Nitrogen Metabolism - Lec. 31

Question 1

where does the urea cycle occur

Answer 1

occurs within the liver cells only because only the liver can perform the urea cycle - mainly occurs within the cytoplasm if the liver cells (nitrogen for the urea cycle comes from the mitochondria)

Question 2

glutamine synthetase

Answer 2

converts glutamate into glutamine

both glutamate and glutamine are in the liver cell

Question 3

glutamate

Answer 3

can be made through transamination with other amino acids

• can also integrate another free form ammonium (NH4+) to become glutamine
• The amino group of alanine is attached to α-ketoglutarate through transamination into glutamate

Question 4

glutamine

Answer 4

glutamine is formed through glutamate through the addition of an ammonium (NH4+)
- glutamine can release an amino group to produce a free ammonium in the liver cell by an enzyme called glutaminase

Question 5

glutaminase (enzyme)

Answer 5

this enzyme removes an NH4+ (ammonium) group from glutamine within the liver cell and converts glutamine back to glutamate –> then glutamate can transport into the liver cells mitochondria

Question 6

glutamine synthase (not the same as glutamine synthetase) - what is is used by and used for?

Answer 6

glutamine synthase is an enzyme that’s used by bacteria and plants to convert glutamine into glutamate

• this is done by taking glutamine and a molecule of alpha-ketoglutarate to form the product, glutamate
• *this enzyme is not found in humans

Question 7

what does glutamate do once transported into mitochondria of liver cell?

Answer 7

when glutamine releases its second amino group (NH4+) to form glutamate by the enzyme glutaminase - its transports to liver cell mitochondria where another enzyme called glutamate dehydrogenase, will remove the amino group from glutamate which produces alpha-ketoglutarate (alpha-keto acid)

Question 8

what enzyme removes the amino group from glutamate within the mitochondria of the liver cell?

Answer 8

glutamate dehydrogenase - this occurs when glutamine loses an amino group from glutaminase enzyme and converts glutamine to glutamate where it shuttles into the liver cell mitochondria –> once there the enzyme glutamate dehydrogenase will remove another amino group from glutamate and produce alpha-ketoglutarate

Question 9

what happens with the excess nitrogen produced? (what cycle is used to get rid of it - simple)

Answer 9

in humans, we use the urea cycle to get rid of the free form ammonia in mitochondria because it is highly toxic to mammals
*key point is that not all animals use the same cycle to handle the removal of nitrogen waste

Question 10

what does the urea structure compose of?

Answer 10

• has two nitrogens (NH2 groups) –> need at least two nitrogens (2 free form ammonia integrations) to combine into one urea molecule
• the two nitrogens are attached to a central carbon and a double bounded oxygen is attached to the carbon as well

Question 11

what’s the first step in making a molecule of urea?

Answer 11

the integration of a free amino or free ammonia group performed by an enzyme called carbonyl phosphate synthetase 1

Question 12

what does the enzyme carbamoyl phosphate synthetase 1 (CPS1) do regarding urea molecule?

Answer 12

this enzyme is what integrates the first amino group to make the building block of the urea cycle –> the very first step of urea cycle, integration of amino group into a molecule of carbamoyl phosphate
- The specific role of carbamoyl phosphate synthetase I is to control the first step of the urea cycle, a reaction in which excess nitrogen compounds are incorporated into the cycle to be processed

Question 13

what three enzymes in human cells can integrate ammonia into molecules?

Answer 13

• glutamine synthetase (integrates a free ammonia into a molecule of glutamate to make glutamine)
• carbamoyl phosphate synthetase 1 (CPS1) (the first starting block material used in the urea cycle)
• glutamate dehydrogenase (release free form ammonia from glutamate producing alpha-ketoglutarate but what’s unique about it is it can be used bi-directionally and add amino group back and make glutamate again

Question 14

carbamoyl phosphate synthetase-1 (what does it do in the urea cycle)

Answer 14

this is the first starting material or the building block of building urea - formed by using a bicarbonate and molecule of ATP –> catalyzes the first and rate-limiting step of the urea cycle
- Carbamoyl phosphate synthetase I transfers an ammonia molecule to a molecule of bicarbonate that has been phosphorylated by a molecule of ATP.

Question 15

where are all the free form ammonias confined to?

Answer 15

within the mitochondria of the liver cell

Question 16

where does the actual urea cycle occur within the liver cell?

Answer 16

occurs within the cytoplasm of the liver cell –> this poses a problem since all the free form ammonia are within the mitochondria (need a mechanism to shuttle them out of mitochondria into cytoplasm)

Question 17

how is free form ammonia shuttled out of the mitochondria and into the cytoplasm of the liver cell?

