13.3 Protein Digestion, Amino Acid Catabolism, the Urea Cycle, Heme Synthesis, Porphyrias & Bilirubin Metabolism

Why this area matters

The TOS lists "protein metabolism (digestion, absorption, catabolism, ammonia intoxication and detoxification, amino acid metabolism), porphyrin biochemistry and metabolism, heme biosynthesis and metabolism, bilirubin metabolism". These pathways explain hepatic encephalopathy, neonatal jaundice, the inborn errors found on Philippine newborn screening (PKU, MSUD, homocystinuria) and lead poisoning.

Digestion, absorption and nitrogen balance

• Stomach: HCl denatures protein and activates pepsinogen to pepsin.
• Pancreas: enteropeptidase (enterokinase) activates trypsinogen to trypsin, which then activates chymotrypsinogen, proelastase and procarboxypeptidases.
• Brush border: aminopeptidases and dipeptidases finish digestion. Free amino acids enter cells by sodium-linked transporters, and di- and tripeptides by PepT1.
• Transport defects:
  • Hartnup disease: neutral amino acid transporter; tryptophan loss can cause pellagra.
  • Cystinuria: cystine, ornithine, lysine and arginine (COLA); causes cystine stones.

Nitrogen balance is nitrogen intake minus nitrogen output; protein is about 16% nitrogen, so nitrogen = protein ÷ 6.25.

• Positive balance: growth, pregnancy, recovery and anabolic states.
• Negative balance: starvation, protein deficiency, fever, sepsis, burns, trauma, uncontrolled diabetes and glucocorticoid excess.
• Lacking even one essential amino acid causes negative balance.

Transamination, Deamination, and Nitrogen Economy

Amino acid catabolism begins with the removal of the $\alpha$-amino group, channeling nitrogen into urea synthesis while liberating the carbon skeleton for energy production.

Transamination Reactions and Pyridoxal Phosphate (PLP)

Transamination transfers an $\alpha$-amino group from an amino acid to an $\alpha$-keto acid acceptor (usually $\alpha$-ketoglutarate), generating a new $\alpha$-keto acid and glutamate. Transaminases (aminotransferases) require Pyridoxal Phosphate (PLP), the active form of Vitamin B6, which forms a covalent enzyme-bound Schiff base intermediate:

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

Oxidative Deamination via Glutamate Dehydrogenase

Glutamate functions as the universal collector of amino nitrogen. In hepatic mitochondria, Glutamate Dehydrogenase oxidatively cleaves the amino group from glutamate, releasing free toxic ammonium ($NH_4^+$) and regenerating $\alpha$-ketoglutarate, utilizing either $NAD^+$ or $NADP^+$:

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

Inter-Organ Ammonia Transport Systems

Free ammonia is highly neurotoxic. Extrahepatic tissues package ammonia into non-toxic carrier molecules for transport to the liver:

• Glutamine Synthetase / Glutaminase Cycle: In muscle and peripheral tissues (particularly the central nervous system), ammonia is condensed with glutamate by glutamine synthetase to form non-toxic glutamine, consuming $1\text{ ATP}$. Glutamine circulates to the liver, where hepatic glutaminase releases free $NH_4^+$ directly into the urea cycle. In the kidneys, renal glutaminase releases $NH_4^+$ into the tubular lumen to buffer excreted metabolic acids.
• The Glucose-Alanine (Cahill) Cycle: In exercising skeletal muscle, branched-chain amino acid transamination produces glutamate. Alanine aminotransferase (ALT) transfers this amino group to glycolytically derived pyruvate, forming alanine. Alanine travels via the bloodstream to the liver, where hepatic ALT transfers the amino group back to $\alpha$-ketoglutarate (forming glutamate for urea synthesis) and regenerates pyruvate for hepatic gluconeogenesis.

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The Hepatic Urea Cycle and Inborn Errors

The Urea Cycle occurs exclusively in hepatocytes, compartmentalized across the mitochondrial matrix (first two steps) and the cytosol (remaining three steps).

