Amino Acid Metabolism: Pathways for Energy and Disposal

Amino Acid Metabolism Overview

• Amino acids serve as building blocks for proteins but can also be utilized for energy production.

• There are four primary metabolic pathways for amino acids, particularly concerning energy utility:

  • Deamination: Removal of the amine group.

  • Transamination: Transfer of an amine group.

  • Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.

  • Protein Turnover/Degradation: (To be covered in a subsequent lecture).

Deamination

• Definition: The enzymatic removal of the amine group ($\text{NH}_2$) from an amino acid, catalyzed by deaminases.

• Products of Deamination:

  • Amine group ($\text{NH}<em>2$): Released as ammonia ($\text{NH}</em>3$). Ammonia is toxic to cells and must be converted to non-toxic urea via the urea cycle.

  • Carbon skeleton: The remaining carbon structure of the amino acid. The specific structure of the carbon skeleton determines the identity of the amino acid.

• Fate of the Carbon Skeleton:

  • Can be converted into glucose (via gluconeogenesis).

  • Can be converted into Acetyl CoA.

  • Can be converted into ketone bodies.

  • Ultimately, the carbon skeleton can be oxidized to $\text{CO}<em>2 + \text{H}</em>2\text{O}$ for energy.

Urea Cycle (aka Ornithine Cycle)

• Purpose: A metabolic pathway occurring primarily in the liver, responsible for converting toxic ammonia into non-toxic urea for excretion.

• Urea ($\text{NH}<em>2\text{-C-\text{NH}</em>2}$) is transported via blood to the kidneys and excreted in urine.

• Key Steps and Enzymes:

  1. Formation of Carbamoyl Phosphate:

     • Ammonia ($\text{NH}3) from glutamine (or directly from glutamate dehydrogenase) and $\text{CO}</em>2$ combine.

     • Requires $2 \text{ATP}$.

     • Catalyzed by Carbamoyl phosphate synthetase 1 (CPS1).

     • Regulator: N-acetylglutamate (NAG) is an allosteric activator of CPS1.

  2. Formation of Citrulline:

     • Carbamoyl phosphate reacts with ornithine.

     • Catalyzed by Ornithine transcarbamoylase.

     • Citrulline then exits the mitochondria to the cytosol.

  3. Formation of Argininosuccinate:

     • Citrulline reacts with aspartate (which provides the second nitrogen atom for urea).

     • Requires $1 \text{ATP}$ (hydrolyzed to $\text{AMP} + \text{PPi}$).

     • Catalyzed by Argininosuccinate synthetase.

  4. Cleavage of Argininosuccinate:

     • Argininosuccinate is cleaved into fumarate and arginine.

     • Catalyzed by Argininosuccinase.

     • Fumarate:

       • Can enter the TCA/Krebs cycle.

       • Can be converted to malate, then oxaloacetate.

       • Oxaloacetate can be used for gluconeogenesis.

  5. Formation of Urea and Regeneration of Ornithine:

     • Arginine is hydrolyzed into urea and ornithine.

     • Catalyzed by Arginase.

     • Urea: Excreted.

     • Ornithine: Returns to the mitochondria to restart the cycle.

• Interconnections:

  • $\alpha \text{-ketoglutarate}$ and Glutamate: Glutamate can be deaminated to $\alpha \text{-ketoglutarate}$ (a TCA cycle intermediate) releasing ammonia. Aspartate, another amino acid, is directly incorporated into the cycle.

  • TCA Cycle: Fumarate, produced in the urea cycle, is an intermediate of the TCA cycle, linking the two pathways.

Transamination

• Definition: The transfer of an amino group ($\text{NH}_2$) from one amino acid to an $\alpha \text{-keto acid}$, forming a new amino acid and a new $\alpha \text{-keto acid}$.

• Catalyst: Transaminases (also known as aminotransferases), which typically use pyridoxal phosphate (PLP), a derivative of vitamin $\text{B}<em>6$, as a cofactor. $\text{Amino acid}</em>1 + \alpha \text{-keto acid}<em>2 \xleftrightarrow{\text{Transaminase}} \alpha \text{-keto acid}</em>1 + \text{Amino acid}_2$

• Purpose: Central for interconversion of amino acids and for funneling nitrogen from amino acids into pathways for excretion or other metabolic uses.

• Key Transaminase Reactions (Examples):

  • Alanine Transaminase (ALT):

    • Transfers an amino group from alanine to $\alpha \text{-ketoglutarate}$ to form pyruvate and glutamate.

    • $\text{Alanine} + \alpha \text{-ketoglutarate} \xleftrightarrow{\text{ALT}} \text{Pyruvate} + \text{Glutamate}$

  • Aspartate Transaminase (AST):

    • Transfers an amino group from aspartate to $\alpha \text{-ketoglutarate}$ to form oxaloacetate and glutamate.

