Urea Cycle

where do amino acids come from and where do they enter

dietary protein, protien turn over, and de novo synthesis- they enter the metabolic amino acid pool

When amino acids are broken down whaty do they undergo?

deamination

When amino acids are broken down , what does this seperate them into

• α-keto acids (carbon skeletons)

• amino group

where does the α-keto acids (carbon skeletons) go once they have been produced

into central metabolic pathways

what comes from the amino group after deamination

ammonia

Since ammonia is highly toxic to the brain , it cannto accumulate, what does the body do to counterract this

convert ammonia in to urea so it is safe to excrete

what is the purpose fo deamination

the removal of the amino group

Amino acids are improtant for transport and distribution of amino groups. Which 4 amino acids are especially important

• glutamate

• glutamine

• alanine

• aspartate

Amino acids are improtant for transport and distribution of amino groups. They are:

• glutamate

• glutamine

• alanine

• aspartate

Why are they used

because they can he,p move nitrogen and keep the carbon skeleton useful by easily converting to central metabolic intermediates

what does glutamate convert to

α-ketoglutarate

what does glutamine convert to

→ glutamate → α-ketoglutarate

what does alanine convert to

pyruvate

what does aspartate convert to

oxaloacetate

why is glutamate converting to α-ketoglutarate important

it is a TCA intermediate

why is glutamine converting to glutamte and then converting to a α-ketoglutarate important

it is a nitrogen carrier

why is alanine converting to pyruvate important

it part of central metabolism

why is aspartatre converting to oxaloacetate important

it becomes a TCA intermediate

Why is it useful that these amino acids can be converted into citric acid cycle intermediates when nitrogen is being transported?

When amino acids carry nitreogen, their carbon skeletons cant be wasted . By converting to TCA cycle intermediates the carbon skleton can enter the TCA cycle, be used for energy production , and be used for gluconeogenesis or othe rmetabolic pathways- this allows the body to seperate nitrgoen disposal from carbon metabolism effeciently

What is the main purpose of the urea cycle, and what toxic molecule is it designed to get rid of?

to safely expel ammonia in the body after it was deaminated during protein breakdown

Where in the cell does the urea cycle start, and where else does it occur?

it starts in the mitochondrial matric of liver cells and then parts of the cycle continue in the cytosol

what is significant about the urea cycle starting in the mitochondrial matrix

its in the mitopchondrial matirx of liver cells, which is where the ammonium ion is first incorporated into carbamoyl phosphate

what is significant about the urea cycle having some parts in the cytosol

it allows intermediates like fumarate to link back into the TCA cycle

what is the first committed step of the urea cycle

the formation of carbamoyl phosphate from ammoinum (NH4+)and bicarbonate HCO3

where does the first committed step of the urea cycle take place

the mitochondria

what catalyzes the formation of carbamoyl phosphate from ammonium and bicarbonate ( first committed step)

carbamoyl phosphate synthethase (CPS1)

why is the formation of carbamoyl phosphate from ammonium and bicarbonate via catalyzation of carbamoyl synthetase (CPS1) , committed

because once carbamoyl phosphate is formed the nitrogen is now destined to go through the urea cycle

What two molecules provide the nitrogen atoms for urea in the cycle

• directly from ammonium

• amino acid of aspartate

How does the first nitrogen come from ammonium

• “direct activation” via carbamoyl phosphate

• when it gets activated in the mitochoindria after it combines with bicarbonate via carbamoyl phosphat synthetase 1 (CPS1) , it forms carbamoyl phosphate which is now a high energy, reactive form of nitogen ready to enter the urea cycle

How does the seconds nitrogen come from the amino group of aspartate

• it comes later in the cytosol, but not the same activation as ammonia.

• “brought in later” using energy from ATP ina synthetase reaction

• Instead, energy from ATP is used indirectly in the reaction catalyzed by argininosuccinate synthetase, which first forms a citrulline AMP intermediate , then the amino group from aspartate attacks, and forms argininosuccinate which carries the secon nitrogen forward

How is the activation of 2 nitrogens a cascade

the cycle only continues once the first nitrogen is activated, and the second nitrogen is added i nthe next step to eventually form urea

Explain how nitrogen gets into the urea cycle

basically we have a free nitrogen floating around after deamination of specific amino acid groups- that can reuse their carbon skeletons, but in order to get rid of it we have to activate it (2 nitrogens) in a 2 stage cascade to process it through the urea cycle

basically the first amino acid can be any of the 4 important amino acids for transport and we'll ghet that free nitrogen and actviate it, but where will the secon nitrogen always come from

aspartate- which donates its amino group when citrulline cxondenses with aspartatre to form argininosuccinate

after argininosuccinate is formed in the urea cycle, it is cleaved in to 2 products, what are these products?

arginine and fumarate

what is the enzyme that catcalyzes the reaction of cleaving argininosuccinate into fumarate and arginine

argininosuccinase (also sometimes called argininosuccinate lyase, abbreviated ASL).

What happens to the arginine product after argininosuccinate is cleaved

it continues in the urea cycle and is eventually cleaved by arginase to produce urea and regenerate ornithine- allowing ornithine to go back into the mitochondira to start the next cycle

What happens to the fumarate product after argininosuccinate is cleaved

it enters the TCA cycle ( or can be converted to malate in the cytosol) and can be used to egenrate enrgy or make other metabolites

After argininosuccinate is cleaved into arginine and fumarate, what is the significance of fumarate entering the TCA cycle? Why does the urea cycle “share” this molecule with the TCA cycle?

By feeding fumarate into the TCA cycle, the cell can recover energy and maintain metabolic efficiency.

