Amino Acid Metabolism

Question 1

List:

The THREE nitrogen distributers/carriers.

Answer 1

• Glutamate
• Glutamine
• Carbamoyl-phosphate

These are the organic compounds that NH₄⁺ gets incorproated into after entering biological systems.

Question 2

Which are the TWO major pathways for incorporation of NH₄⁺ into organic compounds?

Answer 2

The formation of…
- Glutamate from α-ketoglutamate via glutamate dehydrogenase (GDH).

• Glutamine from glutamate via glutamine synthetase (GS).

The first reaction is coupled with the reduction of GDP⁺ to GDPH.

The second reaction consumes 1ATP.

Question 3

Which is the minor pathway for incorporation of NH₄⁺ into organic compounds?

Answer 3

The formation of…
- Carbamoyl-phosphate from bicarbonate via carbamoyl-phosphate synthetase I (CPS I).

This reaction consumes 2ATP.

Question 4

List:

The FIVE amino acid biosynthetic precursor families.

Answer 4

• α-ketoglutarate
• Oxaloacetate
• Pyruvate
• 3-phosphoglycerate
• Aromatic (PEP and erythrose-4-phosphate).

Question 5

What enzymes are required for transamination reactions?

Answer 5

Aminotransferases (transaminases)

Question 6

What cofactor is required for aminotransferase reactions?

Answer 6

Pyridoxal-5-phosphate (PLP)

It is covalently bound to Lys of the enzyme via an imide/Schiff base, as well as non-covalent interactions.

Question 7

What important role does PLP serve as a cofactor to aminotransferase during transamination?

Answer 7

It carries the donated α-NH₃ to form pyridoxamine (PMP) and then reacts with the second α-ketoacid to form the new amino acid.

This involves hydrolysis of the imide bond, and eventually results in the regeneration of the E-PLP complex.

This works via aldimine and ketimine intermediates.

Question 8

What kind of organisms does ornithine synthesis occur in?

Answer 8

Bacteria

Question 9

True or False:

Glutamate is used twice to synthesise ornithine - once as the backbone and second as an amino donor.

Answer 9

True

This pathway exists in bacteria.

Question 10

True or False:

Humans CANNOT form ornithine.

Answer 10

False

However, it is a different mechanism to bacteria, and instead generated during the urea cycle.

Question 11

Why are 2 ATP required for the CPS I reaction pathway?

(To form carbamoyl-P)

Answer 11

It is required twice to activate the OH of the carboxyl group to make it a better leaving group, and then to attach a phosphate in its place.

Question 12

Where is carbamoyl-phosphate synthetase I (CPS I) specifically expressed?

Answer 12

In the mitochondria of liver cells.

Question 13

List:

The FOUR enzymes of the urea cycle.

Answer 13

• Ornithine transcarbamoylase
• Arginosuccinate synthetase
• Arginosuccinate lyase
• Arginase

Question 14

Which TWO sources do the N atoms of urea originate from?

Answer 14

• Carbamoyl-phosphate (originally NH₄⁺)
• Aspartate

Question 15

Where does the α-NH₃ of arginine in the urea cycle originate from?

Answer 15

Ornithine (which comes from glutamate).

Question 16

Which FIVE amino acids are directly derived from glutamate?

Answer 16

• Alanine (via pyruvate and transamination)
• Aspartate (via oxaloacetate and transamination)
• Serine (via conversion of 3-PG to α-ketoacid and transamination)
• Ornithine (glutamate backbone)
• Arginine (via ornithine in urea cycle)

Question 17

List:

The SEVEN amino acid catabolism intermediates.

Answer 17

• Pyruvate
• Oxaloacetate
• α-ketoglutarate
• Succinyl-CoA
• Fumarate
• Acetyl-CoA
• Acetoacetate

Note: the first FIVE are gluconeogenic and the last TWO are ketogenic (ketone and FA synthesis).