Amino Acid Metabolism: Transamination & Deamination

• Amino acids are not stored, necessitating the removal of amino nitrogen from the carbon chain.
• Carbon chains are converted to fat or carbohydrates.
• Amino groups are removed via four enzyme groups, including transaminases, oxidative deaminases, nonoxidative deaminases, and amino acid oxidases.

Transaminases

• Transaminases remove the amino group from one amino acid and transfer it to a keto acid to synthesize a new amino acid and a new keto acid.
• Pyridoxal Phosphate (Vitamin B6) serves as a cofactor for transaminases during amino group transfer.
• Glutamate/oxaloacetate and Glutamate/Pyruvate combinations are used by GOT and GPT enzymes to synthesize Aspartate and Alanine, respectively.
• Lysine, Serine, and Threonine lack corresponding transaminases.

Oxidative Deamination

• Glutamate dehydrogenase serves as the primary enzyme in oxidative deamination, which occurs in the mitochondria.
• Either NAD or NADP is required by the enzyme as a cofactor.
• NADPH-NADP liver ratios are high, while NADH-NAD liver ratios are low, ensuring a pyridine nucleotide coenzyme is available to react.
• Glutamate, α-Ketoglutarate, and NH3 levels regulate the reaction.

Amino Acid Dehydratases

• Serine and Threonine undergo direct deamination by Serine dehydratase and Threonine dehydratase respectively.
• Serine converts to Pyruvate + NH3 via Serine dehydratase, where Threonine converts ot α-ketobutyrate + NH3 via Threonine dehydratase.
• Pyridoxal Phosphate is a cofactor for both enzymes.

L- and D- Amino Acid Oxidases

• Amino acid oxidases catalyze the reaction: L/D Amino Acid +H₂O + O₂ forming α-Keto acid + NH₃ + H₂O₂
• Keto acids can be converted into L-amino acids.
• The tightly bound cofactors required for the reaction are FAD & FMN.
• The physiological signifiance is unknown but amino acid oxidases are present in the kidneys and liver.
• The toxic Hydrogen Peroxide product is decomposed by Catalase in the liver into H₂0 and O₂.

Sources of Free Ammonia

• Free ammonia sources include:

  • Glutamate dehydrogenase
  • Amino acid Dehydratases
  • L- and D-amino acid oxidase enzymes
  • Histidine Lyase
  • Bacterial urease action on gut urea
  • Glutaminase action on glutamine
  • Asparaginase on asparagine
  • Purine nucleotide cycle

• NH₃ is converted into non-toxic UREA by the liver, then excreted via the urine.

Ammonia Transport

• Although tissues constantly produces ammonia, the blood levels of ammonia remain low.
• This is due to the rapid removal of ammonia into urea by the liver.
• Tissues, especially the muscle tissue releases amino acid nitrogen in the form of GLUTAMINE, ALANINE or ASPARAGINE.
• The formualtions are:
  • Glutamate + NH₃ → Glutamine (via Glutamine synthatase)
  • Glutamate + Pyruvate → αketoglutarate + Alanine (via alanine transaminase)
  • Aspartate + Glutamine → Asparagine + Glutamate (via Asparagine synthatase)

Glutamine & Asparagine

• Gluatmine and asparagine are a non-toxic storage and transport form of ammonia.
• Glutamine production happens mainly in muscles and the liver.
• Important in the nervous system as a means of removing ammonia from the brain.
• Circulating glutamate, hydrolyzed by glutaminase, resultss in glutamic acid & NH3.
• Asparaginase hydrolyzes asparagine to aspartic acid & NH3.
• NH3 picks up H+ and is excreted as ammonia.
• The majority of NH3 is converted to urea.

Urea Biosynthesis

• Urea synthesis occurs partly in the mitochondria and partly in the cytosol.

• 300g Carbs, 100g Fat & 100g Protein are consumed daily by a a human who excretes 16.5g of nitrogen per day.

• 95% of the nitrogen waste is expelled via urine and the other 5% through faeces.

• Urea is synthesized in the liver, released into the blood, and cleared by the kidneys.

• The synthesis of urea requires 3 mol of ATP, 1 mol of NH₄+ ions, one mol of c0₂, and the a-amino nitrogen from aspartate.

• Carbamyl Phosphate synthetase 1 (RXN 1) is a rate-limiting enzyme in urea synthesis.

• NOTE ammonium ions are a toxin and can attack/damage the human brain system.

• The urea cycle compromises of reactions 2,3,4 & 5.

