M4: Nitrogen Metabolism L24

Question 1

How is heme produced? What is it important for?

Answer 1

Glycine (amino acid) + succinyl-CoA (CAC) gives you heme. Heme is important for carrying oxygen in the blood.

Question 2

What is glycine used to make?

Answer 2

1. Heme
2. Purine rings: Aspartate and glutamine are also building blocks for the synthesis of the rings
3. Creatine: Energy source for muscles when they contract

Question 3

What is Tyrosine a precursor for? Give examples.

Answer 3

Hormones and neurotransmitters.

Ex: L-Dopa, Dopamine, Epinephrine, Norepinephrine

Question 4

What are the 2 processes to get nitrogen incorporated into our bodies?

Answer 4

1. Fixation (bacteria)

2. Assimilation (plants)

Question 5

What is fixation?

Answer 5

Fixation is a reaction that occurs in Diazotroph bacteria that express the Nitrogenase enzyme. Dizaotroph bacteria have a symbiont relationship with plants in the ground. Without the bacteria and the Nitrogenase enzyme we wouldn’t be able to get nitrogen in our food.
The Nitrogenase reaction uses atmospheric nitrogen along with 16 ATPs to generate ammonia.
Nitrogenase requires ATP to reduce the stable bonds of N2 (triple bond).

Question 6

What is assimilation in plant cells?

Answer 6

Assimilation is the incorporation of ammonia to make other molecules.
The ammonia made from fixation in Diazotroph bacteria can enter the plant cell and be added to glutamate to be converted to glutamine via glutamine synthetase.

Question 7

Describe the uptake of amino acids.

Answer 7

1. In the mouth, chewing starts the mechanical breakdown of proteins
2. In the stomach, chemical digestion of protein begins from HCl and pepsin (enzyme)
3. In the small intestine, polypeptides are broken down into amino acids, dipeptides, tripeptides, by protein digesting enzymes secreted from the pancreas.
4. A variety of different transport proteins move the products from protein digestion into the mucosal cell of the small intestine. Some amino acids share the same transport system.
5. Dipeptides and tripeptides can enter the mucosal cell of the small intestine, once inside they are broken down into single amino acids.
6. Amino acids pass from the mucosal cell into the blood and travel to the liver. The liver regulates the distribution of amino acids to the rest of the body.
7. Some amounts of dietary protein is lost in the feces.

Question 8

Describe where pepsin is found, it’s activated pH, and how it digests dietary proteins.

Answer 8

Pepsin is secreted in the stomach and is activated at a low pH of around 2 (acidic). Whenever in the chain of amino acids, pepsin meets with Phenylalanine, leucine, tryptophan or tyrosine at the Scissile peptide bond, it will break the bond. So it will cut proteins into long chains of polypeptides (long amino acid chains).

Question 9

Describe where trypsin is found, what tissue it digests in, and its role in digestion.

Answer 9

Trypsin is secreted from the pancreas into the duodenum to digest polypeptides that were formed by pepsin. The polypeptides are cleaved to form peptides. It cleaves at the Arginine and Lysine residues at the Scissile peptide bond.

Question 10

Describe where chymotrypsin is found, how it’s activated, what tissue it digests in, and its role in digestion.

Answer 10

Chymotrypsin is secreted from the pancreas into the duodenum and activated by trypsin. It cleaves the Scissile bond at Phenylalanine, tryptophan, and tyrosine to make peptides from polypeptides.

Question 11

Describe where Aminopeptidase and Carboxypeptidase A is found, what tissue it digests in, and its role in digestion.

Answer 11

Aminopeptidase and Carboxypeptidase A are both secreted from the pancreas into the small intestine. They cleave at non specific bonds (it doesn’t matter what amino acid residue is there).
Aminopeptidase cleaves amino acids at the N terminus.
Carboxypeptidase A cleaves at the C terminus.
They cleave peptides to form amino acids that can be taken up by the brush border.

Question 12

Explain the uptake of dietary amino acids at the brush border. Give examples of an amino acid transporter.

Answer 12

The brushborder has transporters that are specific to each amino acid which allows the various amino acids or tripeptides/dipeptides into the enterocyte.
1. Ex of an amino acid transporter…there are MANY others: PEPT1 and PEPT2 can uptake dipeptides, tripeptides, and peptidomimetics. They do it by symport activity (use H+ in the surrounding area and uptake them along with the peptides). They use the H+ gradient which is more abundant in the lumen, to import the peptides into the enterocytes. They couple this with an antiport to recycle the H+ back to the lumen in exchange for importing sodium into the cell.
2. Amino acid sodium symporter.
Once in the enterocyte, there is another amino acid or peptide transporter that get the amino acids and peptides into the blood.

Question 13

How are proteins taken up in neonatal mammals?

Answer 13

Babies might be too young to express some of the amino acid transporters. In babies, there are receptors on the surface of their enterocytes that can take up milk proteins. Then they can either digest milk proteins in lysosomes and then further release the amino acids into the blood. OR the milk proteins can be endocytosed into endosomes and be transported as INTACT proteins into the blood = tanscytosis.

Question 14

What are essential amino acids?

Answer 14

Essential amino acids: need to be ingested from our foods because we cannot produce them.

Question 15

What are non-essential amino acids?

Answer 15

Non essential: even if you dont digest them, it doesnt really matter, because we can synthesize them.

