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Chapter 16
The Citric Acid Cycle
22
Biochemistry
Undergraduate 3
02/15/2016

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Term
pyruvate dehydrogenase (PDH) complex
Definition
oxidizes pyruvate (derived from glucose & other sugars by glycolysis) to acetyl-CoA & CO2; a cluster of enzymes (multiple copies of 3 enzymes) located in the mitochondria of eukaryotic cells & in the cytosol of bacteria
Term
oxidative decarboxylation
Definition
ex) the overall reaction catalyzed by the pyruvate dehydrogenase complex; an irreversible oxidation process in which the carboxyl group is removed from pyruvate as a molecule of CO2 & the two remaining carbons become the acetyl group of acetyl-CoA
Term
thioesters
Definition
when acyl groups are covalently linked to the thiol group
Term
lipoate
Definition
the fifth cofactor of the PDH complex; has two thiol groups that can undergo reversible oxidation to a disulfide bond similar to that between two Cys residues in a protein
Term
3 enzymes in the PDH complex (each is present in multiple copies)
Definition
1. pyruvate dehydrogenase
2. dihydrolipoyl transacetylase
3. dihydrolipoyl dehydrogenase
Term
8 steps to the citric acid cycle
Definition
1. formation of citrate
2. formation of isocitrate via cis-aconitate
3. oxidation of isocitrate to alpha-ketoglutarate & CO2
4. oxidation of alpha-ketoglutarate to succinyl-CoA & CO2
5. conversion of succinyl-CoA to succinate
6. oxidation of succinate to fumarate
7. hydration of fumarate to malate
8. oxidation of malate to oxaloacetate
Term
What is the first reaction of the citric acid cycle (formation of citrate)?
Definition
the condensation of acetyl-CoA with oxaloacetate to form citrate (catalyzed by citrate synthase)
Term
aconitase
Definition
the enzyme that catalyzes the reversible transformation of citrate to isocitrate through the intermediary formation of the tricarboxylic acid cis-aconitate which normally does not dissociate from the active site
Term
What does the iron-sulfur center of aconitase do for the formation of isocitrate via cis-aconitate?
Definition
it acts both in the binding of the substrate at the active site & in the catalytic addition or removal of H2O
Term
isocitrate dehydrogenase
Definition
catalyzes oxidative decarboxylation of isocitrate to form alpha-ketoglutarate
Term
How does the oxidation of alpha-ketoglutarate to succinyl-CoA & CO2 occur?
Definition
an oxidative decarboxylation in which alpha-ketoglutarate is converted to succinyl-CoA & CO2 by the action of the alpha-ketoglutarate dehydrogenase complex; NAD+ serves as the electron acceptor & CoA as the carrier of the succinyl group
Term
divergent evolution
Definition
when the genes for an enzyme with one substrate specificity give rise (during evolution) to closely related enzymes with different substrate specificities but the same enzymatic mechanism
Term
What is indicated in the name of "succinyl-CoA synthetase" (AKA "succinic thiokinase") about the reaction it catalyzes?
Definition
the participation of a nucleoside triphosphate in the reaction
Term
nucleoside diphosphate kinase
Definition
catalyzes the reversible donation of the terminal phosphoryl group of the GTP (formed by succinyl-CoA synthetase) to ADP to form ATP
Term
succinate dehydrogenase
Definition
the flavo-protein that oxidizes the succinate (formed from succinyl-CoA) to fumarate
Term
fumarase (fumarate hydratase)
Definition
catalyzes the reversible hydration of fumarate to L-malate
Term
L-malate dehydrogenase
Definition
an NAD-linked molecule that catalyzes the oxidation of L-malate to oxaloacetate in the last reaction of the citric acid cycle
Term
amphibolic pathway
Definition
ex) the citric acid cycle in aerobic organisms; a pathway that serves in both catabolic & anabolic processes
Term
anaplerotic reactions
Definition
reactions that replenish intermediates of the citric acid cycle as they are removed to serve as biosynthetic precursors
Term
pyruvate carboxylase
Definition
catalyzes the most important anaplerotic reaction in mammalian liver & kidney: the reversible carboxylation of pyruvate by CO2 to form oxaloacetate
Term
biotin
Definition
the vitamin required for the pyruvate carboxylase reaction; the prosthetic group of the enzyme
Term
avidin
Definition
the protein found in egg whites in a large amount; binds very tightly to biotin & prevents its absorption into the intestine
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