Quiz 6 - TCA cycle Flashcards
What are the 4 fates of pyruvate?
Transamination: Alanine
Carboxylation: Oxaloacetate
Oxidative decarboxylation: Acetyl-CoA
Reduction: Lactate
<em>Glucose becomes pyruvate through glycolysis</em>
What are other sources of acetyl-CoA?
What are the 3 fates of acetyl CoA?
Acetyl-CoA enters the:
Tricarboxylic acid cycle
Ketone bodies
Sterol-fatty acids
<em>P<span>Y</span>ruvate, amino acids and fatty acids all create acetyl-CoA</em>
Know the diagram
What is made in TCA cycle?
What is degraded?
Some ATP is made and more NADH is made.
NADH goes on to make more ATP through ETC and oxidative phosphorylation.
Pyruvate(acetate) from glycolysis is degraded to CO2
Where does Krebs cycle occur?
Mitochondria
What is primary function of Krebs cycle?
The oxidation of Acetyl-CoA to Co2.
The energy released from this oxidation is saved as NADH, FADH2 and GTP
What is the overall result of TCA cycle?
What is not required in this pathway?
Acetyl-CoA = 2CO2
3NAD + FAD + GDP +Pi = 3NADH + FADH2
Oxygen is not required in the cycle, the pathway will not occur anaerobically because NADH and FADH2 will accumulte if oxygen is not available for the ETC
What is the only fate of acetyl CoA in TCA cycle?
What does this represent?
The oxidation of acetyl CoA to CO2.
The citric acid cycle does not represent a pathway by which there can be a net synthesis of glucose from acetyl-CoA or the net conversion of acetyl-CoA to citrate, malate or any other intermediate in the cycle.
What is the TCA cycle central to the oxidation of?
Is there any hormonal control of the cycle?
The cycle is central to the oxidation of any fuel that yields acetyl-CoA including:
glucose
fatty acids
ketone bodies
ketogenic amino acids
alcohol
There is no hormonal control of the cycle as control is exerted by energy status of the cell. TCA activity is necessary irrespective of fasting or fed stage
Where are all TCA enzymes located? Which enzyme is not present in this location?
All TCA enzymes are located in the matrix of mitochondria except succinyl dehyrogenase which is found in the inner membrane
List the key enzymes of TCA cycle and their function
- Isocitrate dehydrogenase: major control enzyme, inhibited by NADH and ATP, activated by ADP
- a-ketoglutarate dehydrogenase: similar to pyruvate dehyrogenase complex. Requires thiamine, lipoic acid, CoA, FAD and NAD. Lack of thimaine slows oxidation in TCA cycle
- Succinyl-CoA synthetase(succinate thiokinase): catalyzes substrate level phosphorylation of GDP to GTP
- Succinate dehydrogenase: inner mitochondrial membrane, functions as complex 2 of ETC.
- Citrate synthase: condenses incoming acetyl-CoA with oxaloacetate to form citrate
What other functions do the intermediates citrate, succinyl-CoA and malate have other than in TCA cycle?
Citrate leaves mitochondria (citrate shuttle) to deliver acetyl CoA into cytoplasm for fatty acid synthesis.
Succinyl-CoA is high energy substitute used for heme synthesis and to activate ketone bodies in extrahepatic tissues
Malate leaves mitochondria(malate shuttle) for gluconeogenesis
<em>As intermediates are drawn out of the TCA cycle, the cycle slows and they must be replaced to ensure sufficient energy for the cell</em>
Know the diagram
What two normal ways to cleave C-C bonds and oxidize don’t work for CO2 when oxidizing acetate to CO2?
1. Cleavage between Cs alpha and beta to a carbonyl
2. An alpha cleavage of an alpha hydroxyketone
What is a better way to cleave acetate?
It is better to condense acetate with oxaloacetate and carry out a Beta cleavage. TCA combines this with oxidation to form CO2, regenerate oxaloacetate and capture all the energy as NADH and ATP.
Pyruvate is oxidatively decarboxylated to form what substance?
What does pyruvate use to create this substance?
Acetyl-CoA
Pyruvate uses TPP, CoASH, lipoic acid, FAD and NAD+
Describe 2 reasons why pyruvate from aerobic glycolysis enters mitochondria where it is converted to acetyl-CoA?
1. For entry into TCA if ATP is needed
2. For fatty acid synthesis if sufficient ATP is present
What is pyruvate dehydrogenase inhibited by?
Is the pyruvate dehydrogenase reaction irreversible?
What other mitochndrial enzyme uses pyruvate?
PDH is inhibited by acetyl-CoA.
The PDH reaction is irreversible and cannot be used to convert acetyl-CoA to pyruvate or glucose
The other mitochondrial enzyme is pyruvate carboxylase
What are the cofactors and enzymes used by pyruvate dehydrogenase?
Thiamine pyrophosphate(TPP) from vitamin thiamine
Lipoic acid
Coenzyme A (CoA) from pantothenate
FAD(H2) from riboflavin
NAD(H) from niacin(some synthesized from tryptophan)
What activates Pyruvate dehydrogenase in liver?
Insulin. In the brain and nerves PDH is not responsive to hormones.
Who are most likely to have thiamine deficiency?
What are some symptoms when alcohol interferes with thiamine absorption in the intestine?
Alcoholics and they may develop Wernicke peripheral neuropathy and Korsakoff psychosis.