    • $\text{Aspartate} + \alpha \text{-ketoglutarate} \xleftrightarrow{\text{AST}} \text{Oxaloacetate} + \text{Glutamate}$

• Branched-chain Amino Acid Aminotransferase (BCAAT):

  • Transfers an amino group from branched-chain amino acids (e.g., leucine) to $\alpha \text{-ketoglutarate}$ to form the corresponding $\alpha \text{-keto acid}$ (e.g., $\alpha \text{-ketoisocaproate}$) and glutamate.

  • This is the first step in the catabolism of branched-chain amino acids.

• Glutamate Dehydrogenase: While not a transaminase, it's critical for ammonia metabolism.

  • Can deaminate glutamate to $\alpha \text{-ketoglutarate}$ and free ammonia ($\text{NH}_4^+$).

  • Uses NAD+ or NADP+ as a cofactor.

  • $\text{Glutamate} + \text{NAD}^+ / \text{NADP}^+ \xrightarrow{} \alpha \text{-ketoglutarate} + \text{NH}_4^+ + \text{NADH} + \text{H}^+ / \text{NADPH}$

• Glutamine Synthetase: Synthesizes glutamine from glutamate and ammonia, consuming ATP. This is a crucial way to detoxify ammonia into a transportable form.

  • $\text{Glutamate} + \text{NH}_3 + \text{ATP} \xrightarrow{} \text{Glutamine} + \text{ADP} + \text{Pi}$

• Glutaminase: Hydrolyzes glutamine back to glutamate and ammonia.

  • $\text{Glutamine} \xrightarrow{} \text{Glutamate} + \text{NH}_3$

• Distinguishing Deamination and Transamination:

  • Deamination directly removes an amino group, often releasing free ammonia.

  • Transamination transfers an amino group from one molecule to another, typically forming glutamate, which can then be deaminated or used in the urea cycle.

Conversion of Amino Acids to Carbon Skeletons and Entry into TCA Cycle

• After deamination/transamination, the carbon skeletons of amino acids enter various points of the TCA (Krebs) cycle or related pathways, leading to energy production or synthesis of other molecules.

• Entry Points into TCA Cycle Intermediates:

  • Pyruvate: Alanine, Cysteine, Glycine, Serine, Threonine, Tryptophan.

  • Acetyl-CoA: Leucine, Lysine, Isoleucine, Tryptophan, Phenylalanine, Tyrosine, Threonine.

  • Oxaloacetate: Aspartate, Asparagine.

  • Fumarate: Phenylalanine, Tyrosine, Aspartate.

  • $\alpha \text{-ketoglutarate}$: Glutamate, Arginine, Histidine, Proline, Glutamine.

  • Succinyl-CoA: Isoleucine, Methionine, Valine, Threonine.

Gluconeogenesis (GNG)

• Definition: The metabolic pathway that generates glucose from non-carbohydrate precursors, including the carbon skeletons of most amino acids (glucogenic amino acids).

• Process: The carbon backbone of deaminated amino acids (e.g., pyruvate, oxaloacetate, $\alpha \text{-ketoglutarate}$) can be funneled into the gluconeogenic pathway to produce glucose.

• Link to Urea Cycle: The amine group removed during deamination is processed through the urea cycle.

• Key Enzyme Highlighted: Glucose 6-Phosphatase is the first step in gluconeogenesis (specifically, the final step in the liver to release free glucose into the blood).

  • Expression: Would typically increase during fasting to facilitate glucose production and release, as the body needs to maintain blood glucose levels when dietary carbohydrates are unavailable.

Glucogenic vs. Ketogenic Amino Acids

• Glucogenic Amino Acids: Amino acids whose carbon skeletons can be converted into glucose via gluconeogenesis. Their carbon skeletons typically yield pyruvate, oxaloacetate, or other TCA cycle intermediates that can be channeled into glucose synthesis.

  • Examples: Alanine, Glycine, Threonine, Cysteine, Serine, Tryptophan, Aspartate, Asparagine, Glutamine, Arginine, Histidine, Proline, Valine, Methionine.

• Ketogenic Amino Acids: Amino acids whose carbon skeletons are converted into Acetyl-CoA or Acetoacetyl-CoA, which can then be used to synthesize ketone bodies or fatty acids, but cannot be directly converted into glucose.

  • Examples: Leucine, Lysine.

  • Why Acetyl-CoA cannot make glucose: Acetyl-CoA enters the TCA cycle, but the two carbons are eventually lost as $\text{CO}_2$ before glucose can be formed. The net conversion of acetyl-CoA to oxaloacetate is not possible in mammals.

• Both Glucogenic and Ketogenic Amino Acids: Some amino acids can yield both glucogenic and ketogenic precursors.

  • Examples: Isoleucine, Phenylalanine, Tyrosine, Tryptophan, Threonine.

Coming Up…

• The next lecture will delve into Protein Turnover/Degradation, including how protein metabolism is regulated under different conditions like feasting, fasting, and special physiological states.