Lecture 24 Flashcards
Is the citric acid cycle catabolic or anabolic? Where does it occur?
it is amphibolic (both)
In the mitochondria
How are the TCA cycle and the Kreb’s cycle related to the citric acid cycle? What substance are all energy nutrients converted to so that they may enter the citric acid cycle?
TCA, Kreb’s cycle, and citric acid cycle are all names for the same cycle
The are converted to Acetyl CoA
Name the 3 energy equivalent substances that are generated by the Citric acid cycle. What other products are formed by this cycle?
NADH, FADH2, and GTP
2 CO2 molecules
State the 3 energy nutrients that are converted to Acetyl CoA in order to enter the citric acid cycle. Describe this process for each type of nutrient.
Carbohydrates: Oxidation into pyruvate and decarboxylation into Acetyl CoA
Lipids: Beta oxidation of fatty acids to form Acetyl CoA
Proteins: Breakdown into AA’s and Acetyl CoA
State the 3 steps that occur to form Acetyl CoA from Pyruvate
Decarboxylation
Oxidation
Transfer Acetyl group to coenzyme A
(these reactions are coupled to preserve energy to drive the formation of NADH and Acetyl CoA)
Explain where pyruvate must be in order to be decarboxylated into Acetyl CoA so that it may enter the citric acid cycle. How does it get there? What enzyme carries out the decarboxylation?
Pyruvate must enter the mito in order to be decarboxylated
MPC (mitochondrial pyruvate Carrier) is required to bring Pyruvate into the mito
PDC (Pyruvate Dehydrogenase Complex) catalyzes the decarboxylation of Pyruvate to Acetyl CoA
State the 3 enzymes and 5 coenzymes that are necessary for the completion of the pyruvate Dehydrogenase reaction. Be sure to state which of the 5 coenzymes are catalytic and which are stoichiometric cofactors.
3 Enzymes: E1, E2, and E3
Catalytic Cofactors:
TPP (Thiamine pyrophosphate)
Lipoic Acid
FAD
Stoichiometric Cofactors:
CoA
NAD+
What effect does High acetyl CoA levels have on pyruvate dehydrogenase enzyme(s)? Explain whether this occurs via phosphorylation or dephosphorylation.
High levels of Acetyl CoA directly inhibit the E2 enzyme (one of 3 that composed the pyruvate dehydrogenase enzyme)
Happens via phosphorylation (which inactivates the PDC) of the PDC’s E2 enzyme
State the reactant and product of the PDC (pyruvate Dehydrogenase complex) and then describe why it is considered to be a “link” between 2 important metabolic pathways.
The PDC takes Pyruvate (from the end of glycolysis) and creates Acetyl CoA (the beginning reactant for the citric acid cycle)
This is why the PDC is considered to be the “link” between glycolysis and the Citric Acid cycle
Oxaloacetate + Acetyl CoA + _____ creates ______. What enzyme catalyzes this reaction?
H2O
Citrate
Citrate Synthase catalyzes this reaction
The hydroxyl group of citrate is not in the proper location for oxidative decarboxylation. How is this issue addressed by the citric acid cycle? (include the intermediate that is formed)
Dehydration/hydration moves the OH atoms via the enzyme Aconitase
Cis-Aconitate is the intermediate product that is briefly formed during this
Which enzyme converts Isocitrate into Alpha-ketoglutarate? what is the unstable intermediate that forms and what key reaction occurs at this step?
Isocitrate Dehydrogenase (Rate limiting step of the citric acid cycle)
Oxalosuccinate is the unstable intermediate that is formed and it loses a CO2 molecule (Alpha-ketogluarate is not a 5 carbon molecule)
What Enzyme converts Alpha-ketoglutarate into Succinyl CoA? What are the 2 other molecules involved in this reaction?
Alpha-ketoglutarate Dehydrogenase is the enzyme
Alpha-ketoglutarate + NAD+ + CoA –> Succinyl CoA + CO2 + NADH
What 2 Enzymes are very similar because they are a “complex enzyme” that decarboxylate and create a HIGH ENERGY thioester linkage with CoA?
Alpha-ketoglutarate Dehydrogenase and Pyruvate Dehydrogenase
What enzyme catalyzes the reaction that converts Succinyl CoA into Succinate? What does this step yield and why is it special?