Symptoms of alcohol interfering with thiamine absorption include:
Ataxia
Ophthalmoplegia, nystagmus
Memory loss and cofabulation
Cerebral hemorrhage
What does insufficient thiamine lead to?
Insufficient thiamine impairs glucose oxidation, causing highly aerobic tissues such as brain and cardiac muscle to fail first.
<em>Congestive heart failure may be a complication(wet beri-beri owing to inadequate ATP and accumulation of ketoacids in the cardiac muscle.</em>
<em>Branched chain amino acids are sources of energy in brain and muscle</em>
What other enzyme complexes similar to pyruvate dehydrogenase use thiamine?
1. a-ketoglutarate dehydrogenase(citric acid cycle)
2. Branched-chain ketoacid dehydrogenase(metabolism of branched-chain amino acids)
Coenzyme A structure
Polypeptide chain of E2(dihydrolipoyl transacetylase)
Know the cycle of pyruvate to acetyl-CoA
What are allosteric inhibitors of Citrate synthase?
Does CS have large negative delta G?
NADH and succinyl CoA
Yes it has large negative delta G.
What does enzyme Aconitase do?
What is the stereochemistry of the reaction?
What kind of cluster does aconitase use?
Isomerization of citrate to isocitrate which has secondary OH which can be oxidized because citrate is a poor substrate for oxidization.
The stereochemistry of the reaction is that aconitase removes pro R H of the pro arm of citrate
Aconitase uses Iron-sulfur cluster.
Oxidative decarboxylation of what substance occurs under Isocitrate dehydrogenase?
Oxidative decarboxylation of isocitrate to yield alpha ketoglutarate
Involves NAD+ chemistry(Hydral removal)followed by decarboxylation. IDH is a link to ETC because it makes NADH
What are the 5 coenzymes used by Alpha-ketoglutarate dehydrogenase?
Is this the 1st or 2nd oxidative carboxylation?
Which enzyme is Alpha-ketoglutarate similar to?
The 5 coenzymes are:
TPP, CoASH, Lipoic acid, NAD+, FAD
Second oxidative decarboxylation
The enzyme is similar to pyruvate dehydrogenase
Is Succinyl-CoA synthatase reaction a substrate level phosphorylation?
What is made? What is synthesis driven by? What does it involve?
Yes succinyl-CoA synthatase reaction is a substrate level phosphorylation reaction.
A nucleoside triphosphate is made. Its synthesis is driven by CoA ester and involves a phosphohistidine
What does succinate dehydrogenase involve?
What is the mechanism?
The enzyme is part of which pathway and where?
What happens to the electrons?
An oxidation involving FAD.
The mechanism involves hydride removal by FAD and deprotonation
Succinate dehydrogenase is part of the ETC in inner mitochondrial membrane
The electrons transferred from succinate to FAD to form FADH2 and are passed directly to ubiquinone(UQ) in ETC pathway.
What does fumarase do?
Hydration across double bond
What oxidation does malate dehydrogenase do?
Is the reaction energetically expensive?
Malate dehydrogenase reaction and which other reactions form a reaction triad over and over again?
NAD+ dependent oxidation. The carbon that gets oxidized is the one that received -OH from previous reaction(fumarase).
Yes. Delta G is 30 kj/mol
Succinate dehydrogenase, fumarase and malate dehydrogenase reactions form reaction triad.
Know the diagram
For one acetate through the TCA cycle, how many CO2, ATP and reduced coenzymes are produced?
2 CO2, 1 ATP and 4 reduced coenzymes for 1 acetate
Know the table
What is the fate of carbon in the TCA cycle(Carboxyl C and Methyl C)?
Carboxyl C of acetate turns to CO2 only in second turn of cycle following entry of acetate
Methyl C of acetate survives 2 cycles completely but half of what’s left exits the cycle on each turn after that
What are the intermediates of biosynthesis for the TCA cycle?
Alpha ketoglutarate is transaminated to make glutamate which can be used to make purine nucleotides Arg and Pro
Succinyl-CoA can be used to make porphyrins
Fumarate and oxaloacetate can be used to make several amino acids and pyrimidine nucleotides
Mitochondrial citrate can be exported to be a cytoplasmic source of acetyl CoA and oxaloacetate
What are anploretic reactions?
What are the enzymes involved in this reaction?
Which reaction goes the wrong way? What binds tightly and what binds weakly to the enzyme?
They are filling up reactions.
PEP carboxylase converts PEP to oxaloacetate
Pyruvate carboxylase converts pyruvate to oxaloacetate
Malic enzyme converts pyruvate to malate
PEP Carboxykinase could have been an anaploretic reaction but it goes the wrong way because CO2 binds weakly and oxaloacetate binds tightly to the enzyme.
Know anaploretic reactions 1,2,4
Know the diagram
Know the diagram
Know this diagram for sure!
Know diagram
What is the regulation of TCA cycle?
These 3 reactions are the key sites.
Citrate synthase - ATP, NADH and succinyl-CoA inhibit
Isocitrate dehydrogenase- ATP inhibits, ADP and NAD+ activates
Alpha ketoglutarate dehydrogenase- NADH and succinyl -CoA inhibit, AMP activates
Pyruvate dehydrogenase: ATP, NADH, acetyl-CoA inhibit, NAD+, CoA activate
