The Urea Cycle - Online Tutor, Practice Problems & Exam Prep

The urea cycle converts ammonium ions and carbon dioxide into urea, utilizing ornithine as a carrier. It consists of two phases: preparation (Phase A) and conversion (Phase B). Key reactions include the transfer of the carbamoyl group to ornithine, condensation with aspartate, cleavage to arginine, and hydrolysis to regenerate ornithine and produce urea. The cycle consumes 4 ATPs, with specific enzymes like ornithine transcarbamylase and arginase facilitating each step. Understanding this cycle is crucial for grasping nitrogen metabolism and waste excretion in organisms.

The Urea Cycle Concept 1

In this video, we take a look at the urea cycle in terms of an overview. Now, here we're going to say the urea cycle converts the Ammonium ion to urea using ornithine as a carrier. Now, here we're gonna say the Carbon source comes from Carbon Dioxide, which again comes from the Mitochondrial Matrix. We're gonna say our Nitrogen atom source comes from the Ammonium Ion as well as Aspartate. And we're gonna say here that the energy cost of the process is 4 ATPs. We'll see why that is. Now, if we take a look here, we have our ornithine acting as our carrier, and we're gonna say, remember the Ammonium ion, we're going to have to use Carbon dioxide from the mitochondrial matrix, as well as the consumption of ATP, in order to change it into our Carbamoyl Phosphate. Now, here, to do that we use 2 ATP, which produces 2 ADP. We're going to say here our Carbamoyl Phosphate is created and that goes into the urea cycle. We're gonna say we're gonna go through these different steps of the urea cycle which we'll talk in greater detail later on, but we know that at the end of it, we're gonna produce as a byproduct our urea which is produced within the cytosol.

One important thing is, remember we have our phase a and our phase b. Phase a is our preparation phase, and then here when we're talking about phase b, we're talking about our conversion phase. This is where we're gonna produce our urea from our Carbamoyl Phosphate and Aspartate. If we look here, ATP is also consumed in this step 2 here. But here, ATP becomes AMP. So we've lost 2 inorganic phosphates in this process, and then, aspartate comes into the cycle so that we can make urea at the end.

So, just remember, we are utilizing how many ATP molecules? Three: 2 here, and 1 here. And we are releasing how many inorganic phosphates as a result? Well, we're releasing 2 inorganic phosphates here? Each one of these ATP here releases one of them, together that's 2. And over here, we're still releasing 2 inorganic phosphates but it's coming from 1 ATP. Instead of going from ATP to ADP, we're going from ATP to AMP. Alright. So again, this is just a quick overview of the urea cycle before we go into greater details which each of the steps involves.

The Urea Cycle Example 1

Here it says, give the number of ATP molecules consumed in the urea cycle and the actual energy cost of the process. Now if you can recall the overview of the urea cycle, remember we saw 2 ATPs when we're talking about the Ammonium ion being converted into the Carbamoyl phosphate group. So we know we have 2 ATPs. We also know that another ATP molecule is involved with Aspartate incorporating itself into the urea cycle. So that would be a total of 3 ATP molecules. So that means our answer is going to be either B or D. So, the number of ATP molecules is 3, but what is the energy cost of the process? If we think of the energy cost involved in releasing our inorganic phosphate groups, how many inorganic phosphate groups are released? Well, remember we have 2 ATPs becoming 2 ADPs. So that means we'd have to release 2 inorganic phosphates. We'd have to cut those bonds in order to release those inorganic phosphates. And then we also have that 1 ATP molecule, but it itself becomes an AMP. It loses 2 inorganic phosphate groups as well. So that's 4 inorganic phosphates that are produced. That shows us the actual energy cost involved. So, the actual energy cost would be 4. So that would mean option D is our final answer.

Which amino acid provides the NH₄⁺ for the formation of carbamoyl phosphate?

• A. α-ketoglutarate
• B. Glutamine
• C. Glutamate
• D. Aspartate

Which of the following amino acids is not a part of the urea cycle?

