M4: Amino Acid Synthesis & Breakdown L27

Question 1

All the CO2 generated in metabolism is exhaled from the lungs during respiration.
A) True
B) False

Answer 1

B) False -> urine contains carbonic acid

Question 2

How does the body avoid NH3 toxicity? (select all that applies)
A. Glutamine synthetase incorporates NH3 to glutamine
B. Muscle pyruvate is converted to alanine
C. Carbamoyl-Phosphate Synthetase assimilates NH4
D. Two NH3 moieties are incorporated in urea, which is excreted by the kidney
E. None of the above

Answer 2

A. Glutamine synthetase incorporates NH3 to glutamine
B. Muscle pyruvate is converted to alanine
C. Carbamoyl-Phosphate Synthetase assimilates NH4
D. Two NH3 moieties are incorporated in urea, which is excreted by the kidney

Question 3

How is urea production regulated? (select all that applies)
A. High [arginine] stimulates urea production
B. CPS-1 is downregulated in presence of NAG
C. CPS-2 downregulates CPS-1- mediated production of urea
D. CPS-1 is activated by substrate availability
E. None of the above

Answer 3

A. High [arginine] stimulates urea production

D. CPS-1 is activated by substrate availability

Question 4

What are the metabolic fates of the products of urea cycle? (select all that apply)
A. Deamination of aspartate generates a-ketoglutarate which serves in the Citric Acid Cycle
B. Arginosuccinate generates Succinate wich serves in the Citric Acid Cycle
C. Urea is exported to the kidney
D. The recycling of fumarate to the Citric Acid Cycle permits production of NADH
E. None of the above

Answer 4

A. Deamination of aspartate generates a-ketoglutarate which serves in the Citric Acid Cycle
C. Urea is exported to the kidney
D. The recycling of fumarate to the Citric Acid Cycle

Question 5

What’s left of the amnio acid once the ammonia is removed?

Answer 5

The part of the amino acid that is left is composed mostly of carbons and hydrogens. They have to be recycled to generate glucose, acety-CoA or ketone bodies.

Question 6

What is an amphibolic intermediate?

Answer 6

The term amphibolic is used to describe a biochemical pathway that involves both catabolism and anabolism.

Question 7

Which amino acids form gluconeogenic intermediates? What are they used for?

Answer 7

The ones we focused on:

1. Glutamate
2. Glutamine
3. Aspartate
4. Asparagine
5. Arginine
6. Methionine

The others:

7. Glycine
8. serine
9. Valine
10. Histidine
11. Cysteine
12. Proline
13. Alanine

They will replenish the CAC or make pyruvate to eventually support gluconeogenesis and make glucose which will be exported to other tissues.

Question 8

Which amino acids form ketogenic intermediates?

Answer 8

1. Leucine

2. Lysine

Question 9

Which amino acids form both gluconeogenic and ketogenic intermediates?

Answer 9

1. Threonine
2. Isoleucine
3. Phenlyalanine
4. Tryptophan
5. Tyrosine

Question 10

How is glutamate made into its amphibolic intermediate?

Where does the intermediate go?

Answer 10

1 step degradation via deamination:
Glutamate to a-KG via GDH. release of NH4+ to the urea cycle.
a-KG goes to the CAC.
Glutamate = gluconeogenic amino acid.

Question 11

How is aspartate made into its amphibolic intermediate?

Where does the intermediate go?

Answer 11

1. 1 step degradation via transamination:
   Aspartate + a-KG to oxaloacetate + Glutamate via aspartate aminotransferase (AST).
   Oxaloacetate goes to the CAC and glutamate can be used to make a-KG via GDH which enters the CAC.
2. From the urea cycle:
   Aspartate + citrulline via ASS makes arginosuccinate.
   Arginosuccinate loses an arginine via arginosuccinase to make fumarate.
   Fumarate enters the CAC and arginine can make a-KG and enter the CAC.

Aspartate = gluconeogenic amino acid.

Question 12

How is glutamine made into its amphibolic intermediate?

Where does the intermediate go?

Answer 12

Requires conversion (2 steps)
Glutamine makes glutamate via glutaminase and releases an NH4+ to the urea cycle. Glutamate undergoes deamination via GDH to make a-KG which enters the CAC.

Glutamine = gluconeogenic amino acid.

Question 13

How is asparagine made into its amphibolic intermediate?

Where does the intermediate go?

Answer 13

Requires conversion (2 steps)
Asparagine makes aspartate via asparaginase and releases an NH4+ to the urea cycle. Then a transamination reaction: Aspartate + a-KG to oxaloacetate + Glutamate via aspartate aminotransferase (AST).
Oxaloacetate goes to the CAC and glutamate can be used to make a-KG via GDH which enters the CAC.

Asparagine = gluconeogenic amino acid.

Question 14

What is the fate of the ketogenic amino acids after catabolism?

Answer 14

Leucine and lysine are turned into ketone bodies by being metabolized to either acetyl-CoA or acetoacetate that can be further organized to make b-hydroxybutyrate. These are exported to the blood to serve as an energy source in the heart and brain where they prodcue NADH and FADH2.

Question 15

Where are branched chain amino acids degraded? What are they and which amino acids are they?