Mitochondria:  NH4+ + HCO3- + 2 ATP  -->  Carbamoyl Phosphate  -->  Citrulline
                                              ^                  |
                                            CPS-1               OTC
                                                                 v
Cytosol:       Ornithine  <--  Arginine  <--  Argininosuccinate  <-- Citrulline
                   |               |               ^
              (To Mito)       (Arginase)        (ASS + Aspartate)
                                   v
                                 UREA

The Five Enzymatic Steps of the Urea Cycle

1. Carbamoyl Phosphate Synthetase-1 (CPS-1, Mitochondria): Condenses free $NH_4^+$ with bicarbonate ($HCO_3^-$), consuming $2\text{ ATP}$ to form carbamoyl phosphate. Rate-limiting and committed step. Obligatory allosteric activator: N-Acetylglutamate (NAG), synthesized from acetyl-CoA and glutamate by NAG synthase (which is stimulated by arginine).
2. Ornithine Transcarbamylase (OTC, Mitochondria): Transcarbamoylates ornithine with carbamoyl phosphate to form citrulline. Citrulline exits into the cytosol via the ornithine/citrulline antiporter.
3. Argininosuccinate Synthetase (ASS, Cytosol): Condenses citrulline with aspartate, consuming $1\text{ ATP}$ (cleaved to $\text{AMP} + PP_i$, equivalent to $2\text{ ATP}$ bonds) to form argininosuccinate. Note: The second nitrogen atom of urea is donated directly by aspartate!
4. Argininosuccinate Lyase (ASL, Cytosol): Cleaves argininosuccinate into arginine and fumarate. Fumarate enters the citric acid cycle, linking urea disposal to bioenergetics (the "Krebs bicycle").
5. Arginase-1 (Cytosol): Hydrolyzes arginine to liberate Urea and regenerate ornithine. Ornithine re-enters the mitochondrion to sustain another cycle. Urea is excreted by the kidneys.

Differential Diagnosis of Urea Cycle Enzymopathies

Clinical Management of Hyperammonemia

Ammonia cross the blood-brain barrier, depleting $\alpha$-ketoglutarate (inhibiting the Krebs cycle) and accumulating glutamine in astrocytes, inducing osmotic cerebral edema, asterixis, and coma. Management requires:

1. Immediate cessation of dietary protein.
2. Nitrogen Scavengers: Sodium phenylbutyrate (conjugates with glutamine to form phenylacetylglutamine, excreted in urine) and Sodium benzoate (conjugates with glycine to form hippurate, excreted in urine).
3. Lactulose: Colonic bacteria ferment lactulose into lactic and acetic acids, protonating intestinal ammonia ($NH_3$) into unabsorbable ammonium ion ($NH_4^+$), trapping it for fecal excretion ("ammonia trapping").
4. Hemodialysis for life-threatening refractory hyperammonemic crisis.

Heme biosynthesis and the porphyrias

Heme is made in marrow erythroid cells (about 85%) and the liver (cytochromes). The steps are:

1. ALA synthase (mitochondria; needs pyridoxal phosphate; the rate-limiting step; repressed by heme): glycine plus succinyl-CoA forms delta-aminolevulinic acid (ALA).
2. ALA dehydratase (cytosol; inhibited by lead): two ALA form porphobilinogen (PBG).
3. PBG deaminase (hydroxymethylbilane synthase): forms hydroxymethylbilane.
4. Uroporphyrinogen III synthase, then uroporphyrinogen decarboxylase, form coproporphyrinogen III.
5. Back in the mitochondria, oxidation yields protoporphyrin IX.
6. Ferrochelatase (inhibited by lead) inserts Fe2+ to form heme.

Bilirubin metabolism and jaundice

1. Senescent red cells are broken down in splenic macrophages. Heme oxygenase converts heme to biliverdin, releasing CO and iron. Biliverdin reductase forms unconjugated (indirect) bilirubin, which is lipid-soluble.
2. Unconjugated bilirubin travels bound to albumin to the liver. Drugs such as sulfonamides and ceftriaxone can displace it, which matters in newborns.
3. UDP-glucuronosyltransferase (UGT1A1) in hepatocytes conjugates it into water-soluble direct bilirubin.
4. Conjugated bilirubin is secreted into bile by the MRP2 transporter, the rate-limiting step.
5. Gut bacteria convert it to urobilinogen. Some urobilinogen is reabsorbed and excreted in urine; most leaves in stool as stercobilin (brown color).

Kernicterus occurs when unconjugated bilirubin crosses the immature blood-brain barrier and stains the basal ganglia. Risks rise with prematurity, hemolysis (ABO or Rh incompatibility, G6PD deficiency), acidosis and drugs that displace bilirubin. Phototherapy converts bilirubin into water-soluble isomers that can be excreted.

