Unit 4: Protein Metabolism

1. Catabolism of Amino Acids: Transamination and Deamination

Amino acids are not stored in the human body in the same manner as lipids or carbohydrates. Excess amino acids derived from dietary intake or endogenous protein degradation undergo metabolic breakdown. The catabolism of amino acids proceeds in two primary phases: the removal and disposal of the nitrogenous α-amino group (-NH₂), followed by the degradation of the remaining carbon skeleton (α-keto acid).

Transamination

Transamination is the enzymatic transfer of an α-amino group (-NH₂) from an amino acid to an α-keto acid, forming a new amino acid and a new α-keto acid.

Key Principles and Features:

• Enzymes: Catalyzed by enzymes known as aminotransferases or transaminases.
• Coenzyme Requirement: Transaminases strictly require Pyridoxal Phosphate (PLP), the active coenzyme form of Vitamin B₆. PLP acts as an intermediate covalent carrier of the amino group on the enzyme surface.
• Reversibility: Transamination reactions are freely reversible (ΔG ≈ 0), allowing them to function both in amino acid degradation and in the synthesis of non-essential amino acids.
• Primary Amino Acceptor: α-Ketoglutarate serves as the universal acceptor of amino groups across most transamination reactions, yielding L-glutamate. Glutamate thus functions as a central collector of amino nitrogen in liver cells.

Major Transamination Reactions:

1. Alanine Transaminase (ALT) / Serum Glutamic Pyruvic Transaminase (SGPT):

   Transfers the amino group from alanine to α-ketoglutarate, forming pyruvate and glutamate.

   L-Alanine + α-Ketoglutarate ⇌ Pyruvate + L-Glutamate

2. Aspartate Transaminase (AST) / Serum Glutamic Oxaloacetic Transaminase (SGOT):

   Transfers the amino group from aspartate to α-ketoglutarate, forming oxaloacetate and glutamate.

   L-Aspartate + α-Ketoglutarate ⇌ Oxaloacetate + L-Glutamate

Clinical Diagnostic Significance:

ALT and AST are valuable intracellular liver enzymes used as diagnostic markers:

• Elevated ALT: Specific indicator of acute liver cell damage (such as viral hepatitis or toxic liver necrosis).
• Elevated AST: Associated with myocardial infarction, muscle dystrophies, and severe chronic liver pathology (AST exists in both mitochondrial and cytosolic fractions).

Exceptions: Lysine, threonine, proline, and hydroxyproline do not undergo transamination directly.

Deamination

Deamination is the oxidative or non-oxidative cleavage of an amino group from an amino acid, liberating free ammonia (NH₃ or NH₄⁺) and producing the corresponding α-keto acid.

Unlike transamination, which shifts nitrogen from one molecule to another, deamination releases free nitrogen as toxic ammonia, which is subsequently funneled into the urea cycle.

1. Oxidative Deamination:

Oxidative deamination combines removal of the amino group with oxidation. The primary site is the mitochondrial matrix of liver and kidney cells.

• Glutamate Dehydrogenase (GDH) Reaction:

  Glutamate produced during transamination undergoes rapid oxidative deamination via Glutamate Dehydrogenase. GDH is unique because it can utilize either NAD⁺ or NADP⁺ as cofactors.

  L-Glutamate + NAD⁺ (or NADP⁺) + H₂O ⇌ α-Ketoglutarate + NH₄⁺ + NADH (or NADPH) + H⁺

  Allosteric Regulation of GDH:

  • Inhibitors: ATP and GTP (indicate high cellular energy state).
  • Activators: ADP and GDP (indicate low cellular energy state).
• L- and D-Amino Acid Oxidases:

  Peroxisomal enzymes requiring FMN or FAD coenzymes. They deaminate amino acids to produce an α-keto acid, free ammonia, and hydrogen peroxide (H₂O₂), which is neutralized by catalase.

