MODULE 10: Chapter 10.3 Flashcards
1
Q
What is the primary function of pyruvate in cellular metabolism?
A
Pyruvate serves as a source of reducing power for the cell in the citrate cycle or as a precursor for glucose synthesis in gluconeogenesis.
2
Q
How is pyruvate transported into the mitochondrial matrix?
A
Pyruvate is transported by a transmembrane transporter protein.
3
Q
What enzyme complex metabolizes pyruvate in the mitochondria?
A
Pyruvate dehydrogenase.
4
Q
What is the reaction catalyzed by pyruvate dehydrogenase?
A
The oxidative decarboxylation of pyruvate to form CO2 and acetyl-CoA.
5
Q
How many coenzymes are required for the pyruvate dehydrogenase reaction?
A
Five coenzymes.
6
Q
List the five coenzymes required for the pyruvate dehydrogenase reaction.
A
- NAD⁺
- FAD
- CoA
- Thiamine pyrophosphate (TPP)
- α-lipoic acid (lipoamide)
7
Q
What vitamin is the precursor for nicotinamide adenine dinucleotide (NAD⁺)?
A
Niacin (vitamin B3).
8
Q
What is the daily requirement of niacin for adults?
A
15–20 mg.
9
Q
What disease is caused by severe niacin deficiency?
A
Pellagra.
10
Q
What are the common symptoms of pellagra?
A
- Skin rash
- Diarrhea
- Neurologic problems (depression, memory loss)
11
Q
Why was pellagra common among populations relying on corn as a staple food?
A
Corn contains niacin in a protein-bound form that reduces its absorption.
12
Q
What is the role of flavin adenine dinucleotide (FAD) in metabolism?
A
FAD is a coenzyme that participates in redox reactions.
13
Q
What vitamin is the precursor for flavin adenine dinucleotide (FAD)?
A
Riboflavin (vitamin B2).
14
Q
What is the primary role of Coenzyme A (CoA)?
A
To act as a carrier molecule for an acetyl group in the form of acetyl-CoA.
15
Q
Which vitamin is Coenzyme A derived from?
A
Pantothenic acid (vitamin B5).
16
Q
What is thiamine pyrophosphate (TPP) derived from?
A
Thiamine (vitamin B1).
17
Q
What disease is associated with thiamine deficiency?
A
Beriberi.
18
Q
What are the symptoms of beriberi?
A
- Anorexia
- Cardiovascular problems
- Neurologic symptoms
19
Q
What is the role of α-lipoic acid in metabolic reactions?
A
To provide a reactive disulfide that participates in redox reactions.
20
Q
How is α-lipoic acid linked to the pyruvate dehydrogenase complex?
A
It forms a lipoamide through a covalent linkage to a lysine ε amino group.
21
Q
What is the recommended dietary allowance of thiamine for adults?
A
∼1 mg.
22
Q
Fill in the blank: The reactive thiol in lipoamide accepts an acetyl group from _______.
A
hydroxyethyl-TPP.
23
Q
True or False: Riboflavin deficiency is common in the diet.
A
False.
24
Q
What is one of the main sources of riboflavin in the diet?
A
- Dairy products
- Almonds
- Asparagus
25
What happens to thiamine in the gut before it becomes TPP?
It is phosphorylated by thiamine pyrophosphokinase in the presence of ATP.
26
What is the role of α-lipoic acid in the pyruvate dehydrogenase complex?
It forms a lipoamide through a covalent linkage to a lysine ε amino group on the E2 subunit.
27
What is generated from the transfer of an acetyl group from acetyl-dihydrolipoamide?
Acetyl-CoA.
28
Is α-lipoic acid considered a vitamin?
No, because it is synthesized at measurable levels in humans.
29
List some foods that are high in α-lipoic acid.
* Broccoli
* Liver
* Spinach
* Tomato
30
What is the primary function of the pyruvate dehydrogenase complex?