• A. Asparagine
• B. Argininosuccinate
• C. Arginine
• D. Ornithine

The Urea Cycle Concept 2

Here we have Phase A, which deals with preparation. Here we're going to have the synthesis of our Carbamoyl Phosphate, which we'll abbreviate as CP, from the Ammonium ion and Carbon dioxide, and it represents an energy-consuming step. Now, we've heard that Mitochondria is the energy powerhouse of the cell. The mitochondria also is a site of the creation of carbon dioxide. Remember, through pyruvate oxidation as well as the citric acid cycle or Krebs cycle, we have the generation of Carbon dioxide. This serves as our source of the carbon atom. Here, we're going to say the enzyme Carbamoyl Phosphate Synthase catalyzes the reaction. Remember, a Synthase is going to help link molecules together through the use of energy. We're going to say that 2 ATP molecules are hydrolyzed to produce 2 ADPs as well as 1 inorganic phosphate. Now, here we are creating 2 bonds, one of them being a highly energetic bond; that's why 2 ATPs are required. One ATP would not supply enough energy to convert our Ammonium ion with the incorporation of Carbon Dioxide into our carbamoyl phosphate group. So here, if we take a look, we have our ammonium ion, we have carbon dioxide, through the use of our enzyme, which again is the substrate name followed by the class of enzyme, which is in this case a synthetase. Because we're making links between molecules through the use of energy, we have to use this class of enzyme. We have our 2 ATP which become 2 ADP as well as our inorganic phosphate. So here, we would write the inorganic phosphate within this little box. Alright. So this will give us our carbamoyl phosphate. This will represent the beginning of phase A in terms of preparation.

The Urea Cycle Concept 3

Here in Phase B we deal with conversion. Here we're going to say the urea cycle is a cyclic pathway consisting of 4 reactions. We're going to say reaction 1 happens within the mitochondrial matrix. Here we have Carbamoyl Phosphate being changed into Citrulline. Reactions 2, 3, and 4 occur within the cytosol itself with the end reaction helping us to produce urea. Now, here the cycle basically utilizes 3 non-protein amino acids in order to function. So just remember when it comes to the urea cycle, we have phase A which deals with preparation and then phase B where we have these 4 reactions that is where we talk about conversion. Alright. So now that we know the general overview of the urea cycle, we'll go deeper in terms of these different types of reactions.

The Urea Cycle Concept 4

In this video, we'll talk about reaction 1 when it comes to Phase B of the urea cycle. Here we're dealing with transfer. In it, the carbamoyl group is transferred from CP or Carbamoyl Phosphate to ornithine to produce Citrulline. Now, here it's catalyzed by the enzyme OTC for short. It's a mouthful so it's easier to call it OTC which is ornithine Transcarbamoylase. So we're going to abbreviate that just to OTC. So if we take a look at this reaction, we have our ornithine, which remember represents our carrier. We have our carbamoyl phosphate. It will release our inorganic phosphate group. We have our enzyme being used OTC. Citrulline here, again, it's going to gain the carbamoyl group, which is basically what's shaded here in green. So, now, the nitrogen is going to be connected to the carbonyl, that carbonyl is still connected to its NH₂ group. This represents our citrulline. Now, citrulline itself is transported outside the mitochondria into the cytosol so that we could continue with reactions 2, 3, and 4.

The Urea Cycle Example 2

Here it says, name the amino acid formed when ornithine reacts with carbamoyl phosphate. Now remember, what happens here is that we're going to have our ornithine gain the carbamoyl group. Doing this will help to create the only amino acid in Reaction 1 that's produced, which would be citrulline. Citrulline is then transported outside of the mitochondria into the cytosol so that we can continue with the urea cycle, which encompasses reactions 2, 3, and 4. Remember, at the end of reaction 4, our final product will be urea, which is then excreted in our urine. So here, the only answer is option b.

