The Urea Cycle 1

Question 1

Amino acid transport

Answer 1

Is mostly active transport and driven by ATP hydrolysis
- minor amounts of facilitated diffusion (PCT specifically) exist also

Small intestine (ileum) and the PCT of the kidney have common active transport systems for AA uptake

Question 2

Cistinuria

Answer 2

“COAL” defective uptake in the PCT of the kidneys due to defective active transport systems for Di-basic AAs

COAL = cystine, ornithine, arginine, lysine

**cystine is the primary one and its build up leads to kidney stones

Question 3

Hartnup Disease

Answer 3

Autosomal recessive mutation in active transporters in small intestine and PCT of kidneys that focus on neutral amino acids (especially **tryptophan)

Results in triphosphate and niacin (B3 vitamin) deficiency = NO NADP+

• causes pellagra symptoms (photosensitivity, dementia, dermatitis, diarrhea )
• requires vitamin B3 supplements and dietary changes

most common amino acid transport defect

Question 4

Steps in nitrogen metabolism

Answer 4

1) removal of the a-amino group to glutamine
- requires transamination and oxidative deamination

2) oxidative deamination of glutamine
- requires glutamate dehydrogenase to release amino groups to free ammonia
- requires NAD+ as coenzyme
- converts glutamate -> (a)-ketoglutarate

Question 5

What is transamination?

Answer 5

The transferring of amino groups in between amino acids via aminotransferase enzymes

Most common example:
- aspartate -> glutamate via the aspartate aminotransferase (AST) enzyme

*always generates (a)-ketones as a result (OAA or A-ketoglutarate are most common)

Question 6

Aminotransferases

Answer 6

Are different enzymes that are usually specific for certain AAs de novo production

Localized in the cytosol and mitochondria of cells

ALL require pyridoxal phosphate co enzyme (PLP) which requires vitamin B6

Functions: (all reactions are readily reversible)

• AA degradation through removal of (a)-amino group
• NEAA biosynthesis through addition of amino groups to the carbon skeleton of (a)-keto acids

Question 7

Glutamate dehydrogenase

Answer 7

Enzyme used to produce free ammonium in the process of breaking down glutamate -> (a)-ketoglutarate or by attaching a free ammonium ion onto (a)-ketoglutarate -> glutamate

Expressed at high levels in liver and kidney mitochondria

• uses NAD+ as coenzyme when oxidizing
• uses NADPH as coenzyme when reducing

oxidation vs reduction is based on the levels of glutamate, (a)-ketoglutarate and ammonia ions in the body

Allosteric regulation

• GTP = inhibits
• GDP = promotes

Question 8

Where does the two amino groups in urea and that are used in the urea cycle come from?

Answer 8

1 = comes from glutamate dehydrogenase cleaving a amino group of glutamate in oxidation reaction within th mitochondria and releasing free ammonia into the urea cycle

1 = is carried by aspartate via oxidative transamination of glutamate to the urea cycle
- this is NOT free ammonia

THESE are the nitrogen group sources for the urea cycle

Question 9

What are other sources of ammonia?

Answer 9

Histidine via direct deamination
- forms NH4+ and urocanate

Serine and threonine via direct deamination by dehydration reactions that require PLP coenzyme

Glutamine and asparagine via demindation

Purine nucleotide cycle in muscle and brain tissue that allows amino acids to leave tissues in the form of glutamine

Produced directly by bacteria in the lumen of the GI tract

Question 10

What are the two ways ammonia is transported to the liver from peripheral tissues as a non-toxic metabolite?

Answer 10

As glutamine

• requires glutamine synthase to take NH3 and add it to glutamate and make it glutamine
• comes from all other tissues other than muscles

As alanine

• requires alanine aminotransferase to take pyruvate and add NH3 to it to form alanine
• comes from muscle cells

Question 11

What are the four amino acid transport systems that are shared amoung kidneys and small intestines?

Answer 11

B(0)+

B(0)

X(Ag)-

Imino

Damage to any of these systems = inability to absorb amino acids properly

Question 12

Pellagra

Answer 12

Any pathology that causes a deficiency in niacin (B3) or tryptophan
- results in a deficency of NADP+ since tryptophan cant be converted into NAD+ with niacin

Question 13

Is AST or ALT more sensative or specifc?

Answer 13

AST = sensitive for liver disease
- liver contains mass amounts of AST

Question 14

What form of nitrogen can cross membranes in the mitochondria?

Answer 14

NH3 (ammonia)

NH4+ (ammonium) only stays in aqueous solutions

Question 15

What are all the amino acids that can be used to carry ammonia groups to and from the urea cycle?

Answer 15

Aspartate, alanine and glutamate are MOST common

Histidine 
Serine 
Threonine 
Asparagine 
Glutamine