Flashcards in Lecture 31: Urea Cycle, Metabolic Priorities, and Diabetes Deck (81)
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1
What is the purpose of the urea cycle?
To dispose of excess Nitrogen in the form of urea?
2
What is urea?
H2N-C-NH2
||
O
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What is the only organ that can perform the urea cycle?
Liver
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What is ornithine?
A 5C nonstandard AA: not used in protein synthesis, intermediate or UC
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Is ornithine expended in the UC?
No it's used and then regenerated
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Urea Cycle Step 1: Enzyme?
CPS I: carbamoyl phosphate synthetase I
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Urea Cycle Step 1: Location?
Mitosol
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What is the first committed step of the urea cycle?
Step 1
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What is the main regulatory step of the urea cycle?
Step 1
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Ureal Cycle Step 1a: Reactants/Products?
HCO3- + ATP => Carbonic-phosphoric acid anhydride + ADP
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Ureal Cycle Step 1b: Reactants/Products?
Carbonic-phosphoric acid anhydride + NH4+ => carbamate + Pi
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Ureal Cycle Step 1c: Reactants/Products?
Carbamate + ATP => carbamoyl phosphate + ADP
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What is the purpose of the 2 phosphorylations occurring in Step 1 of the UC?
They prepare the substrates for nucleophilic attacks
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Where does the nitrogen used in step 1 of the UC come from?
Glutamate dehydrogenase: Glutamate + H2O + NADP/NAD+ => alpha-ketoglutarate + NADPH/NADH + NH4+
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What is the center of nitrogen metabolism?
The mito
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What regulates CPS I? How? Why?
N-acetylglutamate by allosteric regulation because high levels represent high levels of glutamate, which mean:
1. Low gluconeogenesis because glutamate is not being used (high levels of Nitrogen in the body)
2. High alpha-ketoglutarate levels = high rate of TCA cycle
=> LOW GLUCONEOGENESIS FROM AAs
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How is N-acetylglutamate formed?
N-acetylglutamate synthase: acetyl-CoA + glutamate => N-acetylglutamate
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Urea Cycle Step 2: Enzyme?
Ornithine transcarbamoylase
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Urea Cycle Step 2: Location?
Mitosol
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Ureal Cycle Step 2: Reactants/Products?
Carbamoyl phosphate + ornithine = citrulline + Pi
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Net reaction of UC step 1?
HCO3- + 2 ATP + NH3 = carbamoyl phosphate + 2 ADP + Pi
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How does the UC happen in both the mitosol and the cytosol?
Ornithine and citrulline can cross the mito membrane via transporters
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Urea Cycle Step 3: Enzyme?
Argininosuccinate synthetase
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Urea Cycle Step 3: Location?
Cytosol
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Urea Cycle Step 3: Reactants/Products?
Citrulline + aspartate + ATP = arginino-succinate + AMP + PPi
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Urea Cycle Step 4: Enzyme?
Argininosuccinase
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Urea Cycle Step 4: Location?
Cytosol
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Urea Cycle Step 4: Reactants/Products?
Arginino-succinate => fumarate + arginine
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Urea Cycle Step 5: Enzyme?
Arginase
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