2. Non-Oxidative Deamination:

Carried out by specific enzymes without electron acceptors:

• Amino Acid Dehydratases: Hydroxy-amino acids (Serine, Threonine) undergo dehydration catalyzed by PLP-dependent enzymes followed by hydrolysis to release free ammonia.
  Serine → Pyruvate + NH₄⁺ (catalyzed by Serine Dehydratase)
  Threonine → α-Ketobutyrate + NH₄⁺ (catalyzed by Threonine Dehydratase)
• Amino Acid Desulfhydrases: Cysteine loses sulfur as H₂S along with ammonia to yield pyruvate.
• Deamidases: Hydrolyze amide bonds of amino acids like Glutamine and Asparagine:
  Glutamine + H₂O → Glutamate + NH₄⁺ (catalyzed by Glutaminase)
  Asparagine + H₂O → Aspartate + NH₄⁺ (catalyzed by Asparaginase)

Comparison of Transamination and Deamination

2. Urea Cycle

The Urea Cycle (Krebs-Henseleit Cycle) is a cyclic metabolic pathway in hepatocytes that converts toxic free ammonia (NH₃) into non-toxic, water-soluble urea for renal excretion.

Site and Location:

• Organ: Liver (hepatocytes).
• Subcellular Compartments: Divided between the Mitochondrial Matrix (Steps 1 & 2) and the Cytosol (Steps 3, 4, & 5).

Steps of the Urea Cycle

1. Step 1: Synthesis of Carbamoyl Phosphate (Mitochondria)

   Free ammonium ion (NH₄⁺) reacts with bicarbonate (HCO₃^-) to synthesize carbamoyl phosphate. The reaction is catalyzed by Carbamoyl Phosphate Synthetase I (CPS-I) and uses 2 ATP molecules.

   NH₄⁺ + HCO₃^- + 2 ATP → Carbamoyl Phosphate + 2 ADP + P_i

   Exam Note: CPS-I is the key rate-limiting enzyme of the urea cycle and absolutely requires N-Acetylglutamate (NAG) as an obligate allosteric activator.

2. Step 2: Formation of Citrulline (Mitochondria)

   The carbamoyl moiety of carbamoyl phosphate is transferred to ornithine by Ornithine Transcarbamoylase (OTC), producing citrulline.

   Carbamoyl Phosphate + Ornithine → Citrulline + P_i

   Citrulline is then transported across the inner mitochondrial membrane into the cytosol via a specific carrier protein.

3. Step 3: Synthesis of Argininosuccinate (Cytosol)

   Citrulline condenses with aspartate in the cytosol to form argininosuccinate, catalyzed by Argininosuccinate Synthetase. Aspartate contributes the second nitrogen atom of urea. This step hydrolyzes 1 ATP to AMP and inorganic pyrophosphate (PP_i), consuming the equivalent of 2 high-energy phosphate bonds (~P).

   Citrulline + Aspartate + ATP → Argininosuccinate + AMP + PP_i

4. Step 4: Cleavage of Argininosuccinate (Cytosol)

   Argininosuccinate Lyase cleaves argininosuccinate into arginine and fumarate.

   Argininosuccinate → Arginine + Fumarate

   Krebs Bicycle Link: Fumarate produced here enters the TCA cycle, connecting carbohydrate and amino acid metabolism.

5. Step 5: Cleavage of Arginine to Form Urea (Cytosol)

   Arginase hydrolyzes arginine to produce urea and regenerate ornithine.

   Arginine + H₂O → Urea + Ornithine

   Ornithine is transported back into the mitochondrion to continue the cycle. Urea is released into blood circulation and filtered by the kidneys into urine.

Overall Reaction and Energetics

Net Chemical Equation:

NH₄⁺ + CO₂ + Aspartate + 3 ATP + 2 H₂O → Urea + Fumarate + 2 ADP + 4 P_i + AMP

Energy Cost:

The synthesis of one urea molecule uses 3 ATP molecules. However, Step 3 produces AMP + PP_i, and subsequent hydrolysis of PP_i (PP_i → 2 P_i) consumes 2 high-energy bonds. Thus, a total of 4 high-energy phosphate bonds (~P) are consumed per molecule of urea synthesized.

