Nitrogen Metabolism: Urea Cycle

Question 1

Biological Molecules that require Nitrogen

Answer 1

• Amino Acids
• Nucleotides
• Amino Sugars
• Coenzymes
• Porphyrins
• Proteins
• DNA
• RNA
• Complex Polysaccharides
• Phospholipids

Question 2

Describe the aminotransferase reactions

Answer 2

• The first step in amino acid degradation
• Using Pyroxidal phosphate (B₆) as a cofactor, they remove the amino group and give it to the corresponding alpha-keto acid.

Question 3

Glutamate + alpha-keto acid

Answer 3

alpha-ketoglutarate + Glutamate

Question 4

Aspartate + A-ketoglutarate

Answer 4

Oxaloacetate + Glutamate

Question 5

Alanine + A-ketoglutarate

Answer 5

Pyruvate + Glutamate

Question 6

Name this molecule:

Answer 6

Vitamin B₆ (pyroxidal)

Question 7

Name this Molecule:

Answer 7

Pyroxidal Phosphate

Question 8

For which types of reactions is pyroxidal phosphate a prosthetic group?

Answer 8

• Transamination reactions
• Chemical change at alpha, beta, or gamma carbons of common amino acids
• Some decarboxylation reactions
• Deamination (Ser —> pyruvate + NH₃)

Question 9

Excrete ammonia into water because it is water soluble

Answer 9

Aquatic Animals

Question 10

Ammonia is converted to urea in the liver by the urea cycle

Answer 10

Mammals

Question 11

Convert ammonia to uric acid; precipitates and can be excreted without water

Answer 11

Birds and Reptiles

Question 12

Describe how nitrogen can travel through the blood to the liver

Answer 12

• In most tissues, ammonia is convered to glutamine using glutamine synthetase, then transported through the blood to the liver.
• Muscle uses the glucose-alanine cycle, wherein pyruvate undergoes transamination w/glutamate to form alanine and A-ketoglutarate. Alanine is then transported to the liver.

Question 13

Describe the Glucose-Alanine Cycle

Answer 13

Pyruvate undergoes transamination with glutamate, which forms alanine and A-ketoglutarate. Alanine is then transported to the liver.