Week 5: TCA Cycle and Pyruvate Dehydrogenase Flashcards
Describe where and how pyruvate is converted to acetyl-CoA, including the vitamins involved in the process and the result of deficiencies in this reaction
Pyruvate enters the mitochondrial matrix after glycolysis and is converted to CO2 and acetyl-CoA by the pyruvate dehydrogenate complex
Acetyl-CoA can then be moved into the TCA cycle or used in other metabolic pathways.
The five cofactors of the pyruvate dehydrogenase complex include: (abbrev.) (precursor)
Thiamine pyrophosphate (TPP) (vitamin B1)
Flavin Adenine Dinucleotide (FAD) (riboflavin/vitamin B2)
Coenzyme A (CoA-SH) (pantothenic acid/vitamin B5, beta-mercaptoethylamine)
NAD+ (tryptophan and niacin)
Lipoic Acid (synthesized naturally, don’t need vitamins)
Describe the conditions that increase the production of acetyl-CoA from pyruvate, and the general mechanism for how this is accomplished, and what physiologic function regulation serves
Acetyl-CoA is generated in the process of the TCA cycle in the second step of aerobic respiration. It is used to carry carbon atoms of the acetyl group back to the TCA cycle to be oxidized for further energy production.
The overall reaction has a high ΔG’ value of -33.4 kJ/mol, and actually involves 5 smaller reactions that generate CO2, NADH, and Acetyl-CoA from pyruvate, CoA-SH, and NAD+
Four of the five reactions are reversible, with only one being irreversible (overall irreversible). This means the overall reaction can be regulated at several points.
Explain the consequences of the TCA cycle being a circular pathway and the inability of acetyl-CoA to contribute to net synthesis of oxaloacetate
Acetyl-CoA can only add an acetyl group to the oxaloacetate group to generate synthase and restart the TCA cycle with a 6C molecule. If you have 100 Acetyl-CoA molecules and only one OAA molecule, you can go through 100 cycles of the TCA cycle and still have an OAA left over because the carbons added by Acetyl-CoA are the only ones that get oxidized over the course of the overall reaction. The cycle without any connecting points means you cannot take “shortcuts” to add any other molecules or elements to mediate the pathway, except for to regulate the process by enzymatic blockade.
Explain the role of each critical enzyme of the TCA cycle and what function it serves as a part of the complete cycle
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Citrate synthase: helps regenerate citrate from OAA and Acetyl-CoA (because [OAA] is very low, a highly negative ΔG is needed to drive this process)
Aconitase: removes or adds water to generate isocitrate
Isocitrate dehydrogenase: first oxidation step, and generates the first NAD(P)H (two different enzymes for each version of electron carrier) + CO2 (CO2 release creates (-) ΔS to drive the reaction–this is a waterfall step)
alpha-Ketoglutarate dehydrogenase complex: similar to pyruvate dehydrogenase complex, and generates CO2 + NADH in the formation of succinyl-CoA. ΔG = -33.5 kJ/mol, last waterfall step
Succinyl-CoA synthetase: severing of thioester bond is conserved by formation of ATP or GTP
Succinate dehydrogenase: becomes oxidixed to produce FADH2 electron carrier intermediates
Fumarase: H2O in the form of -OH and H+ are added on either side of the two-carbon molecule
Malate dehydrogenase: oxidizes malate by removing two hydrogens and reforming the citric acid intermediate (OAA), which can be coordinated with Acetyl-CoA as the process begins again ΔG = 29.7 kJ/mol (low [OAA] helps drive reaction forward)
Explain the multiple functions played by the citric acid cycle and how this can serve to coordinate catabolism of metabolic fuel with biosynthesis
What are the similarities between Wernicke Encephalopathy, Beriberi and Pyruvate Dehydrogenase Complex Deficiency?
What mnemonic helps you remember the TCA cycle intermediates and their enzymes?
Intermediates:
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(Citrate, Isocitrate, alpha-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate)
Enzymes:
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(Citrate synthase, Aconitase, Isocitrate dehydrogenase, Ketoglutarate dehydrogenase complex, Succinyl-CoA synthetase, Succinate dehydrogenase, Fumarase, Malate dehydrogenase)
What are the important elements of CoA and Acetyl-CoA?
Precursors include pantothenic acid and 3’-phosphoadenosine diphosphate, as well as beta-mercaptoethylamine
CoA has a reactive thiol group to form thioesters
It also has a high acyl group transfer potential
How does lipoic acid contribute to overall exchange within the pyruvate dehydrogenase complex?
It has thiol groups that can act as nucleophiles to attack and bind the central carbon group of the pyruvate molecule. The disulfide bond in the oxidized form is the active form. The next step will “hand off” the acetyl group to CoA. In the process, lipoic acid becomes reduced. It is recycled for reuse by later reducing FAD to FADH2, which then reduces NAD+ to NADH