Succinyl CoA Synthetase
This step yields a high energy phospho-transfer compound (ATP or GTP depending on the tissue it’s occurring in) via a substrate level phosphorylation
Explain which tissues will yield GTP and which will yield ATP during the step of the citric acid cycle that is catalyzed by Succinyl CoA Synthetase.
Tissues that perform many anabolic reactions (The Liver) will yield GTP
Tissues that perform a large amount of Cellular Respiration (skeletal and heart muscle) will yield ATP
Which enzyme catalyzed the formation of Fumarate from Succinate? What other molecule is created by this reaction?
Succinate Dehydrogenase catalyzes this reaction
It also creates an FADH2 molecule
Where in the cell does the Succinate Dehydrogenase reaction occur? Explain the significance of this reaction occuring where it does AND what role FADH2 plays in that significance.
This reaction occurs in the inner mito membrane
FADH2 is not actually released from the Succinate Dehydrogenase enzyme, it’s electrons are passed directly to Co-Q to be used in the Electron transport chain.
Which Enzyme Catalyzes the reaction that converts Fumarate to Malate? what Enantiomer of Malate is formed?
Fumarase
L-Malate (specifically)
Which Enzyme converts L-Malate into Oxaloacetate? What type of reaction is this and what level of standard free energy does it exhibit?
Malate Dehydrogenase
This is an Oxidation reaction that has a Positive standard free energy
For the Malate Dehydrogenase step of the citric acid cycle, explain what it means when saying this step is “driven by the use of the products”.
basically, the availability of the products drives the reaction forward
There needs to be Oxaloacetate available for Citrate Synthase to work
There needs to be NADH available for the electron transport chain to occur
Put the following substances in a category of either Activating or Deactivating for the activity of PDC.
Ca2+ Insulin in adipose tissue Catecholamines Acetyl CoA NADH ATP CoA Mg2+ Pyruvate ADP Arsenite NAD+
Activating: Ca2+ Mg2+ ADP CoA NAD+ Pyruvate Insulin in adipose tissue Catecholamines
Deactivating: Acetyl CoA NADH ATP Arsenite
What effect does Ca2+ have on Phosphatases? Why? What about the effect Insulin has on phosphatases? what does this lead to?
Phosphatase are stimulated by Ca2+ in order to increase muscle contractions
Insulin stimulates fatty acid synthesis by activating phosphatases and increasing the conversion of Pyruvate to Acetyl CoA (Fatty acids are synthesized from Acetyl CoA
Which Enzyme in the citric acid cycle prevents the wasteful hydrolysis of Acetyl CoA? what type of reaction does this enzyme carry out?
Citrate Synthase
Carries out a condensation reaction (to yield citrate)
State the 3 “control sites” of the citric acid cycle. (these are basically regulatory sites of the cycle represented by enzymes)
Citrate Synthase
Isocitrate Dehydrogenase
Alpha-ketoglutarate Dehydrogenase
For the Isocitrate Dehydrogenase enzyme step, state the Allosteric stimulant and the 2 reaction product inhibitors that can affect it’s activity.
Allosterically stimulated by ADP
Inhibited by ATP and NADH
For the Alpha-Ketoglutarate Dehydrogenase enzyme step, state the 2 products that allosterically inhibit it.
Succinyl CoA and NADH (ATP also?)
Build up of products can occur from the various control steps in the citric acid cycle. What does the buildup of Citrate cause? What about the buildup of Alpha-ketoglutarate?
Built up Citrate can transport to the cytosol and signal for phosphofructokinase to halt glycolysis
Built up Alpha-ketoglutarate can be used for the synthesis of AA’s and purine bases.
What is the significance of Anaplerotic reactions and the TCA cycle? What are the 2 major anaplerotic reaction types we are concerned with for the citric acid cycle? (same as TCA)
Anaplerotic reactions create intermediates for the citric acid cycle to use
- Degradation of AA’s (creates intermediates for the TCA)
- Carboxylation of Pyruvate (creates intermediates for the TCA)
When energy needs are met, intermediates of the TCA cycle are drawn from in order to be used in the biosynthesis of other molecules. Why doesn’t this have a negative effect on the ability of the TCA cycle to continue to occur?
The intermediates are replenished by the formation of oxaloacetate from pyruvate (TCA cycle takes care of it’s own issue by making more intermediates)
Anaplerotic reactions are required during states of ____ energy.
low
What effect does high levels of ADP and pyruvate have on PDH?
High levels of ADP and Pyruvate activate phosphatases to remove the phosphate from the E2 enzyme of the PDC (activating it)
