AA Catabolism & Urea Cycle

Question 1

What are the 4 processes that AAs are used for?

Answer 1

• Protein synthesis
• Glucose synthesis
• Ketogenesis
• Synthesis of nitrogen-containing compounds

Question 2

What are the two components that make up AAs and what is the fate of each?

Answer 2

• NH3 group: excreted as urea or NH4+ into urine

- Carbon skeleton (used in TCA Cycle or Gluconeogenesis)

Question 3

What are the 3 groups by which AAs are classified?

Answer 3

• Glucogenic: AAs enter TCA Cycle and lead to increase of OAA which is used for gluconeogenesis
• Ketogenic: AAs form Acetyl CoA or Acetoacetate
• Glucogenic/Ketogenic: AAs used for either gluconeogenesis or ketogenesis

Question 4

With Ketogenic AAs, why can’t the Acetyl CoA be used to make glucose?

Answer 4

The 2 Cs from Acetyl CoA are lost to CO2

Question 5

Where are AAs catabolized?

Answer 5

LIVER ONLY

Question 6

What is a transamination reaction? What type of enzymes are used for these reactions?

Answer 6

Transamination is the removal of NH3 from AAs to produce the carbon skeleton of AAs (alpha-ketoacids)

• Uses Aminotransferase enzymes (transaminases)

Question 7

What are the two starting substrates and two end products of every transamination reaction? What accepts the NH3 from the AA?

Answer 7

• Alpha-Ketoglutarate accepts the NH3 to produce Glutamate

- AAs produce the carbon skeleton (alpha-ketoacids)

Question 8

In what part of the body do non-branched chain AAs undergo transamination? In what part of the body do branched chain AAs undergo transamination?

Answer 8

• Non-branched chain AAs: transamination in liver

- Branched chain AAs: transamination in muscle

Question 9

AAs arriving to the liver carry nitrogens from what two sources? Are these AAs arriving to the liver branched chain or non-branched chain AAs?

Answer 9

AAs arriving to the liver carry nitrogen from:

• Dietary AAs
• AAs broken down by muscle in fasting state

These AAs are non-branched chain

Question 10

Why can’t BCAAs be metabolized by the liver? Where are BCAAs metabolized?

Answer 10

Liver lacks the enzymes (aminotransferases) to deaminate BCAAs

BCAAs are metabolized in muscle

Question 11

What are the three AAs metabolized in muscle? (hint: branched chain AAs)

Answer 11

• Valine
• Isoleucine
• Leucine

Question 12

What is the fate of muscle AAs? Describe the process.

Answer 12

1. BCAAs are deaminated (NH3+ removed) and sent to the muscle where they are metabolized
2. The nitrogen is given to Glutamate
3. The nitrogen is then added to either Alanine or Glutamine and enters circulation

Question 13

What happens to Alanine after it is produced in the muscle? What happens to Glutamine after it is produced in the muscle?

Answer 13

They are sent into circulation and taken up by:

• Alanine: taken up by liver and metabolized like non-branched-chain AAs
• Glutamine: taken up by kidneys and serves as an alternative nitrogen excretion mechanism (as NH4+)

Question 14

What is the Glucose Alanine Cycle?

Answer 14

Allows for the transfer of BCAA amino nitrogens from the muscle to liver

Question 15

What is the first step of the Glucose Alanine Cycle? What is the enzyme used in this step?

Answer 15

Glutamate + Pyruvate > Alanine + alpha-ketoglutarate

- via MUSCLE Alanine Aminotransferase (ALT)

Question 16

What is the second step of the Glucose Alanine Cycle? What is the enzyme used in this step?

Answer 16

Glutamate + Pyruvate > Alanine + alpha-ketoglutarate

- via LIVER Alanine Aminotransferase (ALT)

Question 17

What is the fate of the Glutamate produced in the Glucose Alanine Cycle?

What is the fate of the Pyruvate produced in the Glucose Alanine Cycle?

Answer 17

• Glutamate is used as a key intermediate in nitrogen disposal pathway (urea or glutamine)
• Pyruvate is used in gluconeogenesis

Question 18

What are the two ways by which glutamate delivers nitrogen to the Urea Cycle? What is the enzyme used for each process?

