Amino-acid and nitrogen metabolism

Why this matters

There is no store for nitrogen. Fat and glycogen can be put away for later; surplus amino acids cannot, so every gram must be disposed of the day it arrives. That single fact explains the urea cycle, explains why a patient with a metabolic disorder must not be starved before a general anaesthetic, and explains why blood swallowed after an extraction is a metabolic event and not just mess.

The big picture

Nitrogen metabolism is two separate stories about the same molecules. The NITROGEN is collected onto glutamate by transamination, released as ammonia, and disposed of as urea. The CARBON skeleton is a separate matter entirely and ends up as glucose or ketone bodies. Keep the two apart and the whole topic becomes tractable.

The 14 cards in this lesson

1. 01There is no store for nitrogen
2. 02Transamination moves nitrogen, it does not remove it
3. 03Where ammonia is actually made — and how it travels
4. 04The urea cycle: five enzymes, two compartments, two nitrogens
5. 05The bill: three molecules, four bonds
6. 06One activator switches the whole cycle on
7. 07Urea, BUN and ammonia: the units that cost marks
8. 08Why ammonia harms the brain
9. 09Clinical alert — in the lesson
10. 10Carbon skeletons, and the enzyme behind the heel-prick test
11. 11Five disorders and the fact that separates each
12. 12Clinical alert — in the lesson
13. 13Clinical alert — in the lesson
14. 14The mouth runs its own nitrogen metabolism

You have got it when

You are ready to continue when you can say where each of urea's two nitrogens comes from, convert a BUN into a urea without reaching for a table, and name the one enzyme in this lesson that you yourself will inhibit in practice.