BCH 202 Protein metabolism

Question 1

Describe absorptionof amino acids

Answer 1

Free Amino Acids
O Absorption is carried out by Na+-dependent
amino acid cotransport in the luminal
membrane via facilitated diffusion.
O It is analogous to the cotransporter for
glucose and galactose.
O There are four separate carriers for neutral,
acidic, basic, and imino amino acids,
respectively.

Question 2

Describe absorption of peptides

Answer 2

Peptides
O Dipeptides and tripeptides are absorbed by
H+-dependent cotransport in the luminal
membrane.
O Absorption of these peptides occur faster
than free amino acids.
O Cytoplasmic peptidases hydrolyze them to
amino acids which are then transported
from cell to blood by facilitated diffusion.

Question 3

Clinical correlation of protein absorption

Answer 3

Neutral Amino Aciduria (Hartnup Disease)
O Mutation in the epithelial transport of
neutral amino acid.
O Characterized by the inability of renal and
intestinal epithelial cells to absorb neutral
amino acids from the lumen.
This results in malabsorption of free amino acids from the diet.

Question 4

Site of amino acids catabolism

Answer 4

O The principal site of amino acid metabolism
is the Liver
O However, this occurs in other tissues such
as the kidney, muscle, adipose tissue, and
small intestine.

Question 5

Stages of amino acid catabolism

Answer 5

O Transamination
O Oxidative deamination

Question 6

All amino acids undergo
transamination reaction except
which 3?

Answer 6

lysine, proline, and threonine

Question 7

What happens to the ammonium ion formed?

Answer 7

The Ammonium ion formed is converted to Urea, excreted in the urine……..Urea cycle

Question 8

Describe nitrogen excretion

Answer 8

O Nitrogen is removed via urea cycle.
O 2 sources are ammonia and aspartate.
O 4 ATPS are required for this cycle.
O SLC25A15, a transporter for citrulline

Question 9

Clinical correlates of protein catabolism

Answer 9

Hyperammonemia
O Occurs when nitrogen excretion is impaired
due to liver failure or inborn error of
metabolism.
O Ammonia crosses the blood brain barrier,
enters the brain, depletes the brain of alpha
ketoglutarate (what then happens???)
O Can be managed by the use of antibiotics
O Lactulose

Question 10

How is protein catabolism regulated?

Answer 10

REGULATION
O Allosterism: Glutamate dehydrogenase is regulated
allosterically by purine nucleotides
O GTP and ATP are allosteric activators in the
direction of glutamate synthesis
O Enzyme Induction