylation
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• The citric acid cycle is called a “cycle” because it begins and ends with the same molecule:
CHAPTER 14: Metabolism and Bioenergetics
THE CITRIC ACID CYCLE
SH-CoA
The NET reaction is completeoxidation of carbons to CO2
• All eight steps of the citric acid cycle occur in the mitochondrial matrix:
CHAPTER 14: Metabolism and Bioenergetics
CARBON ATOMS IN THE CITRIC ACID CYCLE
Citric acid cycle enzymes are in the mitochondrial matrix
• Same location as beta-oxidation• Reduced products (NADH & FADH2)
feed directly into the electron transport chain in the inner membrane
Reaction Types:
1. Redox reactions:Ø Oxidation of carbonyl
groups produces NADH (Redox A)
Ø Oxidation of an alkane to alkene produces FADH2 (Redox B)
2. Thioester hydrolysis
3. Isomerization4. Hydration
CHAPTER 14: Metabolism and Bioenergetics
DETAILED VIEW OF THE CITRIC ACID CYCLE
Redox A
Redox A
Redox A
Redox B
Thioester hydrolysis
Thioester hydrolysis
IsomerizationHydration
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• After going thru Glycolysis and the Citric Acid Cycle, what are the net products of glucose catabolism?
CHAPTER 14: Metabolism and Bioenergetics
PRODUCT ACCOUNTING FOR GLUCOSE
Glycolysis2 Pyruvates2 ATP2 NADH
Intermediate Step1 Acetyl-CoA1 NADH1 CO2
x 2
Citric Acid Cycle1 GTP 1 FADH2
3 NADH2 CO2
x 2
NET PRODUCTS
4 ATP/GTP 6 CO2
2 FADH2
10 NADH+
CHAPTER 14: Metabolism and Bioenergetics
ATP PRODUCTION BY OXPHOS• Most of the ATP derived from glucose catabolism
comes from the electrons carried by NADH and FADH2:
• Oxidative phosphorylation (OXPHOS) uses energy derived from the electrons of NADH and FADH2 in order to generate ATP:Ø Requires the mitochondrial electron transport chainØ Involves addition of an inorganic phosphate (Pi) to ADP
to create ATP
Ø Only 4 ATP or GTP molecules are made directly…..
CHAPTER 14: Metabolism and Bioenergetics
MITOCHONDRIAL ARCHITECTURE
• Mitochondria have two separate membranes:Ø Inner mitochondrial membrane
Ø Outer mitochondrial membrane
• The membranes define twounique “compartments”:Ø Intermembrane space (IMS),
between the two membranes
Ø Matrix, the region within the inner membrane
Folds in the inner mito membrane (cristae) create more surface area
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CHAPTER 14: Metabolism and Bioenergetics
OXPHOS PROTEIN COMPLEXES• Proteins involved in the electron transport chain
and oxidative phosphorylation are found in the inner mitochondrial membrane (IMM):Ø These are transmembrane protein complexes that
actually span the inner membrane phospholipid bilayer
Electrons from NADH and FADH2 are carried through the chain, ending at oxygen as the final acceptor
4H+ + O2 à 2 H2O
e-
NADHFADH2e-
e-
CHAPTER 14: Metabolism and Bioenergetics
ELECTRON TRANSPORT• Electrons are transferred to the FOUR electron transport
complexes by oxidation-reduction reactions in metal centers of these proteins:Ø The metal centers consist of iron or copper ions, alternately oxidizing
and reducing between Fe2+/Fe3+ or Cu1+/Cu2+
• Transfer of electrons between the complexes involves two extra “electron shuttles”:Ø Coenzyme Q = a small organic co-factor
Ø Cytochrome C = protein with heme-like cofactor (iron ion center)
What sort of redox reaction is involved in Coenzyme Q electron transfer?
CHAPTER 14: Metabolism and Bioenergetics
DIRECTION OF ELECTRON FLOW• Electron flow is determined by relative electron
affinities for each of the components in the electron transport chain:
Ø Electrons always flow from low electron affinity to highelectron affinity: § NADH & FADH2 have low
affinity for electrons§ The final electron acceptor
(O2) has the highest affinity
Ø The difference between these electron affinities is inversely proportional to their potential energy
Low e- affinity
High e- affinity
Pote
ntia
l ene
rgy à
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CHAPTER 14: Metabolism and Bioenergetics
ELECTRON TRANSPORT DRIVES PROTON PUMPING INTO THE IMS
The energy from electron transport is used to “pump” protons across the inner membrane……
…..this creates at pH gradient
CHAPTER 14: Metabolism and Bioenergetics
ANALOGY TO WATER WHEEL PUMPSThe potential energy of flowing water can be harnessed to produce mechanical or electricalenergy using a water wheel
Complex I
Complex III
Complex IV
Complex II
Now imagine using this energy to run a “pump”
protons
CHAPTER 14: Metabolism and Bioenergetics
THE REAL ELECTRON TRANSPORT CHAIN• The electron transport chain complexes are
molecular “proton pumps”• Each complex is composed of many protein
subunits that work together
http://www.nature.com/nrm/journal/v16/n6/images/nrm3997-f1.jpg
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CHAPTER 14: Metabolism and Bioenergetics
INITIATING ELECTRON TRANSPORT• Three of the four complexes in the electron
transport chain also function as “proton pumps”:1. Complex I – receives e- from NADH à pumps H+
2. Complex II – receives e- from FADH2, but does NOT pump
3. Complex III – receives e- from Coenzyme-Q à pumps H+
4. Complex IV – receives e- from Cytochrome C à pumps H+
• Complex IV reduces molecular oxygen with the electrons it receives:
O2 + 4 H+ + 4 e- à 2 H2O Ø The deadly poison cyanide (CN-) blocks the final step
of electron transport, stopping the entire transport chain
CHAPTER 14: Metabolism and Bioenergetics
THE PROTON GRADIENT• Protons (H+) are unable to diffuse through the
inner mitochondrial membrane by simple diffusion because they are charged:Ø There are more protons in the intermitochondrial space
(lower pH) than in the matrix (higher pH).