To convert pyruvate to acetyl-CoA.
31
How many subunits does the eukaryotic pyruvate dehydrogenase complex contain?
More than 90 subunits.
32
What are the three different catalytic enzymes of the pyruvate dehydrogenase complex?
* E1: pyruvate dehydrogenase
* E2: dihydrolipoyl acetyltransferase
* E3: dihydrolipoyl dehydrogenase
33
What is the net reaction catalyzed by the pyruvate dehydrogenase complex?
Conversion of pyruvate to acetyl-CoA.
34
Which coenzyme is covalently linked to the E1 subunit?
TPP (thiamine pyrophosphate).
35
What happens during step 1 of the pyruvate dehydrogenase reaction?
E1 binds pyruvate and catalyzes a decarboxylation reaction, forming hydroxyethyl-TPP and releasing CO2.
36
What is formed in step 2 of the pyruvate dehydrogenase reaction?
Acetyl-dihydrolipoamide.
37
What does the E2 subunit carry from the E1 catalytic site to the E2 catalytic site?
The acetyl group.
38
What does the E3 subunit do with the dihydrolipoamide group?
Reoxidizes it to the disulfide.
39
What is produced when E3-FADH2 is reoxidized?
NADH + H⁺.
40
What is the approximate diameter of the mammalian pyruvate dehydrogenase complex?
∼400 Å.
41
What is the stoichiometry of the E1, E2, and E3 subunits in the pyruvate dehydrogenase complex?
22:60:6.
42
What is the role of the lipoamide moiety in the E2 subunit?
It functions in the transfer of acetyl groups.
43
What does arsenic (As) inhibit in the pyruvate dehydrogenase complex?
The catalytic activity of lipoamide-containing enzymes.
44
What are the consequences of arsenic poisoning?
Irreversible blocking of enzyme activity, leading to potential death.
45
What is the origin of chronic arsenic poisoning in Bangladesh and India?
Arsenic-contaminated drinking water from shallow, hand-pumped wells.
46
What does the 'ball and chain' model of the E2 subunit describe?
The movement of the lipoamide domain between the E1 and E2 catalytic sites.
47
Fill in the blank: The E2 amino-terminal bridge consists of four components: the ball, the chain, the pivot, and the _______.
linker.
48
What toxic element was found in drinking water?
Arsenic
## Footnote
Arsenic contamination has been reported in various regions, particularly near mining activities.
49
What are the three metabolic fates of Acetyl-CoA?
* Metabolized by the citrate cycle to produce ATP
* Stored as fatty acids in adipose tissue
* Used for synthesis of membrane lipids, cholesterol, and ketones
50
Why is the pyruvate dehydrogenase reaction considered essentially irreversible?
Physiologic concentrations of reactants and products cannot overcome the biochemical standard free energy change (ΔG°′ = −33.4 kJ/mol)
51
How does the pyruvate dehydrogenase complex activity relate to carbohydrate availability?
It is fully active when carbohydrate sources are plentiful.
52
What regulates the catalytic activity of the pyruvate dehydrogenase complex?
Phosphorylation
## Footnote
The phosphorylated form of pyruvate dehydrogenase is inactive.
53
What effect do high NADH levels have on the pyruvate dehydrogenase complex?
They reduce efficiency by competing with NAD⁺ for binding to E3.
54
Which metabolites activate pyruvate dehydrogenase kinase?
* NADH
* Acetyl-CoA
* ATP
55
What is the role of pyruvate dehydrogenase phosphatase-1?
To activate the pyruvate dehydrogenase complex by dephosphorylating the E1 subunit.
56
What indicates a high energy charge in the cell?
Increased levels of NADH, acetyl-CoA, and ATP.
57
What indicates a negative energy charge in the cell?
Elevated levels of NAD⁺, CoA, ADP, and Ca²⁺.
58
What is the definition of a coenzyme?
