NBME - 6/10 - Excretion of excess nitrogen: Urea Cycle AND Disorders

Question 1

What are we doing with transamination?

Answer 1

Involves the transfer of an amino group from one amino acid, thus making it an alpha keto acid, and putting it on an alpha keto acid, making that one an amino acid.

We do them in order to synthesize and degrade amino acids.

Question 2

What amino acid does not undergo transamination

Answer 2

Lysine does not

Question 3

What is PLP and what do we use it for

Answer 3

PLP is a cofactor for transamination reactions and is derived from Vitamin B6

Question 4

Vitamin B6 deficiency leads to:

Answer 4

A decrease in synthesis of neurotransmitters, NAD, and heme, resulting in neurologic and pellagra-like symptoms and anemia due to no PLP.

Question 5

Discuss the role of glutamate dehydrogenase

Answer 5

Catalyzes the oxidative deamination of glutamate. The ammonium ion is released and alpha-ketoglutarate is formed.

This is a readily reversible reaction that requires either NAD+ or NADP+

Question 6

Histidase mechanism

Answer 6

Histadase deaminates Histidine to make urocanate and NH4+

Question 7

Serine dehydratase does what?

Answer 7

Converts serine to pyruvate and threonine to alpha keto-butyrate

NH4+ is released

Question 8

What is the purine nucleotide cycle

Answer 8

This cycle serves to release NH4+ from amino acids particularly in muscle.

Glutamate collects nitrogens from other amino acids and transfers them to aspartate via transamination reactions.

Aspartate reacts with inosine monophosphate (IMP) to form adenosine monophosphate (AMP) and generate fumarate.

NH4+ is released from AMP and IMP is re-formed.

Question 9

Ammonia travels from other tissues to the liver in what form?

Answer 9

Alanine and glutamine

Question 10

So we do the urea cycle in order to get rid of waste. This waste begins all the way down at the mitochondrial level, where the mitochondria generate CO2 and NH4+. What is the first synthetic reaction of the urea cycle that involves these waste products?

Answer 10

Carbamoyl phosphate is synthesized in the first reaction from NH4+, CO2, and 2 ATP (2ADP are thus also formed)

The enzyme used is Carbamoyl phosphate synthetase I, which is located in the mitochondria and activated by N-acetylglutamate.

Question 11

So now we’ve turned our waste products into a thing, carbamoyl phosphate. What now?

Answer 11

Ornithine reacts with carbamoyl phosphate to form citrulline, releasing inorganic phosphate in the process.

The enzyme used is ornithine transcarbamoylase which is also found in mitochondria. This product, citrulline, is transported to the cytosol in exchange for cytoplasmic ornithine.

Question 12

Alright so we combined our waste products to save space, and turned that product into citrulline so that we could ship it out of the mitochondria. What do we do with the citrulline?

Answer 12

Citrulline combines with aspartate to form argininosuccinate in a reaction that is driven by hydrolysis of ATP to AMP and inorganic pyrophosphate.

Enzyme used is argininosuccinate synthetase

Question 13

Alright…so we combined waste, made some space, changed the thing to get it out of the mitochondria, and meshed it with something else. Why did we do this, and what is next?

Answer 13

We meshed Citrulline and Aspartate together to make argininosuccinate because we need to break this guy down into the two things we want more than citrulline and aspartate: Arginine and Fumarate.

Argininosuccinate lyase breaks our big guy up into Arginine and fumarate in the cytosol

Question 14

Why did we want to make fumarate?

Answer 14

The carbons of fumarate can be converted to malate or can just link back to the Krebs Cycle.

In the fasting state of the liver, malate can be converted to glucose or to oxaloacetate, which is transaminated to regenerate the aspartate required for the previous step.

Malate to oxaloacetate MUST occur in the mitochondria.

Question 15

Alright so we discussed fumarate but why did we want to make arginine? What is next?

Answer 15

Arginine completes our biochemical cycle. We break it down with arginase to make ornithine (needed for the earlier reaction to make citrulline so that we could leave the mitochondria) and our good new friend urea.

Araginase is located primarily in the liver and is inhibited by ornithine (no sense breaking up all of our arginine and generating waste if we have excess ornithine floating around which indicates we aren’t’ really doing anything with waste at that moment)

Question 16

Urea accounts for ___% of nitrogenous waste products

Answer 16

90

Question 17

Ornithine is recycled, but what do we do if we need more?

Answer 17

When the cell requires additional ornithine, it is synthesized from glucose via glutamate.

Question 18

Is Arginine an essential amino acid?

Answer 18

Nope, we make it ourselves from glucose via ornithine and the first four reactions!