The Urea Cycle Concept 5

Here we're going to say that reaction 2 of Phase B of the urea cycle is dealing with condensation. Here, citrulline undergoes condensation with aspartate to produce arginosuccinate. Now, this is catalyzed by the enzyme arginosuccinate synthetase. Again, remember, energy is being used in order to do this where we have molecules that are mingling together, so synthetase is the class of enzyme required. The name itself, remember, is the substrate followed by the class of enzyme.

Now, in this process, 1 ATP is hydrolyzed to 1 AMP and 2 inorganic phosphates. This is equivalent to saying you have 2 ATP becoming 2 ADP. If we take a look here, we have our citrulline, it gained this carbamoyl group from our ornithine molecule in reaction 1. Here, we have our aspartate. We utilize ATP in order to produce AMP and 2 inorganic phosphates. We have our enzyme that's being utilized. We're going to say as a result of this, we're going to create our connection of arginine to arginosuccinate. Basically, we're going to have this carbon here single-bonding itself to nitrogen. Nitrogen here, ideally wants to make 3 bonds in order to stay neutral, so it's just going to be an NH group. This represents our argininosuccinate group.

Then here, succinate, remember, itself, is the deprotonated form, the part that the carboxylic acids have given away the H+. It's a dicarboxylic acid that has given away its 2 acidic hydrogens. So we have 2 carboxylate ends here. This represents our succinate molecule.

The Urea Cycle Concept 7

The funnel reaction deals with a hydrolysis reaction 4. Here the enzyme Arginase hydrolyzes arginine to ornithine and urea. So here we have our arginine. We're going to utilize our arginase, we're going to bring in water in order to do this hydrolysis. Remember, we're utilizing water in a hydrolysis reaction. When we do this, we're going to create our ornithine and our urea. Remember, our urea is a carbonyl connected to 2 NH₂ groups. Alright. So if we take a look here, we're severing the connection here. This carbon here is this carbon here. It possesses its 2 NH₂ groups still and it's going to gain a carbonyl in order to make urea as our byproduct that's excreted through our urine. The ornithine that's created, it's transported back to the mitochondrial matrix to begin the urea cycle again. So here, the ornithine in this step is being regenerated. We had it earlier in terms of this cyclic process and now it's recreated in reaction 4. Alright. So reaction 4 here, we're doing hydrolysis in order to create our carrier ornithine once again as well as our waste product in the form of urea which again is excreted through our urine.

The Urea Cycle Example 3

Here it says, which of the following statements explains the structure of the carbamoyl group accurately? Alright. So our carbamoyl phosphate, well, it is a carbonyl, and it has an NH₂ group attached to it. And then we'd say that that carbonyl carbon is connected to an oxygen as well as an inorganic phosphate. This is what our CP is, our carbamoyl phosphate group. But here they're not asking for a CP, they're just asking for the carbamoyl group. The carbamoyl group would just be this portion here. We're not looking at the Oxygen with the inorganic phosphate portion. So if we take a look, which one best describes it? Here, 2 NH₂ groups bonded to the carbonyl group. No. That is a definition or a description of the urea molecule, so that doesn't work. 2 OH groups bonded to a carbonyl group. That also doesn't describe this. An NH₂ group bonded to a carbonyl group. Alright. So here's our NH₂ group connected to a carbonyl group. That is what a carbamoyl group is, so this is our answer. But let's look at D just to make sure. A carbonyl group bonded to 1 NH₂ group and 1 OH group. That doesn't describe what a carbamoyl group is. So here the best answer would be option C.

What is the function of aspartate in the urea cycle?

• A. Produces CO₂ for carbamoyl phosphate synthesis.
• B. Provides –NH₂ group as ammonium ion.
• C. Produces succinate for citrulline formation.
• D. Provides the second nitrogen atom for urea.

Which one of the following sentences is an incorrect description of a reaction in the urea cycle?

• A. Citrulline condenses with aspartate to produce argininosuccinate.
• B. Hydrolysis of arginine produces urea and regenerates citrulline.
• C. Transfer of carbamoyl group from carbamoyl phosphate to ornithine produces citrulline.
• D. Argininosuccinate undergoes cleavage to produce arginine and fumarate.