Biomolecules that provide additional functional groups to enzyme active sites and participate in catalytic mechanisms.
59
Why were coenzymes initially discovered as vitamins?
They are required for key metabolic enzymes and their presence or absence can lead to clinical symptoms.
60
What are the five coenzymes required by the pyruvate dehydrogenase complex?
* TPP
* Lipoamide
* CoA
* FAD
* NAD⁺
61
What is the role of TPP in the pyruvate dehydrogenase reaction?
It functions in a decarboxylation reaction to convert pyruvate to CO2 and hydroxyethyl-TPP.
62
What is the function of FAD in the pyruvate dehydrogenase complex?
To reoxidize the dihydrolipoamide group.
63
What is the consequence of limited oxygen availability for the citrate cycle?
NADH and FADH₂ accumulate, leading to feedback inhibition of the citrate cycle.
64
What are the eight enzymatic reactions of the citrate cycle?
* Acetyl-CoA + Oxaloacetate → Citrate (Citrate synthase)
* Citrate ⇌ Isocitrate (Aconitase)
* Isocitrate + NAD⁺ → α-Ketoglutarate + NADH + CO₂ + H⁺
* α-Ketoglutarate + CoA + NAD⁺ → Succinyl-CoA + NADH + CO₂ + H⁺
* Succinyl-CoA + GDP + Pi ⇌ Succinate + GTP + CoA
* Succinate + FAD ⇌ Fumarate + FADH₂
* Fumarate + H₂O ⇌ Malate
* Malate + NAD⁺ ⇌ Oxaloacetate + NADH + H⁺
65
Which reactions in the citrate cycle are highly favorable?
* Citrate synthase reaction
* Isocitrate dehydrogenase reaction
* α-Ketoglutarate dehydrogenase reaction
66
What is the ΔG°′ value for the reaction catalyzed by malate dehydrogenase?
+29.3 kJ/mol
67
What is the purpose of the first reaction in the citrate cycle?
To commit the acetate unit of acetyl-CoA to oxidative decarboxylation.
68
How does the citrate synthase mechanism work?
It follows an ordered mechanism where oxaloacetate binds first, inducing a conformational change for acetyl-CoA binding.
69
What is the reaction type of Citrate cycle reaction 1?
Highly favorable condensation reaction
## Footnote
Catalyzed by the enzyme citrate synthase with ΔG°′ = −31.4 kJ/mol.
70
What is the role of oxaloacetate in the citrate synthase reaction?
Binds first, inducing a conformational change in the enzyme
## Footnote
This facilitates acetyl-CoA binding.
71
What is formed as a transient intermediate in the citrate synthase reaction?
Citryl-CoA
## Footnote
Followed by rapid hydrolysis releasing CoA and citrate.
72
What drives the citrate synthase reaction forward?
Hydrolysis of the thioester bond in citryl-CoA
## Footnote
Contributes to the highly favorable change in free energy.
73
What type of binding is demonstrated by citrate and acetyl-CoA binding to citrate synthase?
Induced-fit substrate binding
## Footnote
Classic example in enzyme kinetics.
74
What is the reaction type of Reaction 2 in the citrate cycle?
Isomerization reaction
## Footnote
Catalyzed by aconitase to form isocitrate.
75
What are the two steps involved in the isomerization reaction by aconitase?
1. Formation of cis-aconitate by dehydration
2. Hydration to form isocitrate.
76
What unique feature does aconitase have in its active site?
Contains an iron-sulfur cluster
## Footnote
Facilitates the removal of an OH group from citrate.
77
What is fluorocitrate and how is it produced?
A toxic compound derived from fluoroacetyl-CoA
## Footnote
Inhibits aconitase and blocks citrate transport.
78
What is the effect of sodium fluoroacetate (Compound 1080)?
Highly lethal poison
## Footnote
Kills most animals at a dose of only 2 mg/kg body weight.
79
What are the two forms of aconitase?
