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Metabolic Pathways > Lecture 2 (1B) - The Urea Cycle > Flashcards

Lecture 2 (1B) - The Urea Cycle Flashcards

1
Q

Ammmonium

A

NH4+

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2
Q

Ammonia

A

NH3

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3
Q

Active nitrogen

A

NH3 (ammonia)

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4
Q

Nitrogen from… microbes and plants

A
  • NO2-
  • NO32-
  • NH3
  • N2
  • involves the enzyme nitrogenase
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5
Q

Nitrogen from… animals

A

from diet, mainly amino acids

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6
Q

Nitrogen uses

A
  • amino acids
  • purines and pyrimidines (DNA bases)
  • haems
  • chlorophylls (for rings)
  • corrins
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7
Q

The urea cycle in animals

A

• nitrogen formed from amino acid breakdown

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8
Q

Animals excrete nitrogen in 3 ways

A
  • ammonia in aquatic animals (ammonotelic)
  • uric acid in birds and reptiles (uricotelic)
  • urea in humans (ureotelic)
  • try to convert ammonia (active, dangerous, toxic) into urea
  • ammonia –> urea UREA IS NOT URINE
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9
Q

The urea cycle - history

A
  • outlined in 1932 by Krebs and Henselieit
  • more detail by Ratner and Cohen (identified components in cycle and details)
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10
Q

Urea cycle - overall process

A

ammonia + bicarbonate + aspartate

–>

urea + fumarate

• uses 3ATP –> 2 ADP + AMP + 2 Pi + PPi (requires energy)

CO2 + NH4+ + 3ATP + aspartate + 2H2O

–>

urea + 2ADP + 2Pi + AMP + PPi + fumarate

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11
Q

Urea cycle uses

A

3 ATP

–>

2ADP + AMP + 2 Pi + PPi

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12
Q

Aquatic animals excrete nitrogen as

A

ammonia

• ammonotelic

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13
Q

Ammonotelic

A

• aquatic animals excrete nitrogen as ammonia

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14
Q

Birds and reptiles excrete nitrogen as

A

uric acid

• uricotelic

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15
Q

Uricotelic

A

• birds and reptiles excrete nitrogen as uric acid

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16
Q

Humans excrete nitrogen as

A

urea

• ureotelic

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17
Q

Ureotelic

A

humans excrete nitrogen as urea

  • try to convert ammonia (active, dangerous, toxic) into urea
  • urea is NOT urine
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18
Q

Urea cycle - basics

A
  • occurs in the liver - NOT KIDNEYS - GENERATED IN THE LIVER
  • urea into blood
  • kidneys extract urea (kidney filters, eg like dialysis)
  • excreted in urine
19
Q

Urea =

A

2 nitrogens from NH3 (ammonia) and from aspartate

20
Q

Urea cycle - mechanics

A

• requires 5 enzymes

  • 2 mitochondrial
  • 3 cytosolic

• specific components therefore transported across mitochondrial membrane (ornithine and citrulline transporters)

21
Q

The priming step

• not strictly part of the urea cycle

A

• carbamoyl phosphate synthetase

(CPS I) (-etase = uses energy)

• CPS I = mitochondrial,

regulated by N-acetylglutamate

2 ATP + Bicarbonate + ammonia

–>

carbamoyl phosphate + 2 ADP + Pi

• 1 phosphate attached to carbamoyl phosphate to activate it

22
Q

The rate-limiting step of the urea cycle

(controls cycle/speed)

A

generally the first step controls/regulates the cycle/process

  • FIRST NITROGEN INCLUDED
  • CPS I reaction in mitochondria
  • 3 step process
23
Q
  1. 3 steps of the rate-limiting step
A

1.

ATP + HCO3-

–>

carboxyphosphate + ADP

2.

NH3 + carboxyphosphate

–>

carbamate + Pi

3.

