Nitrogen Fixation & Amino Acid Metabolism

What is nitrogen fixation?

Conversion of N₂ → NH₄⁺ (ammonia)

Which enzyme performs nitrogen fixation?

Nitrogenase

What does nitrogenase require?

ATP + electrons + metal cofactors (Fe, Mo)

Why is nitrogen fixation energy expensive?

Breaking the N≡N triple bond

Where does nitrogen fixation occur?

Bacteria (often in plant root nodules)

What is the main amino group donor?

Glutamate

What is the main nitrogen storage molecule?

Glutamine

What enzyme forms glutamate?

Glutamate dehydrogenase (GDH)

What enzyme forms glutamine?

Glutamine synthetase

What does glutamine synthetase require?

ATP

What is transamination?

Transfer of amino group to another molecule

What enzyme performs transamination?

Aminotransferase (transaminase)

What cofactor is required for Transamination?

PLP (vitamin B6)

What is the main amino group acceptor in transamination?

α-ketoglutarate

What is produced in transamination?

Glutamate + α-keto acid

What does decarboxylation remove?

CO₂

What cofactor is required for decarboxylation?

PLP

Example of decarboxylation?

Histidine → Histamine

What molecule donates methyl groups?

S-adenosylmethionine (SAM)

What type of group is transferred in methylation?

–CH₃

Cofactor for methylation?

SAM

Cofactor for 1-carbon transfer?

Tetrahydrofolate (THF)

Why is ammonia dangerous?

Toxic to cells

How do humans remove ammonia?

Convert to urea

What do fish excrete?

Ammonia

What do birds excrete?

Uric acid

Where does the urea cycle occur?

Liver

Purpose of the urea cycle?

Remove NH₄⁺ → urea

First step enzyme of the urea cycle?

Carbamoyl phosphate synthetase I (CPS I)

What activates CPS1?

N-acetylglutamate

What does CPS 1 produce?

Carbamoyl phosphate

How many ATP are used in the urea cycle?

~4 ATP equivalents

Where do the nitrogens in urea come from?

Ammonia + Aspartate

What are glucogenic amino acids?

Form glucose

What does glucogenic amino acids become?

Pyruvate or TCA intermediates

What are ketogenic amino acids?

Form ketone bodies

Which AAs are ONLY ketogenic?

Leucine, Lysine

Which AA are glucogenic and ketogenic?

Ile, Phe, Tyr, Trp

Histidine → ?

Histamine

Function of histamine?

Allergy + immune signaling

Tyrosine → ?

Dopamine, epinephrine, norepinephrine

Tryptophan → ?

Serotonin

What molecule carries methyl groups?

SAM

What molecule carries amino groups?

Glutamate

What molecule accepts amino groups?

α-ketoglutarate

What vitamin is PLP derived from?

B6

What reaction type produces histamine?

Decarboxylation

If a molecule goes to α-ketoglutarate, is it glycogenic?

Yes

What are the steps of nitrogen fixation?

1. N₂ binds to nitrogenase

2. Electrons are added (reduction begins)

3. ATP is used to drive the reaction

4. N≡N triple bond is broken stepwise

5. Ammonia (NH₃/NH₄⁺) is released

Bind → Reduce → Use ATP → Break → Release NH₃