Ø This proton gradient is maintained through the action of the electron transport chain
H+
H+H+H+
H+H+
H+
H+
H+
H+
H+H+H+
H+H+
H+H+
H+
H+
H+
CHAPTER 14: Metabolism and Bioenergetics
THE PROTON-MOTIVE FORCE• The proton gradient between the inner membrane
space (IMS) and matrix is another form of potential energy for the cell to use:Ø The energy in this unequal distribution of protons is
called the proton-motive force
Ø The only way for protons to diffuse back down this gradient is through ATP synthase
Ø This flow of protons is harnessed to drive ATP synthesis
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CHAPTER 14: Metabolism and Bioenergetics
PHOSPHORYLATION OF ADP• Phosphorylation of ADP to yield ATP
requires significant energy input:
Energy + ADP + Pi ® ATP + H2O
Ø The energy for this process comes from the proton gradient set up by the electron transport chain across the IMM
• ATP synthase is the enzyme responsible for producing ATP in the mitochondria:Ø Complex protein that spans the inner
mitochondrial membrane (IMM)Ø Contains a “channel” thru which H+ flowØ Proton flow drives rotary motion to combine
ADP with inorganic phosphate (Pi)
ATP synthase is a rotary “machine”
• The electron carrying capacity of redox cofactors can be translated into specific amounts of ATP:Ø Each FADH2 à ~1.5 ATPs Ø Each NADH à ~ 2.5 ATPs
CHAPTER 14: Metabolism and Bioenergetics
ENERGY FROM GLUCOSE OXIDATION
Glycolysis2 ATP2 NADH = 5 ATP
Citric Acid Cycle1 GTP ~ 1 ATP 1 FADH2 = 1.5 ATP3 NADH = 7.5 ATP
2x
Intermediate Step1 NADH = 2.5 ATP2x
7 ATP
5 ATP
20 ATP
32 ATPper glucose
1 glucose
2 pyruvate
2 acetyl-CoA
• The electron carrying capacity of redox cofactors can be translated into specific amounts of ATP:Ø Each FADH2 à ~1.5 ATPs Ø Each NADH à ~ 2.5 ATPs
CHAPTER 14: Metabolism and Bioenergetics
ENERGY FROM PALMITATE OXIDATION
β-oxidation (of C16:0)8 acetyl-CoA7 FADH27 NADH
Citric Acid Cycle1 GTP ~ 1 ATP 1 FADH2 = 1.5 ATP3 NADH = 7.5 ATP
8x
10.5 ATP17.5 ATP
80 ATP
106 ATPper palmitate8 acetyl-
CoA
FA Activation Step+ SH-CoA -2 ATP1 palmitate
1 palmitoyl-CoA
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CHAPTER 14: Metabolism and Bioenergetics
SELF-REVIEW QUESTIONS1. What important role does oxygen play in the
electron transport chain?
2. Explain how cyanide shuts down the electron transport chain.
3. What important chemical reaction is driven by the flow of protons from the intermembrane space back into the matrix?
4. What is the proton-motive force? How does it drive phosphorylation of ADP?
1. How many molecules of NAD+ are reduced to NADH in the citric acid cycle?
2. How many molecules of FAD are reduced to FADH2 in the citric acid cycle?
3. How many molecules of ADP are converted into ATP as a result of one pass through the citric acid cycle, assuming all the NADH and FADH2 are used to phosphorylate ADP to ATP?
CHAPTER 14: Metabolism and Bioenergetics
PRACTICE PROBLEMS
CHAPTER 14: Metabolism and Bioenergetics
METABOLISM REVIEW1. In stage 1, macromolecules
are digested through hydrolysis into molecules small enough to pass into the bloodstream.
2. In stage 2, these molecules are oxidized to form acetyl CoA or other molecules able to enter the citric acid cycle as well as NADH and FADH2, carrying electrons to electron transport.
3. In stage 3, the reduction of oxygen releases energy to drive the proton-motive force used to make ATP.
Central Biomolecule Catabolism