MBIO 2710 / Topic 4: AA Cat — TA + DA

Why don’t we store excess amino acids?

> No dedicated AA storage.

> Excess AAs are converted to cellular respiration intermediates for energy.

Unlike glucose (glycogen) & fats (TAGs), AAs aren’t stockpiled.

What’s the first key step in amino acid catabolism?

> Remove the amino group (transamination or deamination).

> Leaves a carbon skeleton that enzymes can funnel into glycolysis/TCA.

Why is amino acid anabolism limited in humans?

> We’ve lost the ability to synthesize 10 essential AAs.

> Building AAs de novo is energetically costly/complex.

What is pepsin?

Gastric proteolytic enzyme working on acid-denatured proteins’ peptide bonds.

Where does protein digestion begin, and what happens there?

In the stomach; low pH denatures proteins and pepsin hydrolyzes them into smaller polypeptides via protease pepsin.

Where does further polypeptide digestion occur, and by what enzymes?

In the small intestine; catalyzed by proteases like chymotrypsin.

After digestion, where are free amino acids transported, and for what purpose?

To the liver for catabolic processing.

How are amino acids transported from the small intestine to the rest of the body, and why do they go to the liver first?

> After digestion and absorption, amino acids enter capillaries in the small intestine.

> They travel via the hepatic portal vein directly to the liver, bypassing the heart.

> The liver acts as a customs checkpoint — filtering harmful substances, processing amino acids into needed forms, and deciding whether to store, convert, or release them.

> This ensures nutrients are safe, in the right form, and at controlled levels before they reach the general circulation.

> Processed blood leaves via the hepatic vein, flows to the heart, then circulates to tissues.

When does amino acid catabolism happen in animals?

> Normal protein turnover.

> Protein-rich diet → excess amino acids beyond protein synthesis needs (not stored)

> Starvation → breakdown of body proteins for energy.

What does “normal protein cell” turnover mean?

The body’s continuous process of breaking down old/damaged proteins into amino acids and replacing them with new proteins.

Why is amino acid catabolism distinct from carbohydrate & lipid catabolism?

Because amino acids have an amino group, which must be removed before entering catabolic pathways.

What are the two main pathways to consider when breaking down amino acids?

> Fate of α-amino group – Transamination or deamination removes the amino group and stages it for further metabolism (e.g., urea cycle).

> Fate of carbon skeleton – The remaining carbon backbone is converted into pyruvate, TCA intermediates, etc., for energy production.

What is the first step in most amino acid catabolic pathways?

Removal of the amino group via transamination or deamination.

Why is the amino group removed first in amino acid catabolism?

Because nitrogen cannot be directly used for energy and must be processed (often into urea) before the carbon skeleton can enter central metabolism.

Why is oxidative deamination functionally one-way?

Although reversible, it mainly proceeds toward deamination because the enzyme’s K_M for NH₃ is high, favoring ammonia release under physiological conditions.

Why is oxidative deamination important in amino acid catabolism?

It’s the main route for collecting amino groups (from transamination) and funnelling them toward excretion via the urea cycle, while freeing carbon skeletons for energy production.

Why is ala deH₂ase and AA oxidase less significant in oxidative deamination pws?

Glu deH₂ase is main route; most AAs funnelled to Glu first via TA before DA.

How is ammonia from tissues transported to liver for excretion?