The Urea Cycle Concept 8

Now, the urea cycle is made up of 4 reactions, yes, but it has a lot involved with it. We have different types of metabolites and different types of enzymes involved. To help us memorize these, we're going to utilize 2 memory tools. The first memory tool is going to help us memorize the metabolite names. So here, we're going to say, when it comes to memorizing or remembering the urea cycle, the reaction of the urea cycle can be remembered by memorizing the metabolite names. So before we talk about this memory tool, just a little bit of history about me. So I am Jules Bruno Junior. My father's name is Jules Bruno. My father was a cab driver for nearly 30 years in Miami, Florida. At different times after school when he picked me up, I'd be in the cab with him. I'd hear interesting conversations with each of his fares. And this kind of inspires this memory tool. So if we take a look at this memory tool, we're going to say here that ordinary carpooling citizen and we're going to say here, aspires arranged success, and it's going to fuel arguments and utters opinions. So, ordinary carpooling citizen in a cab, we're going to say Aspires Arranged Success. So that can be any success of the people being ridden in the cab. That can mean a lot of things. Fuels arguments. I've heard some interesting debates between the people in the car, the people on the phone with, my dad talking about them with them with different things, utters opinions, strong opinions sometimes being uttered between people in the car. So it's always quite interesting. But how does this fit into the urea cycle? Okay. So, ordinary or we start with ornithine, our carrier molecule. We're going to say Carpooling Car carbamoyl phosphate, carp. Citizen is citrulline. And if we look, these 3 are all part of reaction 1. Here goes 1. Next, we have, aspires, arrange, success, aspartate, aspires, arranged. Here we're talking about argininosuccinate. Success, we have succinate involved. R succ, argininosuccinate. Fuels arguments, fuels fumarate. Arguments arginine. Again, if we go here, this is reaction 2, which is our aspartate. Reaction 2 is our aspartate, and then argininosuccinate. Reaction 3 is fumarate and arginine. And then finally, reaction 4 utters opinions. We have urea and ornithine being regenerated for reaction 4. Remember, arginine, we utilize the enzyme Arginase that helps to produce urea as a product that we excrete in urine and then regenerates ornithine. So, again, utilize this memory tool, think about a cabbie, picking up different fares, hearing interesting conversations, hearing opinions being exchanged, maybe between the cab driver and the passengers, or the passengers amongst one another. Right? So this helps to frame the different types of metabolite names within the urea cycle.

The Urea Cycle Concept 9

Here we're going to say the name of the enzyme can be predicted by knowing the substrate and the type of reaction. Here, our reaction types are transfer, condensation, cleavage, and hydrolysis. And, our memory tool 2 here will be "train, conductor cleans house." So "train conductor cleans house." Here are hints corresponding to the different reactions. So hint 1 is reaction 1. This will be reaction 2, 3, and 4. For reaction 1, which relates to hint 1, we have transfer and it's catalyzed by ornithine transcarbamylase, which is OTC. OTC. Hint 2, we have condensation. It's catalyzed by our synthetase. Cleavage we're cleaning. This is catalyzed by our lyase. And then for reaction 4, hint 4, hydrolysis is catalyzed by our Arginase. Again, use this in conjunction with your memory tools.

For example, hint 4, which is reaction 4, we're talking about our arginine becoming our ornithine being recreated as well as urea. So because it's arginine, we know that arginase has to be the enzyme that's being used. So again, use this memory tool in conjunction with memory tool 1 to help guide you to the different types of metabolites involved and thereby understanding the enzymes needed.

The Urea Cycle Example 4

Here in this example, it says, "What are the products of the 3rd reaction of the urea cycle?" In order to answer this question, we need to think about memory tool 1. So, reaction 3 deals with line 3, and in memory tool 1, line 3 is 'fuels arguments'. And if we take this portion and this portion, that tells us the metabolites involved in that reaction and thus the products formed. So 'Fu' in 'Fuels' stands for fumarate. 'Arg' in 'Arguments' is arginine. That would mean that the answer here would have to be option b, fumarate and arginine, which corresponds to 'fuels arguments'. Although option d has fumarate as well, 'asper tate' is not 'arg' in 'arguments', so it doesn't fit. The other options are not even close. So, again, the final answer would have to be option b: fumarate and arginine.

