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Chapter 12 – Heme Biosynthetic Pathway & Porphyrias

Overview of the Porphyrias

  • Group of inherited/acquired disorders caused by ↓ activity of one of the 8 enzymes in the heme-biosynthetic pathway.
  • Manifestations divide clinically into:
    • Neurovisceral attacks
    • Cutaneous photosensitivity
    • Combined forms (Box 12-1)
  • Parallel “anatomic” classification ➜ hepatic vs. erythropoietic porphyrias.

Porphyrins & Porphyrinogens – chemistry that explains the symptoms

  • Porphyrins = planar, rigid, intensely coloured tetrapyrroles (4 pyrrole rings + 4 methene bridges).
  • Alternating single–double bonds → strong absorption at \sim400\,\text{nm} (Soret band) + red fluorescence.
  • Light-excited porphyrins generate ROS → skin damage in sun-exposed areas.
  • Porphyrinogens = fully reduced, colourless, flexible; easily oxidised to porphyrins → lost as urine/stool pigments.

Cellular logistics of heme synthesis (Fig 12-2)

  • 3 cellular sites:
    1. Mitochondrial matrix (steps 1, 7, 8)
    2. Mitochondrial inter-membrane space (step 6)
    3. Cytosol (steps 2-5)
  • Daily output: ~85 % erythron, ~15 % liver (mostly for cytochrome P450).

Process 1 – Formation of the pyrrole

δ-Aminolevulinate Synthase (ALA-S)

  • Reaction: \text{glycine}+\text{succinyl-CoA} \xrightarrow[\text{PLP}]{\text{ALA-S}} \delta\text{-ALA} + \text{CO}_2 + \text{CoA} (Fig 12-3).
  • Isoenzymes & genes:
    • ALA-S1 (house-keeping): chromosome 3p21.1.
    • ALA-S2 (erythroid): Xp11.21; transcript contains 5’ iron responsive element (IRE).
  • Regulation highlights
    • Heme feedback via heme-regulatory motifs (HRM) in N-terminus → blocks mitochondrial import.
    • Liver: transcription repressed by heme; induced by PGC-1α / Nrf-1 / FOXO-1 during fasting; explains “glucose/hemin therapy”.
    • Erythron: translation depends on iron (IRE/IRP switch); ALA-S2 loss-of-function → X-linked sideroblastic anaemia.
  • Cofactor: pyridoxal-phosphate ⇒ pyridoxine deficiency → hypochromic anaemia.
  • Structure: homodimer; crystal (2BWN) confirms PLP-Schiff-base chemistry.

δ-Aminolevulinate Dehydratase (ALA-D)

  • Reaction: 2\,\delta\text{-ALA} \xrightarrow{\text{ALA-D}} \text{Porphobilinogen (PBG)} + \text{H}_2\text{O} (Fig 12-6).
  • Homo-octamer; each subunit binds Zn²⁺ (4 catalytic + 4 structural).
  • Lead replaces Zn²⁺ ⇒ enzyme inhibition ⇒ lead intoxication mimics porphyria.
  • Morpheeins: high-activity octamer ↔ low-activity hexamer.
  • Gene: chromosome 9q34; alternative erythroid promoter (GATA-1 / KLF-1). Common polymorphism K59N = ALA-D2 allele.

ALA-D Deficiency Porphyria (ADP)

  • Autosomal recessive; < 10 patients reported.
  • Clinical: childhood–adolescent neurovisceral crises (abd. pain, motor neuropathy), very high urinary ALA, normal/near-normal PBG.
  • Triggers & imitators: lead poisoning, hepato-renal tyrosinaemia (succinyl-acetone inhibition), rare acquired inhibitors.
  • Labs: ALA-D ≤ 5 % normal, ↑urinary ALA + coproporphyrin, ↑RBC Zn-protoporphyrin.
  • Therapy: hemin + high-dose glucose; outcomes variable; liver Tx ineffective.

Process 2 – Assembly of the tetrapyrrole

Porphobilinogen Deaminase (PBG-D; hydroxymethyl-bilane synthase)

  • Builds linear tetrapyrrole hydroxymethyl-bilane (HMB) by adding 4 PBG to an active-site dipyrromethane cofactor (Fig 12-7).
  • Monomeric enzyme; gene on 11q23.3; tissue-specific promoters ⇒ housekeeping vs. erythroid isoforms.

