5.3 Amino Acid Catabolism & Urea Cycle

Protein Degradation Pathways

Unlike carbohydrates and lipids, excess amino acids are neither stored in specialized cellular depots nor directly excreted intact. Intracellular proteins undergo continuous turnover—degraded into free amino acids and replaced by new protein synthesis. Cellular protein degradation occurs via two major pathways:

1. Ubiquitin-Proteasome System (UPS)

The Ubiquitin-Proteasome System is an ATP-dependent pathway responsible for the selective targeted degradation of short-lived, regulatory, misfolded, or damaged cytosolic and nuclear proteins. Targeting requires covalent attachment of a 76-amino acid regulatory protein called ubiquitin via a 3-step enzymatic cascade:

1. E1 (Ubiquitin-Activating Enzyme): Activates ubiquitin in an ATP-dependent step, forming a high-energy thioester bond.
2. E2 (Ubiquitin-Conjugating Enzyme): Transfers activated ubiquitin from E1 to E2.
3. E3 (Ubiquitin Ligase): Recognizes specific substrate degradation signals (degrons) and catalyzes the formation of an isopeptide bond between the C-terminus of ubiquitin and the $\epsilon$-amino group of a lysine residue on the target protein.

Polyubiquitin chains linked through Lys48 of ubiquitin target the substrate protein to the 26S Proteasome—a barrel-shaped multisubunit protease complex. The proteasome unfolds the substrate, degrades it into short peptides (3 to 15 amino acids), and recycles free ubiquitin monomers.

2. Lysosomal Proteolysis

Lysosomal degradation is an ATP-independent pathway that degrades extracellular proteins (taken up via endocytosis/phagocytosis) and long-lived organelles/cytosolic complexes (via autophagy). Lysosomes contain acidic hydrolases, primarily cathepsins (cysteine and aspartyl proteases), which operate optimally within the acidic lysosomal lumen (pH ~4.5 to 5.0 maintained by $V$-type $\text{H}^+$-ATPases).

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Nitrogen Processing: Transamination & Oxidative Deamination

Catabolism of amino acids requires the separation of the $\alpha$-amino group ($\text{-NH}_3^+$) from the carbon skeleton. Free ammonia ($\text{NH}_4^+$) is neurotoxic, so terrestrial mammals convert amino nitrogen into non-toxic urea in the liver.

1. Transamination (Aminotransferases)

Transamination is the reversible transfer of an $\alpha$-amino group from an donor amino acid to an $\alpha$-keto acid acceptor (most commonly $\alpha$-ketoglutarate). This reaction converts the donor amino acid into its corresponding $\alpha$-keto acid and converts $\alpha$-ketoglutarate into glutamate:

$\text{Amino Acid} + \alpha\text{-Ketoglutarate} \rightleftharpoons \alpha\text{-Keto Acid} + \text{Glutamate}$

All aminotransferases require Pyridoxal Phosphate (PLP), the active coenzyme form derived from Vitamin $\text{B}_6$. PLP forms a covalent Schiff base (aldimine) linkage with the $\epsilon$-amino group of an active-site lysine residue before transferring the amino group.

Key diagnostic transaminases include:

• Alanine Aminotransferase (ALT): $\text{Alanine} + \alpha\text{-Ketoglutarate} \rightleftharpoons \text{Pyruvate} + \text{Glutamate}$
• Aspartate Aminotransferase (AST): $\text{Aspartate} + \alpha\text{-Ketoglutarate} \rightleftharpoons \text{Oxaloacetate} + \text{Glutamate}$

2. Oxidative Deamination (Glutamate Dehydrogenase)

Glutamate collected from transamination reactions enters the liver mitochondrial matrix. Glutamate Dehydrogenase (GDH) oxidatively deaminates glutamate, releasing free toxic ammonium ($\text{NH}_4^+$) and regenerating $\alpha$-ketoglutarate:

$\text{Glutamate} + \text{NAD(P)}^+ + \text{H}_2\text{O} \rightleftharpoons \alpha\text{-Ketoglutarate} + \text{NH}_4^+ + \text{NAD(P)H} + \text{H}^+$

GDH is unique because it can utilize either $\text{NAD}^+$ or $\text{NADP}^+$ as an electron acceptor. Allosterically, GDH is inhibited by high energy state indicators (ATP and GTP) and activated by low energy state indicators (ADP and GDP).