Answer 18

1. Amino group from glutamate is deaminated to produce NH3 (then protonated to NH4+): Glutamate > NH3 + alpha-ketoglutarate
   - uses Glutamate Dehydrogenase
2. Amino group from Glutamate incorporated into Aspartate: Glutamate + OAA > Aspartate + alpha-ketoglutarate
   - uses Aspartate Aminotransferase

Question 19

What are the sources of the two nitrogens found on urea in the Urea Cycle? (hint: come from glutamate)

Answer 19

1. NH4+ via reaction by Glutamate Dehydrogenase

2. Aspartate via reaction by Aspartate Aminotransferase

Question 20

What is the function of the Urea Cycle and where does it occur?

Answer 20

Urea Cycle disposes of nitrogen in a non-toxic form (urea) and transports it to the kidneys where it is excreted as urine
- Occurs in the liver

Question 21

What is the energy requirement of the Urea Cycle and where does this energy come from?

Answer 21

ATP is required - this ATP comes from B-oxidation

Question 22

What is the rate limiting enzyme of Urea Cycle? How is this enzyme regulated?

Answer 22

CPSI (Carbamoyl Phosphate Synthetase I)
- Activated by high glutamate in the liver (need for nitrogen disposal)
+ N-acetylglutamate
- Acidosis (protons)

Question 23

What are the three fates of the Glutamate produced from AAs (either from muscle or diet)?

Answer 23

1. Glutamate + OAA > Aspartate + alpha-ketoglutarate (via Aspartate Aminotransferase)
2. Glutamate > NH3 + alpha-ketoglutarate (via Glutamate Dehydrogenase)
3. Glutamate + NH4+ > Glutamine (via Glutamine Synthetase)

Question 24

When additional NH4+ is needed to produce glutamine, which enzyme is utilized? Where is this enzyme found in the body?

Answer 24

Glutamate Dehydrogenase

- Found in liver and muscle

Question 25

Which enzyme functions as a “mop-up” and what does this mean? Where does this process specifically occur?

Answer 25

In the perivenous hepatocytes of the liver, Glutamine Synthetase obtains NH4+ from the circulation that was not metabolized by the urea cycle, and uses it to make glutamine

Question 26

Why does the “mop up” reaction only occur in perivenous hepatocytes?

Answer 26

Perivenous hepatocytes are oxygen-poor and found downstream from the periportal hepatocytes, which are oxygen-rich
- Glutamine Synthetase is found in higher levels in the perivenous hepatocytes

Question 27

What is the purpose of periportal hepatocytes?

Answer 27

In oxygen-rich environment that is sufficient for ATP synthesis, allowing Urea Cycle and Gluconeogenesis to occur

Question 28

Can the Urea Cycle occur without Gluconeogenesis?

Answer 28

NO (and vice versa)

Question 29

Why is NH4+ typically utilized in reactions and not NH3?

Answer 29

NH3 is quickly protonated once it crosses the liver membrane, so NH4+ is typically used

Question 30

What are the starting substrates and end products of renal ammoniagenesis?

Answer 30

NH2 (amino group) from Glutamine/Glutamate > NH3

Question 31

What is the fate of NH3 produced by renal ammoniagenesis?

Answer 31

NH3 binds to protons from the blood, producing NH4+

- NH4+ is then filtered into urine

Question 32

What is the carbon skeleton (alpha-ketoglutarate) left by the renal ammoniagenesis reaction used for?

Answer 32

Renal gluconeogenesis

Question 33

In what form do nitrogens enter the urine from the kidneys?

Answer 33

NH4+

- NOT Glutamine

Question 34

What are the three positive regulators of the Urea Cycle?

Answer 34

1. N-Acetylglutamate activates CPSI (rate limiting enzyme of Urea Cycle)
2. Low I/G ratio (glucagon increases synthesis of Urea Cycle enzymes)
3. High substrate availability (high protein diet)

Question 35

How does N-Acetylglutamate act as an activator of the Urea Cycle?

Answer 35

Synthesis of N-Acetylglutamate is increased when Glutamate levels are high AND Acetyl CoA levels are high
- N-Acetylglutamate then activates CPSI and increases activity of the Urea Cycle

Question 36

What are the three negative regulators of the Urea Cycle?

Answer 36

1. Protons inhibit CPSI (acidic blood)
2. Decreased B-oxidation (lack ATP)
3. Low substrate availability (low protein diet)