Regulation of the Urea Cycle

• Allosteric Regulation: CPS-I is inactive in the absence of N-Acetylglutamate (NAG). NAG synthesis from acetyl-CoA and glutamate is catalyzed by NAG synthase, which is stimulated by high intracellular arginine levels.
• Substrate Regulation & Diet: High-protein diets or fasting (increased tissue protein breakdown) increase enzyme transcription and urea cycle turnover.

Clinical Significance & Hyperammonemia

Defects in any urea cycle enzyme lead to Hyperammonemia (accumulation of ammonia in blood), causing neurological impairment, cerebral edema, tremors, seizures, and coma.

• Ornithine Transcarbamoylase (OTC) Deficiency: Most common X-linked urea cycle disorder. Causes excess carbamoyl phosphate to enter pyrimidine biosynthesis, resulting in severe orotic aciduria.
• Hyperammonemia Type I: Caused by deficiency in Carbamoyl Phosphate Synthetase I (CPS-I).

Summary Table of Urea Cycle Steps

3. Fate of C-Skeleton of Glucogenic and Ketogenic Amino Acids

After removal of the amino group, the remaining carbon skeletons (α-keto acids) are converted into major metabolic intermediates. These intermediates are oxidized in the TCA cycle for ATP production, synthesized into glucose via gluconeogenesis, or converted into ketone bodies and fatty acids.

Classification of Amino Acids by Carbon Skeleton Fate

Glucogenic Amino Acids: Amino acids whose carbon skeletons degrade into pyruvate or TCA cycle intermediates (α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate), serving as substrates for net glucose synthesis via gluconeogenesis.

Ketogenic Amino Acids: Amino acids whose carbon skeletons degrade into acetyl-CoA or acetoacetyl-CoA, which yield ketone bodies or fatty acids, but cannot produce net glucose.

Classification of the 20 Standard Amino Acids:

• Purely Ketogenic (2 Amino Acids): Leucine and Lysine.
• Both Glucogenic and Ketogenic (5 Amino Acids): Isoleucine, Phenylalanine, Tryptophan, Tyrosine, and Threonine.
• Purely Glucogenic (13 Amino Acids): Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Histidine, Methionine, Proline, Serine, and Valine.

Entry Points into Central Metabolism

All 20 amino acids break down into 7 fundamental metabolic entry points:

1. Pyruvate (Glucogenic):

   Alanine, Cysteine, Glycine, Serine, Threonine, Tryptophan.

   Example: Transamination of alanine directly yields pyruvate.

2. Acetyl-CoA / Acetoacetyl-CoA (Ketogenic):

   Leucine, Lysine, Isoleucine, Phenylalanine, Tryptophan, Tyrosine.

3. α-Ketoglutarate (Glucogenic):

   Glutamate, Glutamine, Arginine, Histidine, Proline.

   Example: Glutamine is converted by glutaminase to glutamate, which GDH converts into α-ketoglutarate.

4. Succinyl-CoA (Glucogenic):

   Isoleucine, Methionine, Threonine, Valine.

   Mechanism: Methionine and valine catabolism forms propionyl-CoA, which is converted to methylmalonyl-CoA and then succinyl-CoA (requires Vitamin B₁₂).

5. Fumarate (Glucogenic):

   Phenylalanine, Tyrosine.

   Clinical Note: Deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine, results in Phenylketonuria (PKU).

6. Oxaloacetate (Glucogenic):

   Aspartate, Asparagine.

   Example: Asparagine is hydrolyzed to aspartate, which is transaminated directly to oxaloacetate.

Comparison of Glucogenic and Ketogenic Amino Acids

Metabolic and Clinical Significance

• Starvation: During fasting, muscle amino acids are degraded. Glucogenic amino acids enter gluconeogenesis in the liver to maintain plasma glucose, while ketogenic amino acids provide acetyl-CoA for ketone body synthesis.
• Uncontrolled Diabetes Mellitus: Hypercatabolism of amino acids fuels gluconeogenesis and ketogenesis, leading to hyperglycemia and diabetic ketoacidosis.
• Inborn Errors of Carbon Skeleton Metabolism: Defective degradation steps cause pathological conditions, such as Phenylketonuria (PKU) and Maple Syrup Urine Disease (MSUD, caused by branched-chain α-keto acid dehydrogenase complex deficiency).