 Nitrogen Processing Flow:
 Amino Acid + alpha-Ketoglutarate --(Transaminase + PLP/Vit B6)--> alpha-Keto Acid + Glutamate
                                                                                         |
                                                                            (Glutamate Dehydrogenase)
                                                                             In Liver Mitochondria
                                                                                         |
                                                                             NH4+ (Toxic) + alpha-KG
                                                                                         |
                                                                                    Urea Cycle

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The Urea Cycle

The Urea Cycle occurs exclusively in hepatocytes, partitioned between the mitochondrial matrix and the cytosol. It converts toxic free $\text{NH}_4^+$ and bicarbonate into non-toxic, water-soluble urea for renal excretion.

1. Mitochondrial Matrix Steps

• Step 1: Carbamoyl Phosphate Synthetase I (CPS-I) — Rate-Limiting Step CPS-I condenses mitochondrial free $\text{NH}_4^+$ with bicarbonate ($\text{HCO}_3^-$) to form carbamoyl phosphate, consuming 2 ATP: $\text{NH}_4^+ + \text{HCO}_3^- + 2\text{ ATP} \xrightarrow{\text{CPS-I}} \text{Carbamoyl Phosphate} + 2\text{ ADP} + \text{P}_i$ CPS-I requires N-Acetylglutamate (NAG) as an obligatory allosteric activator. NAG is synthesized by NAG synthase from acetyl-CoA and glutamate, a reaction allosterically activated by arginine.

• Step 2: Ornithine Transcarbamylase (OTC) OTC transfers the carbamoyl group from carbamoyl phosphate to ornithine, forming citrulline and releasing $\text{P}_i$. Citrulline is exported from the matrix into the cytosol via a specific transport protein.

2. Cytosolic Steps

• Step 3: Argininosuccinate Synthetase In the cytosol, argininosuccinate synthetase condenses citrulline with aspartate (which donates the second nitrogen atom of urea), consuming 1 ATP and hydrolyzing it to $\text{AMP} + \text{PP}_i$ (equivalent to 2 ATP high-energy phosphate bonds): $\text{Citrulline} + \text{Aspartate} + \text{ATP} \rightarrow \text{Argininosuccinate} + \text{AMP} + \text{PP}_i$

• Step 4: Argininosuccinate Lyase Argininosuccinate lyase cleaves argininosuccinate into arginine and fumarate. Fumarate enters the TCA cycle or gluconeogenesis, providing a biochemical bridge between the Urea Cycle and the TCA Cycle (often termed the "Krebs Bicycle").

• Step 5: Arginase Arginase hydrolyzes arginine into urea and regenerates ornithine: $\text{Arginine} + \text{H}_2\text{O} \xrightarrow{\text{Arginase}} \text{Urea} + \text{Ornithine}$ Ornithine is transported back into the mitochondrial matrix to initiate another turn of the cycle. Urea diffuses into the bloodstream and is filtered by the kidneys into urine.

AAMC MCAT Cost Accounting: Synthesizing one molecule of urea consumes 3 ATP molecules, but requires 4 high-energy phosphate bonds (2 ATP $\rightarrow 2\text{ ADP}$ at CPS-I, plus 1 ATP $\rightarrow \text{AMP} + \text{PP}_i$ at Argininosuccinate Synthetase).

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Fate of Amino Acid Carbon Skeletons

After removing the $\alpha$-amino group, the remaining carbon skeletons of the 20 standard amino acids are funneled into 7 metabolic intermediates: pyruvate, acetyl-CoA, acetoacetate, $\alpha$-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate.

Classification of Amino Acids

1. Purely Ketogenic Amino Acids: Catabolized exclusively into acetyl-CoA or acetoacetate. They cannot contribute to net glucose synthesis because acetyl-CoA carbons are lost as $2\text{ CO}_2$ in the TCA cycle prior to oxaloacetate regeneration. Exactly two amino acids are purely ketogenic: Leucine (Leu) and Lysine (Lys).
2. Dual Glucogenic & Ketogenic Amino Acids: Catabolized into both ketogenic precursors and glucogenic TCA cycle intermediates. Five amino acids belong to this category: Isoleucine (Ile), Phenylalanine (Phe), Tryptophan (Trp), Tyrosine (Tyr), and Threonine (Thr) (Mnemonic: FITTT).
3. Purely Glucogenic Amino Acids: Catabolized into pyruvate or TCA cycle intermediates ($\alpha$-ketoglutarate, succinyl-CoA, fumarate, OAA) that can be converted to oxaloacetate for gluconeogenesis. The remaining 13 amino acids are purely glucogenic.