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15 Cards in this Set
- Front
- Back
Aerobic Fate of Pyruvate
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- Enters mitochondrion to be oxidized into carbon dioxide and water
- Process involved in citric acid cycle and one preparatory reaction - Accounts indirectly for most energy (ATP) produced in cells |
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Pyruvate Dehydrogenase Complex
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- Pyruvate + NAD+ + CoASH to AcCoA + CO2 + NADH + H+
- In mitochondrion - Large multienzyme complex, can be seen in electron microscope |
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PyrDehy Step I
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Decarboxylation
- Requires Thiamine pyrophosphate, TPP. - TPP typically involved in decarboxylation of alpha ketoacids |
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PyrDehy Step II
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Oxidation and Acyl Transfer
- Requires lipoic acid - Lipoic acid in reduced for has 2 SH groups - In oxidized form, there is a disulfide bond. |
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PyrDehy Step III
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Acyl Transfer
- Acetyl group is transferred from reduced lipoic acid to Coenzyme A - CoA forms thioester bonds with acyl groups and makes those groups both good nucleophiles and good electrophiles - CoA derivatives are typically involved in carbon-carbon condensations |
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PyrDehy Step IV
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Re-oxidation of reduced lipoic acid by FAD, flavine adenine dinucleotide
- FAD is an intermediate oxidizing agent in which two hydrogens are accepted as hydrogen free radicals - The reduced for is FADH2 - Generally FAD is a stronger oxidizing agent that NAD+, but this instance is an exception. The protein modifies the oxidation potential of FAD. |
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Properties of the TCA
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- Tricarboxylic acid cycle is the major site of carbon dioxide production in the body
- Occurs in mitochondrial matrix - All enzymes except one are soluble - Common to oxidation of sugars and fatty acids - Requires oxygen |
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Outline of TCA
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- 2 + 4 = 6
- 6 - 1 = 5 - 5 - 1 = 4, but not the same as original 4. However, it can be converted to original. - See notes for illustration |
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Citrate Synthase
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- OAA +AcCoA = Citrate + CoA
- CoA derivatives are both good nucleophiles and electrophiles - In this instance AcCoA makes a nucleophilic attack on the keto group of OAA (oxaloacetate) |
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Tricarboxylate Side
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- Conversion of a alpha-ketoglutarate to succinyl CoA involves the same mechanism as the pyruvate dehydrogenase mechanism
Cofactors: - TPP to decarboxylate - Lipoic Acid to oxidize and serve as an acyl group acceptor - CoA to form a thioester - FAD to reoxidize reduced lipoate - NAD+ to reoxidize FADH2 |
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Dicarboxylate Side
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- Succinate to Fumerate to Malate to OAA
- In this sequence, a methylene group is converted to a keto group. Steps: - Forming a double bond - Adding water across the double bond - Oxidizing the -OH group to a keto group - The same steps occur in oxidation of fatty acids - See lecture for illustration |
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Stoichiometry of TCA
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In each turn of TCA:
- 2 carbons enter as the acetyl groups of AcCoA - 2 carbons are evolved as CO2 - 3 NADH + 3H+ are generated - One FADH2 is produced - One GTP is produced |
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Glyoxylate Cycle (Plants)
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- In animal cells there can be no net synthesis of OAA from AcCoA
- For every 2 C atoms that enter the cycle, 2 are lost as CO2 - The best one can do is to regain the OAA which was used to condense with AcCoA - Plants can get a net synthesis because they contain two enzymes that bypass the decarboxylation steps of TCA |
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TCA Regulation
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-One known allosteric effect is ADP, which activates isocitrate dehydrogenase
- Pyruvate to AcCoA ('prep') is regulated by covalent modification Pyruvate Dehrydrogenase |
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Covalent Modification of PyrDehy
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Enzyme has 2 forms:
- Enz-Ser-OH more active - Enz-Ser-P less active - These 2 are interconverted by a kinase and a phophatase |