Organic Chemistry · The Organic Chemistry of Metabolic Pathways
Catabolism of Proteins: Deamination
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Proteins are not stored like glycogen or fat, so the amino acids released by protein turnover — and especially by the protein breakdown that accompanies fasting, starvation, or muscle wasting — must be dealt with immediately. The first problem is the nitrogen: the cell needs the carbon skeletons of amino acids for fuel and biosynthesis, but free ammonia (\(NH_3\)) is toxic. Catabolism of proteins therefore begins by removing the amino group — a process called Deamination Removal of the amino group from a molecule Full entry → — and then disposing of the nitrogen as urea, while the remaining carbon skeletons enter central metabolism (pyruvate, acetyl CoA, or citric acid cycle intermediates) to make ATP or glucose.
The amino group is usually not removed directly. Instead, Transamination Transfer of an amino group from an amino acid to α-ketoglutarate Full entry → first transfers it to a common acceptor, α-Ketoglutarate The amino-group acceptor that becomes glutamate Full entry →, forming glutamate; glutamate then undergoes Oxidative deamination Deamination coupled to NAD(P)⁺ reduction (glutamate dehydrogenase) Full entry → to release ammonia, which the Urea cycle Five-enzyme liver pathway converting ammonia to urea Full entry → converts to urea:
\[ 2\ NH_4^+ + CO_2 + 3\ ATP + aspartate \rightarrow urea + fumarate + 2\ ADP + 2\ P_i + AMP + PP_i \]
The overall strategy — collect amino groups onto glutamate, then strip them off in the liver — keeps ammonia levels low in peripheral tissues while concentrating nitrogen disposal in one organ. This topic connects protein chemistry (Chapter 26) to the metabolic pathways built in Chapters 25–29: amino acids are both fuel and gluconeogenic substrate.
Why this matters
- Nitrogen must go somewhere: Excess amino groups cannot be stored. If the urea cycle fails (genetic defects, severe liver disease), ammonia rises and causes hepatic encephalopathy — a life-threatening clinical situation.
- Fuel during fasting: After a few days without food, muscle protein is broken down and its amino acids are used for gluconeogenesis (Topic 8) and energy. Deamination is the gate that lets amino acid carbon enter those pathways.
- Glucogenic vs. ketogenic: Knowing whether an amino acid's carbon skeleton becomes pyruvate/TCA intermediates (glucogenic) or acetyl CoA/acetoacetate (ketogenic) predicts whether it can support blood glucose — an important clinical distinction in metabolic disease.
- Transaminases in diagnosis: Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are released into blood when liver or muscle cells are damaged; elevated serum levels are routine clinical markers of liver injury.
- Vitamin B6 connection: Transaminases require Pyridoxal phosphate (PLP) Vitamin B6-derived coenzyme of transaminases Full entry →, the coenzyme form of vitamin B6 — a concrete example of a vitamin deficiency impairing a specific metabolic step.
The college version
Core Concepts
Step 1: Proteolysis
Before any amino group chemistry, proteins must be hydrolyzed to free amino acids. Dietary proteins are digested by proteases (pepsin in the stomach, trypsin/chymotrypsin in the intestine); intracellular proteins are turned over by the ubiquitin–proteasome system. The result is a pool of 20 amino acids whose fates depend on their side chains.
Step 2: Transamination — moving the amino group
Transaminases (aminotransferases) transfer the α-amino group of an amino acid to α-ketoglutarate, converting the amino acid to its α-keto acid and α-ketoglutarate to glutamate. The coenzyme is PLP, which shuttles the amino group through a Schiff base intermediate (an imine formed between the amino group and PLP's aldehyde).
Two enzymes you must know by name:
- Alanine aminotransferase (ALT): alanine + α-ketoglutarate ⇌ pyruvate + glutamate
- Aspartate aminotransferase (AST): aspartate + α-ketoglutarate ⇌ oxaloacetate + glutamate
Transamination is freely reversible and does not release ammonia — it only redistributes amino groups onto glutamate. That is the key: glutamate is the cell's amino-group collection point.
Step 3: Oxidative deamination — releasing ammonia
Glutamate dehydrogenase (GDH), found mainly in the liver, removes the amino group of glutamate as free ammonia, regenerating α-ketoglutarate:
\[ glutamate + NAD(P)^+ + H_2O \rightarrow \alpha{-}ketoglutarate + NH_4^+ + NAD(P)H \]
This reaction is unusual in that it can use either NAD⁺ or NADP⁺ (NAD⁺ for catabolism, NADP⁺ when the cell wants glutamate as a nitrogen donor for biosynthesis). Now the cycle is complete: α-ketoglutarate is recycled to accept more amino groups, and ammonia has been freed. Small amounts of ammonia also come from glutaminase (glutamine → glutamate + \(NH_4^+\)) and from serine/threonine dehydratases.