• 5 Reactions are required to form UREA from Ammonia:

  • Reaction 1: Creation of Carbamyl phosphate (CP) is a key intermediate in the urea cycle and plays a critical role in the detoxification of ammonia in the body. It is synthesized in the mitochondria of liver cells, where it is formed from ammonium ions (NH₄⁺) and bicarbonate (HCO₃⁻) in a reaction catalyzed by the enzyme carbamoyl phosphate synthetase I (CPSI). This reaction is essential as it utilizes the energy derived from ATP to facilitate the conversion of toxic ammonia into a non-toxic molecule that can be processed further in the urea cycle.

    Carbamyl phosphate then reacts with ornithine to form citrulline, an important step that enables the process of converting nitrogen into urea for excretion. By effectively incorporating nitrogen in this manner, the body can flush out excess nitrogen waste, thus preventing accumulation of harmful ammonia levels in the blood, which could lead to neurological issues. Moreover, CP plays a significant role in balancing nitrogen in the body and its synthesis is tightly regulated, demonstrating the intricate mechanisms involved in nitrogen metabolism.

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  • Reaction 2: Synthesis of Citrulline.

  • Reaction 3: Creation of AS.

  • Reaction 4: Synthesis of Arginine.

  • Reaction 5: Creation of UREA.

• Liver regulation of the urea cycle has great capacity to convert toxic NH3 to nontoxic urea which would otherwise accumulate.

• Urea cycle regulation happens by 1. substrate availability; the higher the NH3 rate produced the higher the urea formation regulation.

• Specific allosteric regulation of CPSI by NAG isformed only for the reason.

• NAG has no purpose over that of an allosteric activator of CPS1.

• Induction of protein metabolism enzymes in resposne the human need.

• Protein metabolism is achieved by:

  • increased protein from foods such as chicken, beef or fish .
  • prolonged fasting.

Urea Cycle in Fasting

• When we fast, the muscle protein breaks down to amino acids which are partially oxidised for energy.
• The amino acids either convert to alanine or glutamine and are put into the blood.
• The body has been able to metabolise the tissues int he Glut and kindey with Alanine.
• The final step has the carbons, converted into glucose (or keytone bodies) and travels to the liver.
• The end step as the nitrogen goes to Urea which is then discarded via the urine cycle.

Alanine to Glucose & Urea Conversion

• Key alanine, the gluconeogenic amino acid, is transaminated to form pyruvate.
• It is then converted to glucose.
• Nitrogen transforms into glutamate and then is released by NH4.
• The NH4 (2) is the converted to oxaloacetate for aspartate transformation (3).
• As the urea cycle produces it, NH4 and aspartate are inserted.
• To summarize, the transformation fo alanine into carbons creates glucose.
• two alanine molecules produces ONE of glucose and one of urea.

Ammonia Toxicity

• Hyperammonemia is associated with comatose states as may occur in Hepatic failure.
• (a) Usually, acquired hyperammonemia is just the result of the cirrhosis (fibrosis) in the liver, as this lessens urea sysnthesis. -(b) Genetic defects in the urea cycle enzymes results in the inherited type of hyperammonemia.
• Dangerous as the liver's functions fail to take out NH3 form the portal blood.
• Portal blood-NH3 levels will be naturally extreme than that of it's peripheral blood.
• The increase of NH3 levels is the bacterias work that hydrolisys the gut into ammnonia.

Urea Genetic Defects

• Survival is impossible after birth for infant patients that don't possess the urea enzymes needed for synthisizing it.
• Enzymes for each urea has been noted with only a a small amount available.
• In lots of patients with urea defects, hyperammonemia is one of the most common symtpoms.
  • Deficincies are high especially if reacting wuth CPS1 or Ornithine carbamyl transferase.
• Children's encepthalitis results is episodic if the lack in enzyme urea reaction causes damages in the brain.
  • Stoppage of protein will reduce this effect.
  • Life is uncertain regardless if this is done.

The Glucose Alanine Cycle

• The glucose/alanine cycle-Within the muscle, amino acid degradation.
• Muscle leads to a transfer of nitrogen to alpha ketoglutatrate and pyruvurate where alanine travels to the liver-the carbon of alanine transfers, the alanine nitrogen is used for urea biosynthesis.
• This effect is increased with excercise adn the muscles need blood glucose more.

Role of Gluatmine

• Synthesis of glutamine occurs in peripheral tissues and its transported to the liver.
• Glutaminase is a great conversion for the liver to turn glutamine to glutamate. ketoglutarate is a great acceptor than can accept two molecules of ammonia to form glutamine.
• GDH = glutamate dehydrogenase.