Chapter 19 Flashcards
Overview of Citric Acid Cycle (CAC)
- CAC = biochemical hub of cell
- Receives and delivers compounds
- Has key role in oxidation of various fuel sources
- Before fuel sources can enter cycle, MUST be converted into acetyl-CoA (acetyl-CoA is entry point into cycle)
- Oxidation reactions happening in cycle generate high energy electrons captured as FADH2 and NADH, which are used to make ATP in oxidative phosphorylation through ETC
- 1 ATP generated from 1-turn of cycle
CAC – First Stage of Cellular Respiration
- CAC constitutes the first stage in cellular respiration
– Various fuel source can channel into cycle as acetyl-CoA
–> Fuel sources include glucose, fatty acids, or amino acids
- From the carbon fuel sources, high energy electrons are removed for oxidative phosphorylation
–> These electrons are used to reduce oxygen, this generates a proton gradient
–> This proton gradient is used to synthesize ATP (green)
–> Reduction of oxygen + synthesis (creation) of ATP = oxidative phosphorylation
- CAC located within mitochondrial matrix
General over of the CAC: 2-Parts
PART 1: Oxidation of carbon atoms to Co2
- Joining 2-carbon acetyl from acetyl-CoA to a 4-carbon oxaloacetate molecule to create one 6-carbon molecule of citrate
- As citrate moves through first part of cycle, 2 CO2 molecules are liberated (released) in process of oxidative decarboxylation, which yields a 4-carbon carbon molecule and 2 NADH molecules
PART 2: Regeneration of 4-carbon Oxaloacetate
- To regenerate oxaloacetate there’s a rearrangement of the 4-carbon compound through series of reactions
- Energy is harvested as ATP (GTP), NADH and FADH2 also produced
Stage 1 - Step 1: Citrate Synthase
- Cycle begins w/ condensation of oxaloacetate (4-C), and acetyl CoA (2-C), to generate citrate. Reaction is catalyzed by enzyme citrate synthase
Oxaloacetate reacts w/ acetyl-CoA and H2O to yield citrate and CoA
- Oxaloacetate condenses w/ acetyl CoA to form citryl CoA (a molecule that’s energy-rich b/c it contains the thioester that originated in acetyl CoA)
- Thioester hydrolysis of citryl-CoA drives the entire reaction and is specific to citryl-CoA ~ enzyme citrate synthase functions in induced fit manner, leading to conformational change when oxaloacetate binds to active site of enzyme citrate synthase
- Citryl-CoA is cleaved (split) to form citrate and CoA
Stage 1 - Step 2: Citrate Isomerization
- Citrate is isomerized into isocitrate, catalyzed by enzyme aconitase
- Isomerization of citrate is accomplished by a dehydration step followed by a hydration step ~ aconitase moves hydroxyl from central carbon closer to terminal carbon (interchange of an H and an OH)
**citrate --> cis-aconitate --> isocitrate**
- Enzyme catalyzing both steps is aconitase b/c cis-aconitate is an intermediate
- Isomerization ensures that isocitrate is set up to undergo oxidative decarboxylation
Stage 1 - Step 3: Oxidative Decarboxylation
- Isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to generate alpha-ketoglutarate
- Hydroxyl group from isocitrate is converted into carboxyl group on oxalosuccinate ~ an intermediate of the reaction and an unstable alpha-ketoacid
- While oxalosuccinate is bound to the enzyme isocitrate dehydrogenase, it loses CO2 ~ this forms alpha-ketoglutarate
- During the reaction, high energy electrons are captured by NAD+ to form NADH
- During reaction, high energy electrons, through oxidation, are captured by NAD+ to form the first high-transfer-potential electron carrier in cycle, NADH
Stage 1 - Step 4: Succinyl CoA formed from a-ketoglutarate