Answer 15

They are degraded in adipose, kidney, and brain tissue because the branched-chain aminotransferase enzyme is located there. (NOT IN THE LIVER LIKE THE OTHER AMINO ACIDS)

Branched chain amino acids contain tertiary carbons:
Valine, Isoleucine, leucine.

Question 16

How are branched chain amino acids degraded?

Answer 16

1. Using the branched chain amino-transferase (BCAT) enzyme, valine, isoleucine, and leucine will be transaminated into their corresponding alpha-keto acids. The transaminase pair used in the reaction will be a-KG and Glutamate.
2. The alpha-Keto acids are metabolized to make acyl-CoA derivatives that can fit into glycolysis or the CAC. This is done by the BCKD (branched chain alpha-keto acid dehydrogenase complex) enzyme.

Question 17

What happens when you have a deficiency in BCKD?

Answer 17

Maple syrup urine disease.

Question 18

How is BCKD regulated? What does the E1 subunit of the BCKD complex do?

Answer 18

BCKD is an enzyme complex like the PDC.
This enzyme can be regulated through phosphorylation by kinase. When phosphorylated, it is inactive. When the phosphatase dephosphorylates it, it is active.
Leucine’s keto-acid will inhibit the kinase so that the enzyme is in the active form.

The E1 of the BCKD Complex is turned on to process branched ketoacids.

Question 19

What are the non-essential amino acids?

Answer 19

1. Glutamate (glutamine is conditionally essential)
2. Asparagine
3. Alanine
4. Aspartate

Question 20

How is alanine synthesized?

L27, slide 22(Alessia), 23(Isabella)

Answer 20

Pyruvate is converted to alanine via an aminotrasferase using an amino acid and alpha-keto acid pair.

Question 21

How are aspartate and asparagine synthesized?

L27, slide 22(Alessia), 23(Isabella)

Answer 21

Oxaloacetate is converted to aspartate via an aminotrasferase using an amino acid and alpha-keto acid pair.
Aspartate + Glutamine + ATP makes Asparagine + Glutamate + AMP + PPi using asparagine synthetase.

Question 22

How are glutamate and glutamine synthesized?

L27, slide 22(Alessia), 23(Isabella)

Answer 22

a-KG is converted to Glutamate via an aminotrasferase using an amino acid and alpha-keto acid pair.
Glutamate + ATP makes an intermediate + ADP via glutamine synthetase. Then the intermediate + NH4 makes glutamine + Pi.

Question 23

Why is methionine an important amino acid?

Answer 23

Methionine has a sulfur atom therefore it can serve in many cellular processes:

1. Protein synthesis: The first amino acid that gets incorporated into the protein being translated is methionine.
2. Synthesis of cysteine: another amino acid
3. One-carbon metabolism: source of carbons for rxns that require methylation.
4. Regulation of gene expression: through the methylation of DNA and the regulation of transcription of genes.

Question 24

Study all of methionine from ppt

Answer 24

Isa: Lecture 27 slide 28 to the end.
Ale: Lecture 27 slide 27 to the end.

Question 25

What is Hyper-homocysteninemia? What can cause it? L27 S33

Answer 25

• Elevated homocysteine plasma levels (Imbalance between the rate of production and breakdown of Homocysteine.)
  Causes:
  a) B12, Folic acid deficiencies.
  b) mutations in the enzyme converting Homocysteine to Cystathionine.
  (Reaction #5: “Cystathionine beta-Synthase”)

Question 26

What co-factors are required in methionine metabolism? Why are they important? L27 slide 36.

Answer 26

• Vit. B6 required (precursor) for PLP,
which is a coenzyme in reactions 5 and 6.

• Vit. B8 (a.k.a. Biotin) is a coenzyme in reaction 8.
• Vit. B9 (a.k.a. Folic Acid)
• Required for the MTHFR reaction
• Reaction 4 also requires folic acid.

• Vit. B12
The only 2 enzymatic reactions requiring Vitamin B12 in mammals are involved in Met metabolism (reactions 4 & 10).

Question 27

What human diseases are associated with hyper-Homocysteinemia? Explain each.

Answer 27

1. Cardiovascular disease
   • Homocysteine interferes with
   the formation of connective tissue. • Causes blood vessel defects.
2. Cognitive impairment / Dementia
   • General reduction in the ability to reason, judge, concentrate • May be associated with some types of depressions
3. Developmental defects
   a) Neural tube defects (e.g. Spina Bifida)
   - Characterized by the failure of the neural tube to form or close properly.
   - Usually severe and can result in paralysis.
   b) Anencephaly (failure of the brain to develop) - Invariably fatal
   - Leading cause of infant death due to congenital anomalies
   This is why we recommend pregnant women to consume enough folic acid & B12–to keep a normal balance of homocysteine.

Question 28

What is vitamin B12 deficiency? What does it lead to? is it common? what are the mild and severe symptoms?

Answer 28

• Vitamin B12 deficiency is when you don’t have enough vitamin B12 in your body.
• Very little Vitamin B12 is needed so deficiency is relatively rare. It’s Produced by bacteria and Sufficient amounts are usually provided from a normal diet. (Animal products or Vegan food sources: Fortified Soy Products, cereals, nutritional yeast)
• The deficiency leads to elevated homocysteine levels and health problems.
• Mild B12 deficiency symptoms: Anemia, Nausea, Constipation, Gas
• Severe B12 deficiency: Neurological damage, memory loss