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18 Cards in this Set
- Front
- Back
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Name 5 alpha-amino acids that are required in the urea cycle.
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There are 3 traditional, arginine, aspartate, and glutamate, as well 2 non-traditional: ornithine and citrulline. (traditional meaning it appears in proteins)
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What is the function of glutamate in the cycle?
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Glutamate is needed to regenerate aspartate from oxaloacetate. The glutamate becomes alpha-ketoglutarate.
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How many ATPs are needed to make one molecule of urea?
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Three are needed . Two to make carbamoyl-PO4 and one to provide energy for the aspartate condensation with citrulline.
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Why is the urea cycle referred to as a “bicycle”?
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There are actually 2 cycles going on. One takes ornithine to arginine and returns arginine to ornithine. The second takes fumarate from the argininosuccinate and returns it to aspartate.
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If you put ornithine labelled on the terminal N into the urea cycle, where does the label end up?
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On ornithine.
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If you use labelled ammonia for the urea cycle, where does the label end up?
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The urea.
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What complex is formed to replace the oxygen of citrulline with the NH3 from aspartate?
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Argininosuccinate.
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When we split succinate from argininosuccinate, what are we left with?
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A pair of electrons went with the NH3 group and we are left with “oxidized” succinate, better known as “fumarate”
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Name the intermediates on the pathway to produce urea
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ornithine
citrulline argininosuccinate arginine |
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Name the intermediates you go through to regenerate aspartate.
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argininosuccinate
fumarate malate oxaloacetate aspartate |
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The urea molecule is ________ free from arginine using the enzyme _________. The oxygen in the urea molecule comes from ____.
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The urea molecule is hydrolyzed free by the enzyme arginase
oxygen is obtained from H2O during hydrolysis |
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Carbamoyl-PO4 is assembled from ________ using ______ as an energy source. This is catalyzed by the enzyme ________.
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NH4+ and CO2
uses 2ATPs carbamoyl-PO4 synthetase I |
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________ reacts with ornithine to form citrulline, liberating ________.
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Carbamoyl-PO4
Phosphate is liberated |
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What is the purpose of the urea cycle?
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To dispose of amino groups & prepare them for excretion. Excess ammonia will aminate keto-glutarate so the TCA won't work anymore - this is toxic.
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Where is the ammonia coming from?
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Oxidative deamination of glutamate (funnel) by glutamate dehydrogenase using NAD+ or NADP+. The amino groups of most amino acids get funneled to glutamate by transamination of alpha-ketoglutarate.
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What is transamination? What catalyzes this?
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Transfer of amino groups from one carbon skeleton to another, catalyzed by transaminases (e.g. alanine transaminase, aspartate transaminase). This is a step in catabolism of almost all amino acids - NOT lysine or threonine.
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What special role does alpha-ketoglutarate play in amino acid metabolism?
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It accepts the amino groups from other amino acids, becoming glutamate so nitrogen can enter the urea cycle and eventually be excreted.
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How does the structure of the liver contribute to the urea cycle?
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B/c ammonia is toxic need low concentration in circulation, but for the cycle to run at speed it must be high enough to saturate carbamoylphosphate synthetase
-ammonia must be released as blood enters the liver and picked back up again before it enters systemic circulation. Enzymes that release ammonia (glutaminase, glutamate dehydrogenase are mostly in the periphery of the liver so there is a higher [ammonia] in the bulk of the lobule. Scavenging escaped ammonia is the job of glutamine synthetase so it's mostly around the central veins of the lobule. |