Question 16

What are conditionally essential amino acids?

Answer 16

Conditionally essential: we can produce them from essential amino acids but we need so much that we need to ingest them from the diet because our metabolism will not produce sufficient amounts.

Question 17

What stereochemistry orientation of amino acids are used in our body?

Answer 17

We use L amino acids NOT D amino acids. L amino acids have their side chain on the left.

Question 18

What is the amino acid pool in the circulation used for? Where does it come from?

Answer 18

Dietary proteins are digested into an amino acid pool that can then be used for different things. Also, body proteins can be digested to add more amino acids to the amino acid pool if they are needed.

1. To make body proteins (ex: building muscle for body building)
2. Synthesis of non-protein molecules that contain nitrogen (ex: heme, purines, creatine, neurotransmitters).
3. Synthesis of glucose or fatty acids
4. Broken down for energy production

Question 19

What are the characteristics of the amino acid pool in the body?

Answer 19

1. It must remain homogeneous (the amino acids are the same no matter the source it comes from. SO glutamine from corn is the same as glutamine from meat)
2. The amino acid pool is dynamic (breakdown and synthesis)
3. The free amino acid pool MUST remain constant at all times. So, when you eat you need to use the amino acids to generate different things (ex: body proteins), and when you starve, you need to break down amino acids sources to fill the free amino acid pool.
4. All amino acids must be available in the pool at the same time for protein synthesis, meaning all the different types need to be in a constant balance (can’t be missing any).

Question 20

What does it mean to be in an amino acid equilibrium? Positive balance? Negative balance?

Answer 20

Equilibrium: Intake = Outflow (normal healthy adult on a balanced diet)
Positive balance: Intake > Outflow (growth, pregnancy, weight lifting, etc.)
Negative balance: Intake < Outflow (Illness, surgery trauma, cancer cachexia)

Question 21

Can animals store nitrogen containing molecules?

Answer 21

No. Animals must replenish nitrogen supplies through the diet to replace nitrogen lost through catabolism.

Question 22

What are the 3 types of amino acids used for catabolism? What are they?

Answer 22

1. Gluconeogenic amino acids: used for gluconeogenesis
2. Ketogenic amino acids: used for ketogenesis
3. Those that can do both

Question 23

What happens to an amino acid once you remove their amino group?

Answer 23

If you remove the amine group from amino acids, the rest of it is composed of carbons and hydrogens which can feed back into the CAC as a form of one of the intermediates to become gluconeogenic or ketogenic amino acids.

Question 24

What is a problem generated from amino acid breakdown?

Answer 24

a.a deamination forms a carbon skeleton + NH3 (ammonia). High [NH3] or [NH4+] is highly toxic and can cause disturbance of consciousness, coma, convulsions, and death. Therefore, both ammonia and ammonium need to be detoxified.

Question 25

What are the nitrogen assimilation reactions?

Answer 25

1. alpha-KG + NH3 + NAD(P)H + 2H+ to Glutamate + NAD(P)+ + H2O. Enzyme: glutamate dehydrogenase
2. Glutamate + ATP + NH3 to Glutamine + ADP + Pi. Enzyme: Glutamine (& Aspargine) synthetase
   The NH3 comes from the deamination of amino acids during their breakdown

Question 26

Why is glutamine a good ammonia carrier?

Answer 26

Because it has 2 NH3 groups, so it can carry a lot of ammonia from tissues to the liver where the opposite of the assimilation reaction happens.

Question 27

Describe the toxic pathway of ammonia starting from tissue.

Answer 27

Ammonia (toxic) produced in the various tissues from the breakdown of amino acids. Needs to be transported to liver for detoxification by urea cycle:

1. Ammonia combined to glutamate by glutamine synthetase into glutamine to be exported into the blood.
2. Glutamine carries ammonia through the blood to the liver.
3. Absorbed in liver
4. In the liver, the glutaminase reaction will reverse the pathway to regenerate NH3 from glutamine
5. NH3 enters urea cycle in liver to be detoxified
6. Urea is exported out of liver and secreted by the kidney into the urine
7. Urine is degraded by bacteria in the environment which regenerates N2 in the atmosphere and the cycle restarts.

Question 28

What are the 2 most abundant amino acids in the blood and why?

Answer 28

Alanine and glutamine are the most abundant 2 amino acids in the blood because they serve as excellent carriers for NH3 groups back to the liver.

Question 29

What enzymes regulate the pool of glutamine?

Answer 29

Glutamine synthetase and Glutaminase.

Question 30

Describe the glutaminase reaction. Where does it occur?

Answer 30

Glutamine + H2O to Glutamate + NH4+ via Glutaminase enzyme. This happens in the liver. The ammonium released is detoxified in urea cycle.
The Glutamine is made in the tissues via glutamine synthetase and is sent to the liver via the blood to be turned back into glutamate.
The glutamate released goes to various tissues to be turned into glutamine.

Question 31

Describe the glutamine synthetase reaction. Where does it occur?

Answer 31

The glutamine synthase reaction occurs in various tissues and prevents over accumulation of NH3 n the blood.

1. ATP is added to glutamate to generate a primed high energy intermediate, ADP is released.
2. NH3 is added to the intermediate to form glutamine, the phosphate is released.
3. Glutamine travels through the blood to the liver where the glutaminase deamination reaction will occur.