Step 4: The urea cycle — disposing of ammonia safely
Ammonia is too toxic to accumulate, so the liver converts it to urea, a neutral, highly water-soluble molecule excreted in urine. The urea cycle (Krebs–Henseleit cycle) has five enzymes spanning the mitochondrial matrix and cytosol:
- Carbamoyl phosphate The activated \(NH_4^+ + CO_2\) intermediate made with 2 ATP Full entry → synthetase I (mitochondrial): \(NH_4^+\) + \(CO_2\) + 2 ATP → carbamoyl phosphate.
- Ornithine transcarbamoylase (mitochondrial): carbamoyl phosphate + ornithine → citrulline (citrulline leaves the mitochondrion).
- Argininosuccinate synthetase (cytosolic): citrulline + aspartate + ATP → argininosuccinate (uses the second nitrogen, from aspartate).
- Argininosuccinate lyase: argininosuccinate → arginine + fumarate (fumarate rejoins the citric acid cycle).
- Arginase: arginine + \(H_2O\) → urea + ornithine (ornithine returns to the mitochondrion).
Net: two nitrogens per urea — one from ammonia (via carbamoyl phosphate), one from aspartate — and one carbon from \(CO_2\). The cycle costs 3 ATP (4 high-energy bonds, because ATP → AMP + PPᵢ counts as two).
Step 5: Carbon skeletons enter central metabolism
After deamination, the α-keto acids feed into the pathways built earlier in this chapter:
- Glucogenic amino acids (e.g., alanine → pyruvate; aspartate → oxaloacetate; glutamate → α-ketoglutarate): their carbons can make glucose via gluconeogenesis.
- Ketogenic amino acids (leucine, lysine): their carbons become acetyl CoA or acetoacetate and can make ketone bodies or fatty acids, but not glucose.
- Many amino acids are both (e.g., isoleucine, phenylalanine, tyrosine, tryptophan).
The ATP yield of amino acid catabolism comes from oxidizing those carbon skeletons — through pyruvate, the citric acid cycle, and electron transport — not from the deamination steps themselves.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Transamination | Deamination | Transamination moves the amino group (no free ammonia, reversible); deamination removes it as \(NH_3/NH_4^+\) (irreversible) |
| Glutamate dehydrogenase | Transaminases | GDH releases free ammonia (oxidative deamination); transaminases only shuffle amino groups between substrates |
| Urea | Uric acid | Urea is the nitrogen waste from amino acid catabolism (mammals); uric acid is the purine breakdown product (birds/reptiles, and gout in humans) |
| Carbamoyl phosphate synthetase I | Carbamoyl phosphate synthetase II | CPS I is mitochondrial, uses ammonia, feeds the urea cycle; CPS II is cytosolic, uses glutamine, feeds pyrimidine synthesis |
| Glucogenic amino acid | Ketogenic amino acid | Glucogenic → pyruvate/TCA intermediates → glucose; ketogenic → acetyl CoA/acetoacetate only (leucine, lysine) |
| ALT | AST | ALT couples alanine to pyruvate; AST couples aspartate to oxaloacetate — both PLP transaminases, different substrates |
| Arginase step | Transamination | Arginase hydrolyzes arginine to urea + ornithine; it is not a transaminase and needs no PLP |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Amino acids are like snack boxes with a tasty snack (the carbon part) and a wrapper you must throw away (the nitrogen part). You can't just toss the wrapper anywhere — it's toxic. So the cell collects all the wrappers onto one special box (glutamate), carries them to the liver, and the liver seals them into a safe trash bag called urea that your body pees out. The tasty snack part goes into the energy factory to make fuel or sugar.
Worked example
Example 1: Transamination stoichiometry
Write and balance the ALT reaction and verify atom conservation.
Write the reaction with formulas:
\[ alanine\ (C_3H_7NO_2) + \alpha{-}ketoglutarate\ (C_5H_6O_5) \rightleftharpoons pyruvate\ (C_3H_4O_3) + glutamate\ (C_5H_9NO_4) \]
Check atoms: carbons 3 + 5 = 3 + 5 (8 = 8); nitrogens 1 + 0 = 0 + 1 (the amino group simply moved); oxygens 2 + 5 = 3 + 4 (7 = 7). Answer: the reaction is a group transfer, not an oxidation — no NAD⁺/FAD involved, no \(CO_2\) released. That is why transamination is freely reversible.
Example 2: Nitrogen budget of the urea cycle
One urea molecule contains two nitrogen atoms. If 1.00 g of ammonia nitrogen (14.01 g/mol N) is processed, what mass of urea (\(CH_4N_2O\), 60.06 g/mol) can be formed — a hypothetical upper bound if both nitrogens came from ammonia?
Convert mass of nitrogen → moles N → moles urea → grams urea:
\[ 1.00\ \cancel{g\ N} \times \frac{1\ mol\ N}{14.01\ \cancel{g\ N}} \times \frac{1\ mol\ urea}{2\ mol\ N} \times \frac{60.06\ g\ urea}{1\ \cancel{mol\ urea}} = 2.14\ g\ urea \]
Answer: 2.14 g urea — the theoretical maximum. In the real cycle the second nitrogen comes from aspartate, so ammonia nitrogen accounts for at most half of each urea's nitrogen. This calculation is a good check on the "2 N per urea" stoichiometry.