What happens to arginine at the end of the urea cycle, and why is this step important for the cycle to continue?

Arginase cleaves arginine into urea and ornithine. Urea is excreted safely, and ornithine is recycled back into the mitochondria to kick off the next round of the urea cycle.

Why is the recycling of ornithine critical

without regenerating ornithine, the cycle would halt and ammonia would accumulate, which is toxic.

How does the urea cycle connect to the TCA cycle, and why is this connection metabolically useful?

• ornithine is required to start the urea cycle—it combines with carbamoyl phosphate to make citrulline in the mitochondria.

• Without ornithine, the cycle would stop, ammonia couldn’t be safely disposed of, and you wouldn’t produce argininosuccinate → fumarate → malate, which links to the TCA cycle.

The connecrtion to the TCA cycle form the urea cycle is mainly via

fumarate

How does fumarate link the TCA cycle and the urea cycle

• Argininosuccinate is cleaved into arginine and fumarate.

• Fumarate can be converted to malate, which can either re-enter the mitochondria for the TCA cycle or stay in the cytosol.

• This generates intermediates that can produce NADH, feeding electrons into the ETC for ATP production.

What are the main ways the urea cycle is regulated?

• enzyme levels

• CPS1 activation

• Substrate availibility

How do enzyme levels regulate the urea cycle

The liver regulates the urea cycle mainly by how much of the four urea cycle enzymes are present.

What are the four enzymes that regulated the urea cycle

• Carbamoyl phosphate synthetase I (CPS1)

• Ornithine Transcarbamoylase (OTC):

• Argininosuccinate Synthetase (ASS1)

• Argininosuccinate Lyase (ASL)

In terms of regulating the urea cycle- if we have more enzymes what will be the result

a faster cycle

In terms of regulating the urea cycle- what induces enzyme sysnthesis that allows us to have more enzymes and a faster cycle

high protein intake

How does CPS1 activation regulate the urea cycle

• it is the first and committed step- formation of carbamoyl phosphate by CPS1—is allosterically activated by N-acetylglutamate (NAG).

• NAG itself is made from glutamate + acetyl-CoA, and its production is activated by arginine, which signals high amino acid level

How does substrate availibility regulate the urea cycle

Ammonia and aspartate availability affect the cycle becasue more nitrogen means the cycle runs faster to safely remove it

What molecule allosterically activates CPS1, and why does this make sense physiologically?

NAG- because NAG is onlysynthesized when there is plenty of glutamate and acetyl-CoA (signals that amino acids are available) so CPS1 is activated only when there’s nitrogen to get rid of.

Which enzyme produces NAG, and what else is needed for its activation?

Arginine- it is the allosteric activator of N-acetylglutamate synthase (NAGS)- the enzyme that makes NAG from glutamate + acetyl-CoA.

Why does it make sense that glutamate and arginine signal a high amino acid load for the liver?

Glutamate: It’s central because it’s a main amino group donor and can be converted into α-ketoglutarate, feeding the carbon skeleton into the TCA cycle. So high glutamate signals that there are lots of amino groups available for disposal, i.e., high amino acid catabolism.

Arginine: Its high concentration also signals that the body has plenty of amino acids around, and it allosterically activates NAGS, making more NAG → activating CPS1 → kickstarting the urea cycle.

Why do you think it’s important for the urea cycle to only run when there’s a high amino acid load rather than all the time?

Running the urea cycle all the time would waste ATP unnecessarily because the cycle consumes energy (2 ATP just to make carbamoyl phosphate!). Plus, the body only needs to detoxify ammonia when amino acid breakdown is high

CPS1 is allosterically activated by N-acetylglutamate (NAG). Why do you think it’s clever that the body uses NAG as an activator instead of just running CPS1 at full speed all the time?

Using NAG as an allosteric activator lets the body only “turn on” CPS1 when there’s actually amino acid abundance that needs nitrogen disposal, instead of wasting 2 ATP every time uselessly.

Why is arginine important in this regulation, even though it’s not a substrate in the first CPS1 reaction?

arginine senses high amounts of amino acids present so it turns on NAGS which then turns on NAG which then turns on CPS1 to make carbamoyl phosphate which then continues the urea cycle

Why do you think glutamate is necessary in this regulation cascade?

• Glutamate is a major donor of nitrogen in amino acid metabolism—it’s like the “hub” amino acid for collecting and transferring nitrogen.

• In the NAGS reaction, glutamate provides the amino group that combines with acetyl-CoA to form N-acetylglutamate (NAG).

• So without glutamate, NAG can’t be made, and CPS1 won’t get activated → the urea cycle can’t efficiently dispose of ammonia

What is so significant about Glutamate

it has a double rile in nitrogen metabolism

what is glutamates double role in nitrogen metabolism

• nitrogen donor: It can donate its amino group for transamination or deamination reactions, helping to release nitrogen from other amino acids.

• urea cycle activator: Its amino group is also used by N-acetylglutamate synthase (NAGS) to make NAG, which allosterically activates CPS1, the “starter enzyme” of the urea cycle.

Glutamate both provides nitrogen to be disposed of and ensures the disposal system (urea cycle) is turned on when nitrogen is abundant. It’s like it’s saying: “I’ve got nitrogen, let’s safely get rid of it!”

In the NAGS reaction what does glutamate provide and react with to form NAG

it provides the amino group and reacts with acetyl coa to form NAG

If glutamate wasn’t present in sufficient amounts, what do you think would happen to ammonia levels in the liver?

the ammonia levels would rise becasue it would not be able to form sufficient amounts and work with acetyl coa to form NAG during NAGS, therefore not allowing for the activation of CPS1'