Question 19

N-Acetylglutamate does what to the urea cycle? What is it made of?

Answer 19

This guy is an activator of Carbamoyl phosphate synthetase I, the first enzyme we used for the urea cycle.

Arginine stimulates N-Acetylglutamate synthase, which makes N-Acetylglutamate from acetyl-CoA and glutamate.

The idea is that more arginine means we have waste products to get rid of, so lets keep the cycle going.

Question 20

What type of diet induces the urea cycle?

Answer 20

High protein diet of at least 4 days

OR

Starvation

Question 21

Loss of activity of any urea cycle enzyme leads to what?

Answer 21

Hyperammonemia

Question 22

What is the recommended protein intake per day?

Answer 22

0.8 g/kg body weight

Question 23

What is nitrogen balance?

Answer 23

Degradation of body protein equals synthesis of new protein (amount of nitrogen excreted = intake)

Ex. Negative = More nitrogen excreted than consumed (degradation exceeds synthesis)

Question 24

Defective carbamoyl phosphate synthetase I yields what?

Answer 24

Ammonia accumulation in blood and urine.

This is a very rare disorder that is fatal with no treatment.

Question 25

Defective Ornithine Transcarbamoylase yields what?

Answer 25

Accumulation of Ammonia and Orotic acid.

This is an X-linked disorder and is the most common of all of the urea cycle disorders.

The reason we get the orotic acid is because the carbamoyl phosphate produced in the mitochondria gets bored, diffuses into the cytoplasm, and starts activating pyrimidine synthesis.

Question 26

Defective Argininosuccinate synthetase yields what?

Answer 26

Ammonia and citrulline buildup.

This is the second most common defect. Mental retardation can result if this is not effectively treated

Question 27

Defective Argininosuccinate lyase?

Answer 27

Get a build up of ammonia and argininosuccinate (big stretch there huh)

This ammonia buildup isn’t as bad as the other urea cycle defects since two nitrogens have now been slapped into argininosuccinate which can be excreted. This just makes arginine an essential amino acid since we can no longer generate it.

Question 28

Defective arginase yields what?

Answer 28

Ammonia and arginine (seriously, if any of these are questions folks, you better nail them).

Same as the argininosuccinate lyase since we can just excrete arginine like urea. Again, makes arginine essential since we’re just dumping it.

Question 29

How many ATP per Ammonium do we need for the urea cycle

Answer 29

3 ATP

Question 30

Discuss the metabolic changes that occur in the body due to starvation after 3 - 5 days

Answer 30

Muscles decrease use of KBs and oxidize fatty acids as primary energy sources, and because of this decreased use, blood KBs can rise.

In response to extra KBs in the blood, the brain decides to start grabbing them up and oxidizing them for energy, allowing it to decrease its use of glucose.

Liver says hey, guess we can get by, and slows down on the gluconeogenesis, which spares the muscle protein, which also leads to a decreased protein level.

Question 31

Before we go into this protein sparing prolonged starvation mode, we eat our own muscles. Describe this and what stimulates this.

Answer 31

The major trigger for all of this is acetyl CoA.

During early fasting, muscle protein is degraded to make amino acids which get partially metabolized by muscle to form alanine and glutamine for transport to the liver (tissues like the gut and kidney tend to munch on the glutamine a little when it’s in circulation).

In the liver, these guys get turned into glucose or ketone bodies with their nitrogen parts getting turned to urea.

Question 32

During fasting, muscle ___________________.

During exercise, muscle ___________________.

(In regards to energy use specifically)

Answer 32

Fasting: Muscle oxidizes fatty acids released from adipose tissue and ketone bodies produced by the liver

Exercise: Muscle can also use its own glycogen stores as well as glucose, fatty acids and ketone bodies from the blood.

Question 33

Albumin is made by _____

Answer 33

the liver

Question 34

What’s so great about NH3, which can convert between itself and NH4+?

Answer 34

Ammonia is released into the urine where it forms NH4+ to buffer the hydrogen ions produced by phosphoric acid, sulfuric acid, (made from cysteine), and various metabolic acids

Question 35

What is the glucose-alanine cycle?

Answer 35

Glucose leaves the liver, enters the muscle and gets turned into pyruvate. Pyruvate and NH3 come together to make alanine, which can now travel back to the liver. Alanine breaks into pyruvate and urea. Pyruvate becomes glucose again and urea goes bye bye

Question 36

What is the rate determining enzyme of gluconeogenesis and what stimulates it?

Answer 36

RDS is Pyruvate carboxylase activated by an increase in acetyl CoA (this was on our last exam)