Deamination

Where it happens

Deamination happens in the liver, which processes the excess amino acids absorbed from digested protein that the body cannot store.

The liver is positioned to do this job. Amino acids absorbed by the villi of the ileum enter the blood capillaries and travel first along the hepatic portal vein straight to the liver, before reaching the general circulation.

Liver cells take up the amino acids the body cannot use and carry out deamination in their cytoplasm and mitochondria. The urea formed leaves in the hepatic vein and is carried to the kidneys.

The liver is therefore the organ of formation, and the kidney is the organ of excretion; both are named in the syllabus under excretion and the role of the liver.

Inputs and outputs

• Input: excess amino acids from the digestion of dietary protein, delivered by the hepatic portal vein.
• Input: oxygen and enzymes in liver cells, which remove the amino group and convert ammonia into urea.
• Input: carbon dioxide, which combines with ammonia in the liver to build the urea molecule.
• Output: ammonia, formed briefly and converted at once because of its high toxicity.
• Output: urea, released into the blood and later filtered by the kidneys into urine.
• Output: a keto acid, the carbon-containing remainder, which is respired or converted into glycogen or fat.

The steps

1. Protein in food is digested into amino acids, which are absorbed in the ileum and carried by the hepatic portal vein to the liver.
2. Liver cells release the amino acids needed for protein synthesis into the general circulation and retain the excess, since amino acids cannot be stored.
3. Deamination: enzymes in the liver cells remove the amino group (-NH2) from each excess amino acid.
4. The removed amino group forms ammonia, a highly toxic and very soluble compound.
5. The liver immediately combines ammonia with carbon dioxide to form urea, which is far less toxic and can be carried safely in the blood.
6. The keto acid left behind enters respiration to release energy or is converted into glycogen or fat for storage.
7. Urea leaves the liver in the hepatic vein, circulates, and is removed from the blood by ultrafiltration in the kidney nephrons.
8. Urea is concentrated in urine, stored in the bladder and excreted from the body.

Why it matters and how it is controlled

The body cannot store protein or amino acids the way it stores carbohydrates as glycogen or fat, so any excess must be broken down. Deamination allows the nitrogen-containing part to be safely removed as urea, while the remaining organic acid can be respired for energy or converted into glycogen or fat.

Control of deamination follows the supply of amino acids. After a protein-rich meal, more amino acids reach the liver than the body can use, so the rate of deamination and urea formation rises; during fasting the rate falls, and the liver may deaminate amino acids released from the breakdown of body protein to supply energy.

The kidneys respond by adjusting how much urea is removed, so the concentration of urea in blood stays within a narrow range in a healthy person.

Deamination is also the link between protein metabolism and the other energy stores. The keto acid produced can enter the same pathways as glucose, which is why protein can be used as a fuel when carbohydrate and fat are scarce.

This connection is examined alongside the role of the liver in regulating blood glucose, since both processes happen in the same cells.

How it is examined

You may be asked to describe what happens to excess amino acids, to explain why ammonia is converted to urea rather than excreted directly, or to name the organ where deamination occurs and the organ that excretes the urea formed.

Structured items often present a table of the composition of blood entering and leaving the liver, or entering and leaving the kidney, and ask why urea concentration is higher in the hepatic vein than in the hepatic portal vein, or lower in the renal vein than in the renal artery. The answer must name the process in each organ: deamination and urea formation in the liver, ultrafiltration and excretion in the kidney.

Essay items ask you to trace the fate of excess amino acids from a meal to urine, and full marks require the sequence amino acid, amino group removed, ammonia, urea, blood, kidney, urine, with the reason for converting ammonia to urea stated clearly.

Common misconceptions

Worked exam-style question

Question. Table W shows the relative concentration of amino acids and urea in blood samples taken from the hepatic portal vein and the hepatic vein of a healthy person 2 hours after a meal of grilled fish. The amino acid concentration is higher in the hepatic portal vein than in the hepatic vein, and the urea concentration is higher in the hepatic vein than in the hepatic portal vein.

(a) Explain the difference in amino acid concentration between the 2 vessels. (b) Name the process responsible for the difference in urea concentration and describe how it takes place.

(c) Explain why the liver does not release the ammonia it forms directly into the blood. (d) State what happens to the part of the amino acid that remains after the amino group is removed.

Model answer. (a) The hepatic portal vein carries amino acids absorbed from the ileum after digestion of the fish protein. In the liver, excess amino acids are removed from the blood because the body cannot store amino acids, so the hepatic vein carries fewer.

(b) The process is deamination. Liver cells remove the amino group (-NH2) from each excess amino acid, forming ammonia; the ammonia is combined with carbon dioxide to form urea, which is released into the blood leaving in the hepatic vein.

(c) Ammonia is highly toxic to cells even at low concentration, so it is converted at once into urea, which is much less toxic and can be transported safely to the kidneys for excretion. (d) The remaining keto acid is respired to release energy or converted into glycogen or fat for storage.

Source:SRC-DSKP-EN

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