biochem exam 2 - citric acid cycle Flashcards
Which of the following is not a cofactor in the Pyruvate Dehydrogenase Complex?
A
TPP
B
Biotin
C
FAD
D
NAD
E
Lipoate
B
Biotin
From what vitamin is NAD derived?
A
Thiamine
B
Niacin
C
Riboflavin
D
Pantothenate
E
Folate
B
Niacin
Which CAC reaction requires the same 5 cofactors as the pyruvate dehydrogenase reaction?
A
3 (Formation of α-KG)
B
4 (Formation of Succinyl-CoA)
C
5 (Formation of Succinate)
D
6 (Formation of Fumarate)
E
7 (Formation of L-malate)
B
4 (Formation of Succinyl-CoA)
In which CAC reaction is GTP (or ATP) formed?
A
3 (Formation of α-KG)
B
4 (Formation of Succinyl-CoA)
C
5 (Formation of Succinate)
D
6 (Formation of Fumarate)
E
7 (Formation of L-malate)
C
5 (Formation of Succinate)
Which intermediates of the CAC are formed in anaplerotic reactions?
A
Citrate and isocitrate
B
Citrate and malate
C
Malate and oxaloacetate
D
Succinate and oxaloacetate
E
Succinate and fumarate
C
Malate and oxaloacetate
What effect does an elevated level of ADP have on the activity of isocitrate dehydrogenase (Rxn #3)?
A
ADP activates the enzyme
B
ADP inhibits the enzyme
A
ADP activates the enzyme
and indicators of high energy inhibit the citric acid cycle
and indicators of low energy promote the citric acid cycle
Notes:
Isocitrate is oxidized and then decarboxylated (CO2 formed) to form α-ketoglutarate Exergonic; Regulated [ATP] – energy inhibitor, NADH - product inhibitor NAD(P)H is an electron carrier that will help make ATP (~2.5) in the ETC (later)
Fates of Glucose: Glycolysis…and more?
in glycolysis
- glucose to pyruvate
in full oxidation
- glucose to 6CO2 + 6H2O
Glycolysis captures only a small amount of the available energy in glucose… How do we get the rest out? Cellular respiration!
Cellular Respiration
Process in which cells consume O2 and produce CO2
Provides more energy (ATP) from glucose than glycolysis
Captures energy stored in lipids and amino acids
produces 32-38 ATP rather than 2 ATP which is what glycolysis produces
Three Steps:
1. Acetyl CoA production (Now - from pyruvate (sometimes not))
2. Acetyl CoA oxidation (Citric Acid Cycle – Now)
3. Electron transfer and oxidative phosphorylation
The CAC Occurs in Mitochondria in Eukaryotes
Glycolysis occurs in the cytoplasm
CAC occurs in the mitochondrial matrix *succinate dehydrogenase is in the inner membrane
oxidative phosphorylation occurs in the inner membrane
glycolysis: 2 ATP
CAC: 2 ATP
Oxidative phosphorylation: 34 ATP
Step 1: Pyruvate to Acetyl-CoA
Net Reaction: Completely irreversible!
Oxidative decarboxylation of pyruvate (remove CO2)
the 5 co-factors of pyruvate dehydrogenase
- TPP - thiamine pyrophosphate from thiamine
- FAD - from riboflavin
- coenzyme A (COA; from pantothenate)
- NAD (from niacin)
- lipoate
pyruvate is transported from the cytosol into the mitochondria via the pyruvate carrier
pyruvate + CoA-SH + NAD+ + pyruvate dehydrogenase complex (E1 + E2 + E3) = NADH + Acetyl-CoA + CO2
- the thioester in the acetyl-CoA is high-energy storage! coupling
pyruvate dehydrogenase complex
- is a 3-enzyme complex
- has 5 cofactors for the 3-enzyme complex
vitamin deficiencies?
- B1 (beriberi)
- B2, B5
- B3 (pellagra)
Step 1: Pyruvate Dehydrogenase Complex
very large complex with multiple copies of
E1 - pyruvate dehydrogenase w TPP
E2 - dihydrolipoyl transacetylase with lipoate
E3 - dihydrolipoyl dehydrogenase with FAD
- 2 regulatory proteins:
- protein kinase
- phosphoprotein phosphatase
Advantages of multienzyme complexes:
- Short distance b/t catalytic sites allows for channeling of substrates b/t catalytic sites
- Channeling minimizes side-reactions
- Regulation of activity of one subunit affects the entire complex (everything is so close!)
Step 1: So what happens?
Five Steps:
1. Pyruvate ox.→CO2. Remaining 2-C molecule binds to TPP.
- 2-C molecule is oxidized and transferred to lipoic acid→ acetyl group created.
- Acetyl group transferred to CoA. Lipoic acid is left reduced.
- FAD can reoxidize lipoic acid (FAD→FADH2) so everything can go again.
- NAD+ oxidizes FADH2 → FAD. NADH created (electrons!)
Lipoate or lipoic acid is also known as a biological tether
focus on the 3 enzyme and 5 cofactors
Step 1 of CAC: Citrate Formation
Notes:
Only reaction with C-C bond formation
Acetyl from the CoA is added to the oxaloacetate → citrate
CoA-SH is lopped off…and energy is released…what does that mean? Forward!
Exergonic; Regulated – citrate – product inhibition; [ox] substrate availability; covalent (later) Activity depends on [oxaloacetate] (required for Acetyl-CoA binding to the enzyme)
acetyl-COA + oxaloacetate + citrate synthetase = citrate
delta G = -32.2 kJ/mol
Step 2 of CAC: Citrate→Isocitrate
Notes:
Enzyme in the next reaction only binds isocitrate
Dehydration and then rehydration isomerizes citrate to isocitrate Endergonic: delta G = 13.3 kJ/mol
Coupled exergonic reactions to make it go
citrate + aconitase - remove water
cis-aconitate + aconitase - add water
= isocitrate
H + OH effectively reversed
Step 3 of CAC: α-Ketoglutarate and CO2 Formation!
Notes:
- Isocitrate is oxidized and then decarboxylated (CO2 formed) to form α-ketoglutarate
- Exergonic; Regulated [ATP] – energy inhibitor, NADH - product inhibitor
NAD(P)H is an electron carrier that will help make ATP (~2.5) in the ETC (later)
isocitrate + NAD(P) + isocitrate dehydrogenase = NAD(P)H + H+ = a-ketoglutarate
notes:
NAD(P)H formed
CO2 released