A second oxidative decarboxylation reactions occurs, where the removal of CO2 from a-ketoglutarate (5-C) occurs to form succinyl-CoA (4-C); catalyzed by a-ketoglutarate dehydrogenase (KD) complex
During the reaction, NADH is produced, along with a thioester w/ high energy transfer potential
- oxidative decarboxylation takes place by the interactions of the 3 subunits of the complex
- assembly of 3 enzymes in KD complex is similar to pyruvate dehydrogenase complex
Stage 2 - Step 1: High Energy Transfer from Succinyl CoA
Cleavage (split) of the thioester of succinyl CoA is coupled to the phosphorylation ADP (a purine nucleoside diphosphate), catalyzed by succinyl CoA synthetase (succinate thiokinase)
- 1 ATP is created
- Formation of ATP by succinyl-CoA synthetase is an example of substrate-level phosphorylation (succinyl phosphate donates a phosphate to ADP)
- In stage 2, oxaloacetate is regenerated, and, in the process, energy rich electrons will be harvested
Stage 2 - Step 1: Histidine residue in active site of enzyme facilitates the transfer of an orthophosphate from succinyl phosphate to ADP. Explain the steps to this reaction
- Orthophosphate displaces CoA, which generates another energy-rich compound, succinyl phosphate
- A histidine residue removes the phosphoryl group w/ the concomitant generation of succinate and phosphohsitdine
- Phosphohistidine residue swings over to a bound nucleoside diphosphate
- Phosphoryl group is transferred to form the nucleoside triphosphate
Stage 2 - Step 2, 3, 4: Oxidation of Succinate to Oxalacetate
Methylene group (-CH2) is converted into carbonyl group(C=O) in 3 steps: an oxidation, a hydration, and second oxidation reaction. Oxaloacetate is regenerated for another round of the cycle, and more energy is extracted in form of FADH2 and NADH
- First reaction, catalyzed by succinate dehydrogenase, is an oxidation reaction that converts succinate into fumarate
- hydrogen acceptor is FAD rather than NAD+ b/c free-energy change is insufficient to reduce NAD+
- The high energy electrons are captured by FADH2
- Succinate dehydrogenase enzymes are located in inner mitochondrial membrane. Due to its location, the FADH2 that is generated is directly fed into the ETC at coenzyme Q.
- The next step the hydration of fumarate into L-malate, catalyzed by fumarase ~ a H2O molecule is required in this step
- Finally, another oxidation reaction occurs, where malate is oxidized to form oxaloacetate, catalyzed by malate dehydrogenase
- hydrogen acceptor here is NAD+, which is reduced to NADH
- Reaction is unfavorable (+29.7 kJ/mol) but is driven by the rapid use of products NADH and oxaloacetate, shifting the equilibrium
CAC Summary
- CAC is strictly aerobic
acetyl-CoA + 3 (NAD+) + FAD + ADP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + ATP + 2 (H+) + CoA
- Carbons: 2-C in (acetyl CoA) and 2-C out (2 CO2)
- Hydrogens/electrons: 4 pairs:
–> 3-NADH (each will generate 2.5 ATP)
–> 1-FADH2 (each will generate 1.5 ATP) - Water: 2 molecules
- 1-cycle:
–> 3-NADH = 7.5-ATP
–> 1 FADH2 = 1.5-ATP
–> + 1-ATP
= 10-ATP / turn of the citric acid cycle
Regulation of entry of acetyl-CoA into CAC occurs via regulation of ___________ and regulation of ________
pyruvate dehydrogenase complex (PDH); fatty acid oxidation
True or False: Within CAC, there’s regulation of isocitrate dehydrogenase and a-ketoglutarate dehydrogenase (KD)
True
Regulation at isocitrate dehydrogenase is activated by ____ and inhibited by ______ and ______
ADP; NADH and ATP
Regulation at a-ketoglutarate is with inhibition by _______
a) NADH, ATP and succinyl CoA
b) FADH2, ATP, and succinyl CoA
c) NAD+, ADP, succinyl CoA
d) FAD+, ADP, acetyl CoA
a) NADH, ATP and succinyl CoA