ATP + carbamate

–>

carbamoyl phosphate + ADP

(once again phosphate added as activation)

24
Q

CPS I and CPS II

A

CPS I relevant - in mitochondrion

CPS II not relevant - in cytosol

25
Urea cycle steps after rate-limiting
**in mito** 2. ORNITHINE TRANSCARBAMOYLASE ornithine + carbamoyl phosphate --\> citrulline + Pi **cytosolic** 3. ARGININOSUCCINATE SYNTHETASE ATP + citrulline --\> citrulline-AMP + PPi citrulline-AMP + Aspartate --\> argininosuccinate + AMP (second N added from aspartate, urea created) 4. ARGININOSUCCINASE argininosuccinate --\> arginine + fumarate 5. ARGINASE arginine --\> urea + ornithine
26
2. Ornithine transcarbamoylase
ornithine + carbamoyl phosphate --\> citrulline + Pi * carbamoyl transferred to ornithine to get citrulline * ornithine is out in the cytosol but then transported to mitochondrial membrane * reaction in the mitochondrion, ornithine brought in from cytosol * next reaction cytosolic, citrulline transported out
27
3. Argininosuccinate synthetase
ATP + citrulline --\> citrulline-AMP + PPi citrulline-AMP + Aspartate --\> argininosuccinate + AMP * in cytosol * bonds AMP to citrulline shown by 18-O labelling * SECOND NITROGEN (amino) INCLUDED (from aspartate) * urea now fully assembled * in creation of argininosuccinate urea has been created
28
4. Argininosuccinase
argininosuccinate --\> arginine + fumarate * arginine = amino acid precursor to urea * fumarate can be used for aspartate cleaves argininosuccinate into arginine and fumarate * carbon skeleton of aspartate is preserved in the form of fumarate
29
5. Arginase
arginine --\> urea + ornithine * ornithine recreated = cycle (originally put in to make citrulline) * passed to mitochondrion * hydrolysis * ornithine goes to mitochondrion * urea is excreted
30
Regulation of the Urea Cycle
* N-acetylglutamate * glutamate * all other enzymes controlled by substrate concentration
31
Regulation of the Urea Cycle N-acetylglutamate
allosteric activation of carbamoyl phosphate synthetase I (priming step)
32
Regulation of the Urea Cycle Glutamate
* glutamate increase in amino acid breakdown * hence increased in N-acetylglutamate * hence urea cycle upregulated in turnover
33
Regulation of the Urea Cycle All other enzymes controlled by
substrate concentration
34
Urea cycle diseases
* hyperammonemia * ornithine transcarbamoylase deficiency (OTCD) * argininosuccinase deficiency
35
Urea cycle disease Hyperammonemia
* major problems with the brain (makes brain switch off, lethargic) * probably interferes with production of - glutamate - GABA * hence neurological defects result - brain starts to switch off - morbidity
36
Urea cycle disease Ornithine transcarbamoylase deficiency (OTCD)
ornithine + transcarbamoyl phosphate --\> citrulline + Pi • elevated levels of - glutamate - orotic acid - ammonia • low levels of citrulline
37
Urea cycle disease OTCD treatment
diet supplement - benzoate, phenylacetate • take up ammonia in different ways of excretion, different products excreted
38
Urea cycle disease argininosuccinase deficiency
argininosuccinate --\> arginine + fumarate * usually severe hyperammonemia * very few other levels altered
39
Urea cycle disease arginninosuccinase deficiency treatment
diet supplement * arginine to help with the cycle * limited protein intake
40
The urea cycle begins with the coupling of
free NH3 (ammonia) with HCO3 - (bicarbonate) to form carbamoyl phsophate
41
NH3 is a strong base that normally exists as
NH4 + in aqueous solution but carbamoyl phosphate synthetase only uses NH3 as a substrate
42
Parts of the urea cycle that happen in the mitochondrial matrix
* formation of NH4 + by glutamate dehydrogenase * its incorporation into carbamoyl phosphate as NH3 * synthesis of citrulline next 3 reactions of the urea cycle in the cytoplasm
43
Urea cycle - picture
44
Recreating aspartate from fumarate

Metabolic Pathways (11 decks)

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