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S2B5: Biochem > Urea Cycle > Flashcards

Urea Cycle Flashcards

1
Q

What is the fate of the carbon skeleton of amino acids?

What is the fate of the NH4+ of amino acids?

How are these two processes connected?

A
  • Carbon skeleton
    • citric acid cycle
  • NH4+
    • Urea cycle
  • Interaction
    • Aspartate-arginino-succinate shunt of citric acid cycle
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2
Q

Where in the body are most amino acids metabolized?

A

liver

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3
Q

What is the human excretory form of nitrogen?

A

urea

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4
Q

What four amino acids play a major role in connecting the urea cycle to the citric acid cycle?

What is an important characterisitc about these animo acids?

A

glutamate

glutamine

alanine

aspartate

They are all easily converted to citric acid cycle intermediates

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5
Q

The urea cycles uses nitrogen from what two places to produce urea?

A

First nitrogen can come from any amino acid (NH4+)

Second nitrogen comes from aspartate

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6
Q

What are hte 4 fates of the amino acid nitrogen?

A
  1. Transamination reactions
  2. Removal as ammonia
  3. Role of Glutamate
  4. Role of alanine and glutamine
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7
Q

What is the general tranamination reaction?

What amino acid is commonly involved?

What two amino acids do not undergo transamination?

A
  • Amino group from one amino acid is transferrd to another
    • converting an alpha-keto acid to an amino acid and an amino acid to a alpha-keto acid
    • readily reversible & uses PLP as cofactor
  • alpha-ketoglutarate and glutamate are usually one of the pairs
  • All amino acids EXCEPT: lysine and threonine
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8
Q

PLP is derived from what vitamin?

A

vitamin B6

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9
Q

Cells in the body and bacteria in the gut releast nitrogen from amino acids in what form?

Which reactions are reversible & which are irriversible?

What type of reaction occurs with serine & threonine?

A
  • ammonium ion (NH4+)
    • source for urea cycle
    • brain & muscle: glutamate transfers amino group to oxaloacetate to form aspartate- release fumerate & NH4+
  • All of the reactions are irreversible
    • EXCEPT glutamate dehydrogenase (GDH)
  • Dehydratase reactions produce NH4+ from serine and threonine
    • require PLP
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10
Q

In what form can nitrogen cross cell membranes?

In what form can it no longer freely diffuse?

A

ammonia (NH3)

once in the interstitial space, it forms NH4+ and can no longer freely diffuse

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11
Q

What is special about asparagine and glutamine?

A

in addition to the nitrogen availble in their skeleton, they have an additional nitrogen in their side chain

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12
Q

Describe the glutaminase reaction in the kidney

A

glutaminase acts on glutamine, forming glutamate and NH4+

this forms salts with metabolic acids

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13
Q

What is the difference between deamidation and deamination?

A
  • deamidation
    • The conversion of glutamine or asparagine to glutamate or aspartate acid via transamidase or deamidase.
  • deamination
    • The removal of an amino group from a compound
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14
Q

What are the 3 enzymes that can fix ammonia into an organic molecule?

A

glutamate dehydrogenase

glutamine synthetase

carbomoyl phosphate synthetase I

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15
Q

What enzymes and cofactors are required to convert glutamate to alpha-ketoglutarate

A

glutamate dehydrogenase

NAD(P)+

Releases: NAD(P)H and NH4+

readily reversible reaction

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16
Q

What is the important role that glutamate plays in the metabolism of amino acids?

A

It is involved in both synthesis and degradation of amino acids

17
Q

Describe the role of glutamate in urea production

A
  • when amino acids are degraded and urea is formed, glutamate collects nitrogens from other amino acids via transamination reactions
    • then that nitrogen is released from glutamate via glutamate dehydrogenase and it is able to enter the urea cycle as ammonium
  • Glutamate can also undego transamination to produce aspartate that will contribute a nitrogen to the urea cycle
18
Q

Where are urea cycle enzymes most active?

why is this important to know?

A

the liver

because there must be a way to transport the nitrogen from amino acid metabolism to the liver

19
Q

Describe the glucose-alanine cycle

A
  • Alanine is primarily exported by muscles because the muscles are metabolizing glucose through glycolysis & therefore pyruvate is available to be transaminated to alanine
  • Alanine serves as a nitrogen carrier, and travels to the liver where it is degraded into carbon (recycled- gluconeogenesis) and nitrogen (enters urea cycle)
  • movement of glucose from liver to muscle is called, glucose-alanine cycle
20
Q

Describe how glutamine can act as a nitrogen carrier

A
  • muscles convert alpha-ketoglutarate to glutamate via glutamate dehydrogenase (NH4+ + NADPH)
  • this glutamate is then converted to glutamine via glutamine synthetase (NH4+ + ATP)
  • Glutamine then serves as a nitrogen carrier and traves to the liver where it is degraded to glutamate and NH4+ via glutaminase
    • that glutamate is converted to alpha-alphaketoglutarate & NH4+ via glutamate dehydrogenase
    • both NH4+ produced enter the urea cycle
21
Q

What does it mean to be in nitrogen balance?

How is this accomlished?

A

the amount of nitrogen ingested equals the amount excreted

excreatory product is urea (produced in liver)

22
Q

Why is it important to excrete nitrogen?

How is it present in the body?

A

Ammonia is toxic to brain & nervous system

there is a little ammonia NH4+ present in the blood (30-60 micromolar)

It is also present in the blood vi carriers: alanine and glutamine

23
Q

What are the 5 steps to the urea cycle?

Include location where each step takes place, enzymes, products & if ATP is required for the reaction

A
  • Mitochondria
    • (1) Ammonium will react with bicarbonate via enzyme carbamoyl phosphate synthetase I (and use of 2 ATP) to produce Carbamoyl phosphate
    • (2) Carbamoyl phosphate is acted on by ornithine transcarbamylase and use an ornithine to produce citruline
    • Citruline will leave the mitochondria
  • Cytosol
    • Citruline will become part of the urea cycle
    • (3) Citruline acts with aspartate via arginosuccinate synthetase (use of 1 ATP) to produce arginosuccinate
    • (4) Arginosuccinate is acted on by arginosuccinate lyase which will release a molecule of fumerate and produce arginine
    • (5) Arginine is acted on by arginase, which release urea (excreted in the urine) and produces ornithine
    • ornithine again enters the mitochondria

S2B5: Biochem (3 decks)

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