Example 3: Glucogenic vs. ketogenic prediction
Alanine is glucogenic; leucine is ketogenic. Predict where each carbon skeleton enters metabolism after deamination.
Write the products of deamination:
\[ alanine \rightarrow pyruvate \rightarrow oxaloacetate \rightarrow glucose\ (glucogenic) \]
\[ leucine \rightarrow acetyl\ CoA\ and\ acetoacetate \rightarrow ketone\ bodies\ (ketogenic) \]
Answer: pyruvate and oxaloacetate are gluconeogenic precursors (Topic 8), so alanine supports blood glucose; acetyl CoA cannot make net glucose (Topic 8, Example 4), so leucine cannot. Rule of thumb: if an amino acid degrades to pyruvate or a TCA intermediate, it is glucogenic; if it degrades to acetyl CoA or acetoacetate alone, it is ketogenic.
Example 4: Energy cost of urea synthesis
Calculate the ATP cost of the urea cycle in high-energy phosphate bonds.
Write the two ATP-consuming steps:
\[ Carbamoyl\ phosphate\ synthetase: 2\ ATP \rightarrow 2\ ADP + 2\ P_i \]
\[ Argininosuccinate\ synthetase: ATP \rightarrow AMP + PP_i\ (equivalent\ to\ 2\ ATP) \]
Sum: \(2 + 2 = 4\) high-energy bonds, usually quoted as "3 ATP" because the cycle consumes 3 ATP molecules (one split to AMP). Answer: 4 high-energy phosphate bonds per urea — a significant investment that explains why the body routes nitrogen disposal through one organ rather than doing it everywhere.
Key takeaways
- Protein catabolism = proteolysis → transamination → oxidative deamination → urea cycle → carbon skeletons to metabolism.
- Transamination: amino group moved to α-ketoglutarate forming glutamate; PLP (vitamin B6) is the coenzyme; no free ammonia released; reversible.
- Oxidative deamination: glutamate dehydrogenase releases \(NH_4^+\) from glutamate; regenerates α-ketoglutarate.
- Urea cycle (liver): 2 N per urea (one from \(NH_4^+\), one from aspartate), 1 C from \(CO_2\), cost 3 ATP (4 high-energy bonds).
- Key intermediates: ornithine → citrulline → argininosuccinate → arginine → urea (+ ornithine regenerated).
- Fumarate from argininosuccinate rejoins the citric acid cycle — a direct link between urea cycle and TCA cycle.
- ALT and AST (PLP-dependent transaminases) are clinical markers of liver damage.
- Glucogenic amino acids → glucose precursors; ketogenic (leucine, lysine) → acetyl CoA/acetoacetate only.
- Ammonia toxicity: urea cycle failure or liver disease → hyperammonemia → hepatic encephalopathy.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What are the five stages of protein catabolism, in order?
Show answer
Proteolysis → transamination → oxidative deamination → urea cycle → entry of carbon skeletons into central metabolism.
Why is glutamate the "collection point" for amino groups, and which enzyme releases ammonia from it?
Show answer
Transaminases move amino groups from every amino acid onto α-ketoglutarate, forming glutamate; glutamate dehydrogenase then removes the group as \(NH_4^+\) while regenerating α-ketoglutarate.
Which coenzyme do all transaminases require, and what vitamin is it derived from?
Show answer
Pyridoxal phosphate (PLP), derived from vitamin B6.
How many nitrogen atoms are in each urea molecule, and where do they come from?
Show answer
Two nitrogen atoms: one from ammonia (via carbamoyl phosphate) and one from aspartate (via argininosuccinate synthetase).
What is the ATP cost (in high-energy bonds) of the urea cycle?
Show answer
3 ATP molecules = 4 high-energy phosphate bonds (2 ATP → 2 ADP; 1 ATP → AMP + PPᵢ).
Alanine is glucogenic; leucine is ketogenic. Which one can support blood glucose during fasting, and why?
Show answer
Alanine: its deamination product, pyruvate, can be converted to oxaloacetate and then glucose; leucine's product, acetyl CoA, cannot make net glucose.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Transamination
- Transfer of an amino group from an amino acid to α-ketoglutarate
- Deamination
- Removal of the amino group from a molecule
- Oxidative deamination
- Deamination coupled to NAD(P)⁺ reduction (glutamate dehydrogenase)
- Pyridoxal phosphate (PLP)
- Vitamin B6-derived coenzyme of transaminases
- α-Ketoglutarate
- The amino-group acceptor that becomes glutamate
- Urea cycle
- Five-enzyme liver pathway converting ammonia to urea
- Carbamoyl phosphate
- The activated \(NH_4^+ + CO_2\) intermediate made with 2 ATP
- Glucogenic amino acid
- Amino acid whose carbon skeleton can make glucose
- Ketogenic amino acid
- Amino acid whose carbon skeleton becomes acetyl CoA/acetoacetate
- ALT / AST
- Transaminases coupling alanine/aspartate to glutamate
Sources & references
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