L31 - N Transport and AA Transamination

Question 1

Kwashiorkor and Marasmus are both conditions resulting from inadequate protein intake (negative balance). What is the difference between them?

Answer 1

Kwashiorkor: A_dequate total calories_ with deficiency in protein content in diet.

Marasmus: Inadequate total calories with respect to protein and carbohydrate consumption.

Question 2

Discuss the role of Transamination (aminotransferase) reactions in nitrogen handling.

Answer 2

Using pyridoxal phosphate (PLP), a cofactor dependent on vitamin B6, an amino acid can be converted to an a-keto acid and vice versa (are reversible). The predominant amino acid/a-keto acid pair is glutamate/a-ketoglutarate, where the NH3+ is on Glu. Aspartate/Oxaloacetate is another example (NH3+ on Asp).

• **Lysine **and **Threonine **do NOT undergo transamination rxns.

Question 3

Discuss the significant characteristics of Vitamin B6 deficiency.

Answer 3

Cannot make PLP: Pyridoxine (vit. B6) <– [NAD+/NADH+] –> Pyridoxaldehyde [ATP] –> PLP

• *aldehyde functional group critical in catalysis

Sx: dermatitis, microcytic hypochromic anemia, weakness, irritability, convulsions

Marker in Urine: Xanthurenic acid (degradation product of Trp)

Cause of Sx?

• microcytic hypochromic anemia from decreased ability to synthesize Heme from Gly
• convulsions from decreased ability to decarboxylate AAs to form neurotransmitters

Question 4

Describe the Alanine Aminotransferase (ALT) Reaction.

Answer 4

Alanine (w NH3+) + a-ketoglutarate <– [PLP] –> Pyruvate + Glutamate (w NH3+)

*Amino acid degradation in liver:

• Ala + a-ketoglutarate —> Pyruvate + Glu

Question 5

Describe the **Asparate Aminotransferase (AST) **Reaction.

Answer 5

Aspartate (w NH3+) + a-ketoglutarate <– [PLP] –> Oxaloacetate + Glutamate (w NH3+)

*Amino acid degradation in liver:

Asp + a-ketoglutarate <— Oxaloacetate + Glu

Question 6

Discuss the central role of Glutamate reactions in amino acid transport.

Answer 6

1. a-ketoglutarate <– [L-amino acid/a-keto acid] –> **Glu **req. enzyme: PLP
2. Glu <– [NAD(P)+/NAD(P)H, NH4+]** **–> a-ketoglutarate req. enzyme: **GDH **
3. **Glu **– [ATP/ADP, NH4+/Pi] –> Gln req. enzyme: Glutamine Synthetase
   
   1. ​NOT reversible – must use Glutaminase
4. Gln – [H2O] –> **Glu + NH4+ **req. enzyme: Glutaminase
   
   1. ​NOT reversible

*The third enzyme capable of “fixing nitrogen” in the human body is CPSI

Question 7

Why are high levels of ammonia associated with neurotoxicity?

Answer 7

1. Ammonia (NH3), not ammonium ions (NH4+), can traverse blood-brain barrier. Increased ammonia = increased pH.
2. High ammonia drives glutamine synthetase rxn: Glu + ATP + NH3 – [Glutamine synthetase] –> Gln + ADP +Pi
   
   1. depleted Glu stores alters neurotransmission as Glu is both a neurotransmitter and a precursor of GABA
   2. high Glu causes astrocytes to swell, altering their fxn
3. High ammonia and depleted Glu drives the GDH rax to form Glu: Glu <– [GDH] –> a-ketoglutarate + NH4+ means the equilibrium shifts LEFT, thus depleting a-ketoglutarate, causing OAA to fall and TCA cycle to cease (absence of aerobic oxidative phosphorylation)

Question 8

What are the two major carriers of nitrogen in the blood?

Answer 8

Glutamine (primary) and Alanine

*due to their prominent roles in ammonia transportation, free alanine and glutamine are more abundant compared to other AAs

Question 9

Describe how Glutamine transports nitrogen to the liver from peripheral tissues.

Answer 9

In muscle and peripheral tissues:

• a-ketoglutarate – [GDH: NADPH, 1st NH4+] –> Glutamate – [Glutamine Synthetase: ATP, 2nd NH4+] –> Glutamine —-[transported to liver via blood]—->

In liver mitochondria:

• Glutamine – [Glutaminase: yields 1st NH4+] –> Glutamate – [GDH: yields 2nd NH4+] –> a-ketoglutarate

The two NH4+ released in the liver form urea –> urine

Question 10

What is the Glucose/Alanine Cycle?

Answer 10

Alanine is an ammonia transporter in the blood to the liver during vigorous exercise. Glutamate reacts with pyruvate and alanine aminotransferase to form the N carrier, Alanine, and an a-ketoglutarate. Alanine is released into blood and travels to liver, where the N is extracted by reversing the rxn (using alanine aminotransferase and a-ketoglutarate). Glutamate then releases the NH4+ to enter urea cycle while the pyruvate that is generated is converted to glucose via gluconeogenesis.

• Also occurs during fasting and blood glucose is produced for brain in this case
• 2 Ala provide the Cs and Ns to form 1 glucose and 1 urea