Answer 17

a carrier molecule known as ornithine shuttles into the mitochondria to pickup a molecule of carbamoyl phosphate that has an amino group attached to it already - together the carrier molecule and carbamoyl phosphate combine to create a new molecule called citrulline (with the attached ammonia) which then shuttles out of the mitochondria and into the cytoplasm

Question 18

ornithine (amino acid)

Answer 18

a shuttling carrier molecule (also an amino acid) that shuttles into mitochondria to pickup a molecule of carbamoyl phosphate, which has the amino group attached, and combines with the carrier molecule to form a new molecule called citrulline which can then be shuttled out of the mitochondria and into cytoplasm

Question 19

citrulline (amino acid)

Answer 19

this is the molecule that forms when the carrier molecule, ornithine, shuttles into the mitochondria to pickup a molecule of carbamoyl phosphate and the two combine in the mitochondria to produce a new molecule known as citrulline which then shuttles out of the mitochondria and into the cytoplasm of the liver cell –> this ultimately provides the first nitrogen for the urea cycle

Question 20

what provides the second nitrogen into the urea cycle?

Answer 20

aspartate

- recall that aspartate results from the transamination of oxaloacetate (OAA) and glutamate via aspartate transaminase

Question 21

what are the two major building blocks of the urea cycle and what do those building blocks combine and form into?

Answer 21

• the two major building blocks are citrulline and aspartate
• they will combine through their functional groups and form arginosuccinate

Question 22

what is arginosuccinate composed of? (two molecules combined)

Answer 22

arginine and succinate =)

Question 23

what does arginosuccinate break apart into during the urea cycle?

Answer 23

a cut will be made within the molecule of arginosuccinate to form a molecule of arginine and a molecule of fumarate

Question 24

once arginosuccinate is broken down into arginine and fumarate what happens next? (what happens to the arginine)

Answer 24

the next step is to get a molecule of urea out of the molecule of arginine through an enzyme called arginase
- so the arginine gets broken down into urea and ornithine

Question 25

arginase (enzyme) - what does it breakdown in the urea cycle

Answer 25

this enzyme will kick out a urea group from the molecule of arginine - ultimately producing a molecule of urea and a molecule of ornithine and the urea cycle is completed
- Urea produced by the liver is then released into the bloodstream, where it travels to the kidneys and is ultimately excreted through our urine

Question 26

REVIEW: where do the two nitrogrens come from to be used in the urea cycle?

Answer 26

both nitrogens will come from the mitochondria of the liver cell (the liver cell houses all the free form ammonia) - one comes from ornithine shuttle carrier which picks up a molecule of carbamoyl phosphate and forms citrulline; the other comes from aspartate
- both will transport into the cytoplasm for the urea cycle to occur

Question 27

What happens if we do not have enough oxaloacetate or too much fumarate (fumarate gets produced from the oxaloacetate) during urea cycle?

Answer 27

too much fumarate or too little oxaloacetate will ultimately caused the urea cycle to become blocked and not function properly because without oxaloacetate there will be no aspartate either which is crucial for the addition of its nitrogen required for the urea cycle

Question 28

why is urea cycle not a closed cycle?

Answer 28

technically it will be a closed cycle because it will coupled with another cycle called TCA cycle (citric acid cycle) which creates a closed loop cycle - we couple it together because urea is extremely energy consuming so it needs to be coupled with an energy producing cycle to ultimately keep the urea cycle going

Question 29

what is the shared component of the TCA cycle and urea cycle? (what are they sharing to make them a loop cycle)

Answer 29

arginosuccinate and aspartate - and the coupling process is called aspartate-argininosuccinate shunt of citric acid cycle also known as the Kreb’s bi-cycle

Question 30

what molecules can and cannot pass through the mitochondria membrane

Answer 30

able to pass through mitochondria: malate and aspartate
not able: oxaloacetate and fumarate
- oxaloacetate needs to be converted into aspartate to transport out of mitochondria and into cytoplasm to be used in urea cycle or convert back to oxaloacetate to be used in TCA cycle to produce the energy that is needed to power urea cycle
- fumarate needs to be converted into malate to transport back into the mitochondria through the TCA cycle and be used in the Kreb’s cycle

Question 31

how is urea excreted

Answer 31

once urea is produced the urea will be transported into the blood and then the kidney will pickup the urea from our blood and excrete it through our urine

Question 32

beginning of urea cycle - explain it

Answer 32

The urea cycle begins in the mitochondria with the transfer of ammonia from either glutamate or glutamine to a phosphorylated molecule of bicarbonate by the enzyme carbamoyl phosphate synthetase 1, creating carbamoyl phosphate