Citric Acid Cycle & Electron Transport Chain Flashcards
Citric Acid Cycle
- occurs in the matrix of the mitochondria
- indirectly requires oxygen to work – aerobic
- purpose is to complete oxidation of carbons in intermediates to CO2 so that reduction reactions can be couple with CO2 formation thus forming energy carriers such as NADH and FADH2 for ETC
Synthase
- enzyme that forms new covalent bonds without input of outside energy
- lyase
Citrate Formation
- requires oxaloacetate, acetyl CoA and H20 to form citrate
- intermediate of reaction: citryl CoA (formed through condensation reaction then H2O hydrolyzes this to form citrate)
- catalyzed by Citrate Synthase
Citrate Isomerization to Isocitrate
- catalyzed by Aconitase – metalloprotein that requires Fe2+ as a cofactor
- isomerization involves removing then adding H2O to create Citrate
Alpha-Ketoglutarate Formation
- catalyzed by Isocitrate Dehydrogenase – enzyme that oxidizes Isocitrate and reduces NAD+ to NADH
- decarboxylation reaction also occurs which leads to a release of a CO2, forming this 5 carbon product
Dehydrogenase
- enzyme that transfers H to an electron acceptor (ex. NADH, FADH2)
- subtype of oxidoreductases – enzymes that catalyze redox reactions
What is the rate-limiting step of the CAC?
isocitrate -> alpha-ketoglutarate
isocitrate dehydrogenase is the enzyme needed for this transition
Succinyl-CoA Formation
- catalyzed by alpha-ketoglutarate dehydrogenase complex
- NAD is reduced to NADH
- CO2 is produced, forming this 4 carbon product
Succinate Formation
- catalyzed by Succinyl-CoA Synthetase – hydrolysis of thioester bond yields succinate and CoA-SH which is coupled to phosphorylation of GDP -> GTP
- GTP can then go and transfer its phosphate to ADP to create ATP
- this is the only place in the CAC where energy currency is directly produced
Synthetases
- enzymes that create new covalent bonds but require energy input to do this
- Ligase
Fumarate Formation
- only step of CAC that occurs in inner mitochondria membrane
- catalyzed by Succinate Dehydrogenase – involved in transferring 2 H’s from succinate to FAD+
- succinate is oxidized and FAD is reduced to form FADH2
Where is the enzyme Succinate Dehydrogenase also used?
ETC – it is an essential enzyme of Complex II where it is involved in oxidizing FADH2 (donating high energy electrons to ETC)
Malate Formation
- catalyzed by Fumarase – hydrolyzes alkene bond in fumarate to form malate
- only forms L-malate
Oxaloacetate Formation
-catalyzed by Malate Dehydrogenase – malate is oxidized to OAA while NAD+ is reduced to NADH
What are the substrates in the CAC?
- Please, Can I Keep Selling Sex For Money, Officer?*
- Pyruvate
- Citrate
- Isocitrate
- alpha-Ketoglutarate
- Succinyl-CoA
- Succinate
- Fumarate
- Malate
- Oxaloacetate
Where are the 3 checkpoints of the CAC?
- citrate synthase (inhibitors: ATP, NADH, succinyl CoA, citrate)
- isocitrate dehydrogenase (inhibitors: ATP, NADH; stimulators: ADP, NAD+)
- alpha-ketoglutarate dehydrogenase (inhibitors: ATP, NADH, succinyl CoA; stimulators: ADP, NAD+, Ca2+)
What happens in a metabolically active cell?
ATP and NADH are converted to ADP and NAD+
Rise in ____ and ____ induces activation at all cell checkpoints
ADP and NAD+
Net Reaction of CAC (two turns for one glucose molecule)
2 Acetyl-CoA + 6 NAD+ + 2 FAD + 2 GDP + 2 Pi + 4 H2O -> 4 CO2 + 2 CoA-SH + 6 NADH + 6 H+ + 2 FADH2 + 2 GTP
How many ATP are produced per molecule of NADH?
2.5 ATP / NADH
How many ATP are produced per molecule of FADH2?
1.5 ATP / FADH2
How many ATP are produced per molecule of GTP?
1 ATP / 1 GTP
How many ATP are produced per pyruvate?
12.5 ATP / Pyruvate so 25 ATP / Glucose
What is the net yield of ATP for one glucose molecule from glycolysis through oxidative phosphorylation?
30-32 ATP
Electron Transport Chain
- occurs in inner membrane of mitochondria
- directly uses O2 – aerobic
- use of a proton motive force to produce ATP – protons moved from matrix to intermembrane space
- 2 goals: oxidize (“empty”) electron carriers, and make ATP
- from glycolysis and CAC, there are 10 NADH and 2 FADH2 that are used in ETC
Proton-Motive Force
- facilitates ATP generation
- as intermembrane space becomes more concentrated with H+ ions, the pH decreases (acidic) and the potential difference grows – creation of electrochemical gradient here is used by ATP Synthase to generate ATP
Overall, ETC is _____ [delta G _ 0] while ATP formation is ____ [delta G _ 0]
Overall, ETC is EXERGONIC [delta G < 0] while ATP formation is ENDERGONIC [delta G > 0]
-spontaneous exergonic process of electrons moving down their gradient is coupled to non-spontaneous endergonic process of ATP generation
What are the protein complexes in the ETC?