High-Yield Inborn Errors of Amino Acid Catabolism

Phenylketonuria (PKU)

• Etiology: Autosomal recessive deficiency of Phenylalanine Hydroxylase (PAH) ($>98\%$ of cases) or Tetrahydrobiopterin ($BH_4$) deficiency (due to dihydropteridine reductase defect, causing malignant PKU).
• Biochemical Defect: Inability to convert phenylalanine to tyrosine. Phenylalanine accumulates and is diverted into alternative transamination pathways, generating neurotoxic phenylketones (phenylpyruvate, phenyllactate, and phenylacetate). Tyrosine becomes an essential amino acid.
• Clinical Manifestations: Profound intellectual disability, microcephaly, developmental delay, seizures, eczematous dermatitis, fair skin, light blonde hair, blue eyes (lack of tyrosine for melanin), and a characteristic musty or mousy body and urinary odor (due to phenylacetate).
• Screening and Treatment: Newborn screening at 24–48 hours of life after protein feeding. Lifelong restriction of dietary phenylalanine, tyrosine supplementation, avoidance of aspartame (Equal / NutraSweet), and $BH_4$ (sapropterin) supplementation in responsive variants.

Maple Syrup Urine Disease (MSUD)

• Etiology: Autosomal recessive deficiency of the Branched-Chain $\alpha$-Ketoacid Dehydrogenase (BCKDH) multi-enzyme complex.
• Biochemical Defect: Inability to decarboxylate the branched-chain amino acids (Isoleucine, Leucine, Valine; Mnemonic: "I Love Vermont maple syrup"). Requires the same five cofactors as PDH: TPP (B1), Lipoic acid, CoA (B5), FAD (B2), and $NAD^+$ (B3).
• Clinical Manifestations: Neonatal onset of poor feeding, vomiting, lethargy, alternating hypotonia and hypertonia, dystonia, seizures, cerebral edema, and progressive encephalopathy. Urine and earwax feature a pathognomonic sweet, burnt caramel or maple syrup odor (derived from isoleucine metabolites).
• Treatment: Dietary restriction of isoleucine, leucine, and valine; thiamine (Vitamin B1) supplementation in thiamine-responsive variants.

Alkaptonuria (Ochronosis)

• Etiology: Autosomal recessive deficiency of Homogentisate 1,2-Dioxygenase in the tyrosine degradation pathway.
• Biochemical Defect: Accumulation of homogentisic acid (homogentisate), which spontaneously oxidizes and polymerizes into a black-brown ochronotic pigment that binds collagen in connective tissues.
• Classic Diagnostic Triad:
  1. Urine that turns dark brown or black upon prolonged standing or exposure to alkaline air.
  2. Ochronosis: Blue-black macular and cartilage pigmentation (visible in sclerae, ear helix, and nasal cartilage).
  3. Severe, debilitating ochronotic arthropathy of large weight-bearing joints and spine (calcification and narrowing of intervertebral discs resembling ankylosing spondylitis).
• Prognosis: Benign in childhood; severe debilitating arthropathy manifests in the 3rd and 4th decades of life.

Homocystinuria

• Etiology: Most commonly autosomal recessive deficiency of Cystathionine $\beta$-Synthase (CBS) (which requires PLP/Vitamin B6); less commonly methionine synthase deficiency (requires Vitamin B12) or MTHFR deficiency.
• Biochemical Defect: Accumulation of homocysteine and methionine in blood and urine.
• Clinical Manifestations:
  • Ocular: Downward and inward ectopia lentis (subluxation of the lens; high-yield differentiator from Marfan syndrome, which features upward and outward lens subluxation).
  • Skeletal: Marfanoid habitus (tall stature, long slender extremities, arachnodactyly), severe osteoporosis, pectus excavatum or carinatum, kyphoscoliosis.
  • Vascular: Premature, life-threatening arterial and venous thromboembolism (DVT, pulmonary embolism, stroke, myocardial infarction) at a young age.
  • Neurological: Intellectual disability and psychiatric disturbances.
• Treatment: Pharmacologic doses of Pyridoxine (Vitamin B6) in CBS-responsive forms; dietary restriction of methionine and cysteine supplementation (cysteine becomes essential).