1. Mitochondrial form
2. Cytosolic form.
80
What dual role does cytosolic aconitase play?
Acts as an enzyme and RNA binding protein
## Footnote
Regulates synthesis of ferritin and transferrin receptor proteins.
81
What does isocitrate dehydrogenase convert isocitrate to?
α-Ketoglutarate, CO2, and NADH
## Footnote
First decarboxylation step in the citrate cycle.
82
What is the standard free energy change (ΔG°′) for the isocitrate dehydrogenase reaction?
−20.9 kJ/mol
## Footnote
Considered the rate-limiting step in the cycle.
83
What is the effect of ADP and Ca2⁺ on isocitrate dehydrogenase activity?
Positive effectors
## Footnote
Stimulate activity.
84
What does α-ketoglutarate dehydrogenase convert α-ketoglutarate to?
Succinyl-CoA, CO2, and NADH
## Footnote
Uses a similar mechanism as pyruvate dehydrogenase.
85
What is the standard free energy change (ΔG°′) for the α-ketoglutarate dehydrogenase reaction?
−33.5 kJ/mol
## Footnote
Allosterically regulated by energy charge.
86
What does succinyl-CoA synthetase produce from succinyl-CoA?
GTP and succinate
## Footnote
In a substrate-level phosphorylation reaction.
87
What is the net biochemical standard free energy change for the succinyl-CoA synthetase reaction?
−2.9 kJ/mol
## Footnote
Coupled phosphoryl transfer reaction.
88
What high-energy intermediates are formed in the succinyl-CoA synthetase reaction?
1. Succinyl-phosphate
2. Phosphohistidine.
89
What is the role of the β subunit in nucleotide specificity?
The β subunit donates phosphate to GDP or ADP to generate GTP or ATP respectively.
90
How does the succinyl-CoA synthetase reaction relate to energy?
It converts high potential energy of the thioester bond of succinyl-CoA into two high-energy intermediates before phosphorylating GDP.
91
What is the significance of the succinate dehydrogenase reaction?
It links the citrate cycle to the electron transport system through the redox pair FAD/FADH2.
92
What was critical in Hans Krebs’s elucidation of the citrate cycle?
Inhibiting succinate dehydrogenase with malonate allowed measurement of the accumulation of cycle intermediates.
93
What is the outcome of succinate oxidation in the succinate dehydrogenase reaction?
It results in the generation of fumarate and FADH2.
94
What type of reaction does fumarase catalyze?
Fumarase catalyzes the hydration of fumarate to generate the L isomer of malate.
95
What is the proposed reaction mechanism for fumarase?
It involves the formation of a carbanion intermediate through hydroxylation, followed by a protonation step.
96
What rare genetic disorder is associated with mutations in the fumarase gene?
Fumarase deficiency leading to devastating neurologic impairments.
97
What mutations are commonly found in the human fumarase protein?
A308T and H318Y.
98
What is the effect of the A308T and H318Y mutations on fumarase activity?
They disrupt homotetramer formation, leading to decreased enzyme activity.
99
What does malate dehydrogenase do in the citrate cycle?
It oxidizes malate to form oxaloacetate and reduces NAD⁺ to NADH.
100
What is the free energy change (ΔG°′) for the malate dehydrogenase reaction?
+29.3 kJ/mol.
101
How can the fate of individual carbon atoms in the citrate cycle be tracked?
Using radioactive tracer techniques, specifically 14C labeling.
102
Why isn't the pyruvate dehydrogenase reaction considered part of the citrate cycle?
It is not directly replenished with pyruvate from the citrate cycle.
103
What other sources can supply acetyl-CoA for the citrate cycle?
From the metabolism of fats and amino acids.
104
What is fluorocitrate?
A toxin that blocks aconitase activity, halting the citrate cycle.
105
Define hydration in the context of chemical reactions.
A reaction in which H2O is added to convert a carbon–carbon double bond to an alcohol.