Which of the following metabolites is hydrolyzed in the urea cycle to produce ornithine and urea?

• A. Citrulline
• B. Arginine
• C. Aspartate
• D. Argininosuccinate

Which enzyme catalyzes the reaction of citrulline with aspartate to produce argininosuccinate?

• A. Succinate dehydrogenase
• B. Argininosuccinate lyase
• C. Argininosuccinate synthetase
• D. Arginine oxidase

Write the total number of amino acid metabolites in the urea cycle. How many of those amino acids are not found in proteins?

• A. 5, 2
• B. 4, 3
• C. 4, 2
• D. 5, 3

Here’s what students ask on this topic:

What is the urea cycle and why is it important?

The urea cycle is a metabolic pathway that converts toxic ammonium ions (NH₄⁺) and carbon dioxide (CO₂) into urea, which is then excreted in urine. This cycle is crucial for detoxifying ammonia, a byproduct of protein metabolism, and maintaining nitrogen balance in the body. The cycle involves two phases: preparation (Phase A) and conversion (Phase B), and it consumes 4 ATP molecules. Key enzymes like ornithine transcarbamylase and arginase facilitate the reactions. Understanding the urea cycle is essential for grasping nitrogen metabolism and waste excretion in organisms.

What are the main steps involved in the urea cycle?

The urea cycle consists of two phases: preparation (Phase A) and conversion (Phase B). In Phase A, carbamoyl phosphate is synthesized from ammonium ions and carbon dioxide, consuming 2 ATPs. In Phase B, the cycle involves four reactions: (1) transfer of the carbamoyl group to ornithine to form citrulline, (2) condensation of citrulline with aspartate to form argininosuccinate, (3) cleavage of argininosuccinate to produce arginine and fumarate, and (4) hydrolysis of arginine to regenerate ornithine and produce urea. The cycle consumes a total of 4 ATPs and involves key enzymes like ornithine transcarbamylase and arginase.

How many ATP molecules are consumed in the urea cycle?

The urea cycle consumes a total of 4 ATP molecules. In the preparation phase (Phase A), 2 ATPs are used to convert ammonium ions and carbon dioxide into carbamoyl phosphate. In the conversion phase (Phase B), 1 ATP is hydrolyzed to AMP and 2 inorganic phosphates during the condensation of citrulline with aspartate to form argininosuccinate. This is equivalent to using 2 ATPs. Therefore, the total energy cost of the urea cycle is 4 ATPs.

What enzymes are involved in the urea cycle?

The urea cycle involves several key enzymes: (1) Carbamoyl Phosphate Synthetase I (CPS I) catalyzes the formation of carbamoyl phosphate from ammonium ions and carbon dioxide, (2) Ornithine Transcarbamylase (OTC) transfers the carbamoyl group to ornithine to form citrulline, (3) Argininosuccinate Synthetase catalyzes the condensation of citrulline with aspartate to form argininosuccinate, (4) Argininosuccinate Lyase cleaves argininosuccinate to produce arginine and fumarate, and (5) Arginase hydrolyzes arginine to regenerate ornithine and produce urea. These enzymes facilitate the various steps of the urea cycle, ensuring efficient conversion of toxic ammonia into urea.

What is the role of ornithine in the urea cycle?

Ornithine acts as a carrier molecule in the urea cycle. It plays a crucial role in the transfer and conversion phases of the cycle. In the first reaction of Phase B, ornithine combines with carbamoyl phosphate to form citrulline, facilitated by the enzyme ornithine transcarbamylase (OTC). Later, in the final reaction of the cycle, arginase hydrolyzes arginine to regenerate ornithine and produce urea. The regenerated ornithine is then transported back to the mitochondrial matrix to begin the cycle again. Thus, ornithine is essential for the continuous operation of the urea cycle.