- Complex I
- Complex II
- UQ
- Complex III
- Complex IV
Complex I
- NADH is oxidized and UQ is reduced
- 2 H+ are transferred to coenzyme Q (UQ)
- 4 protons expelled into intermembrane space
- hydrogen ion pump
What is the net reaction at Complex I?
NADH + H+ + UQ -> NAD+ + UQH2
Complex II
- Succinate-CoQ Oxidoreductase
- FADH2/succinate are oxidized and UQ is reduced
- succinate transfers electrons to UQ (aka succinate dehydrogenase)
- complex serves as direct link to CAC
- 2 H+ are transferred to UQ
- no H+ pumping occurs here
What is the net reaction at Complex II?
succinate + UQ + 2 H+ -> fumarate + UQH2
Complex III
- CoQH2-Cyotochrome C Oxidoreductase
- UQ is oxidized and cytochrome C is reduced
- 2 electrons are shuttled from a molecule of UQH2 near the intermembrane space to a molecule of UQ near mitochondrial matrix, and another 2 electrons are attached to heme moieties and reduce two molecules of cytochrome c
- hydrogen ion pump
What is the net reaction at Complex III?
UQH2 + 2 cytochrome c [with Fe3+] -> UQ + 2 cytochrome c [with Fe2+] + 2 H+
Complex IV
- Cytochrome C Oxidase
- cytochrome c is oxidized and oxygen is reduced
- facilitates transfer of electrons (2e-) from cytochrome c to oxygen which is reduced in the process to form H2O
- 2 protons are pumped across the membrane
- hydrogen ion pump
What is the net reaction at Complex IV?
4 cytochrome c [with Fe2+] + 4 H+ + O2 -> 4 cytochrome c [with Fe3+] + 2 H2O
What molecule is the Mitochondria impermeable to?
NADH
NADH from ____ needs to use the NADH shuttle whereas NADH from ____ does not need to use this shuttle
- glycolysis
- CAC
Glycerol 3-Phosphate Shuttle
- NADH shuttle
- catalyzed by Glycerol 3-Phosphate Dehydrogenase
1. in the cytosol, DHAP (from glycolysis) is reduced to glycerol 3-phosphate, while NADH is oxidized to NAD+
2. glycerol 3-phosphate crosses membrane into matrix
3. in the matrix it is oxidized to DHAP, while reducing FAD to FADH2
4. FADH2 now enters ETC at Complex II where it transfers its electrons via UQ
5. DHAP cross the membrane and enters the cytosol where it can restart the process
Malate-Aspartate Shuttle
- NADH shuttle
1. cytosol contains Malate Dehydrogenase which reduces OAA to malate
2. malate cross membrane into matrix
3. malate is acted on by a second Malate Dehydrogenase that catalyzes the reverse reaction and produces OAA and NADH
4. NADH can now directly enter the ETC at Complex I
5. OAA is converted into aspartate via Aspartate Transaminase
6. aspartate crosses the membrane and enters the cytosol where it can be reconverted into OAA to restart process
What is the ATP yield from one NADH using the Glycerol 3-Phosphate Shuttle?
1.5 ATP
What is the ATP yield from one NADH using the Malate-Aspartate Shuttle?
2.5 ATP
Chemiosmotic Coupling
- process where chemical gradient is created (from ETC) and energy from gradient is used to phosphorylate ADP into ATP (ATP Synthase)
- promotes indirect relationship between proton gradient and synthesis of ATP by inducing conformational coupling – ATP is released from ATP Synthase as a result of conformational change caused by gradient
ATP Synthase
- protein complex that links the ETC with the final ATP product
- spans the entire mitochondrial membrane and sticks out into the matrix
- requires 4 H+ to make 1 ATP
- contains 2 portions: Fo and F1
- spare H’s created as a byproduct from ATP production here get stuck on O2 in matrix and create metabolic H2O which prevents the creation of an acidic environment from buildup of H’s
Fo Portion
- spans mitochondria membrane
- functions as an ion channel – protons travel along their gradient into the matrix
- exergonic process (-delta G)
F1 Portion
- spins within a stationary compartment to facilitate the harnessing of gradient energy for chemical bonding
- uses energy from Fo proton gradient to phosphorylate ADP to ATP
- endergonic process (+delta G)
What are the 2 regulators of Oxidative Phosphorylation?
- limited ADP or low O2
- abundant ADP or adequate O2
What happens when short on O2?
decrease in oxidative phosphorylation -> electron carriers (FADH2 and NADH) from CAC get backed up -> inhibition of CAC
What happens when their is a buildup of ADP?
activation of CAC -> elevated levels of electron carriers (NADH and FADH2) -> increased ETC activity -> increased oxidative phosphorylation
What is the result of a pH increase in the mitochondrial intermembrane space?
decreased proton gradient and decreased ATP synthesis
What is commonly associated with accumulation of NADH in respiratory control?
CAC inhibition