Acute Intermittent Porphyria (AIP)

  • Autosomal dominant; > 385 mutations; penetrance ≈ 1 %.
  • Clinical (post-puberty F>>M): recurrent severe abd. pain, vomiting, constipation, tachycardia, hypertension, hyponatraemia, motor neuropathy, seizures, psychiatric disorders; no photosensitivity.
  • Triggers: barbiturates, AEDs, rifampin, sulfonamides, fasting, infection, hormones.
  • Pathogenesis: hepatic ALA-S1 induction ⇒ excess ALA+PBG; toxic to nervous system > heme lack.
  • Labs during attack: urinary PBG 220-880 µmol/day; ALA 100-450 µmol/day; plasma/urine tests via LC-MS/MS or bedside Trace PBG Kit (>6 mg L⁻¹).
  • Treatment
    • Acute: IV hemin 3!\text{–}4\,\text{mg kg}^{-1}!\times4 d (albumin-stabilised) ± 10 % glucose drip, symptom control (opioids, β-blockers, benzodiazepines), correct Na⁺/Mg²⁺.
    • Prevention: avoid unsafe drugs, adequate carbs, GnRH analogues for luteal attacks, prophylactic hemin, liver Tx for intractable cases.
    • Prognosis: ↑HCC risk (35–70×); monitor liver.

Uroporphyrinogen III Synthase (URO3-S)

  • Cyclises HMB → uroporphyrinogen III (flips D ring; Fig 12-8); monomeric.
  • Gene: 10q25.2; housekeeping + erythroid promoters (multiple GATA-1 sites).

Congenital Erythropoietic Porphyria (CEP; Günther)

  • Autosomal recessive; ~200 cases; > 40 mutations.
  • Phenotype: infancy onset blistering photosensitivity, hypertrichosis, red-brown teeth (erythrodontia), haemolytic anaemia, splenomegaly, fluorescent diapers (Fig 12-9/10).
  • Pathogenesis: < 10 % URO3-S ⇒ non-enzymatic HMB → uroporphyrin I & coproporphyrin I (dead-end) ⇒ RBC damage & photosensitisation.
  • Labs: urine porphyrins ↑↑ (uroporphyrin I dominant), stool coproporphyrin I ↑, RBC porphyrins, gene sequencing definitive.
  • Therapy: stringent sun avoidance, transfusion ± splenectomy, curative allogeneic stem-cell Tx; lentiviral/ iPSC gene correction promising.

Process 3 – Modification of side-chains

Uroporphyrinogen Decarboxylase (URO-D)

  • Sequentially removes 4 acetates → methyls producing coproporphyrinogen III (Fig 12-11). Homodimer; gene 1p34; no erythroid promoter.

Porphyria Cutanea Tarda (PCT)

  • Most common porphyria; largely adult onset.
  • Variants
    • Familial (F-PCT): heterozygous URO-D mutation (~25 %).
    • Sporadic (S-PCT): hepatic inhibitor without URO-D mutation (~70 %).
    • Hepato-erythropoietic Porphyria (HEP): biallelic URO-D mutations (< 40 cases).
    • Toxic PCT: chlorinated hydrocarbons (e.g., hexachlorobenzene epidemic, TCDD).
  • Risk factors: iron overload (C282Y HFE), chronic Hep C, HIV, alcohol, oestrogens.
  • Skin: fragility, tense bullae, erosions, milia, hyper-/hypopigmentation, facial hypertrichosis (Fig 12-13/14).
  • Pathogenesis: iron-dependent hepatic oxidation of uroporphyrinogen → uroporphomethene (potent URO-D inhibitor) ⇒ hepatic porphyrin build-up (Fig 12-15).
  • Lab pattern: plasma porphyrins ↑, urine: uroporphyrin + hepta- > hexa- > pentacarboxyl isomers (Fig 12-16); stool isocoproporphyrin; ALA/PBG normal.
  • Treatment
    • Phlebotomy: 500 mL q2w until ferritin ≈ 50 µg/L.
    • Low-dose chloroquine/hydroxychloroquine 125–250 mg wk⁻¹ if iron modest.
    • Stop risk factors; sunscreen.
    • HEP responds poorly to above; manage like CEP.

Coproporphyrinogen Oxidase (CPO)

  • Oxidative decarboxylation of propionates on rings A,B ⇒ vinyls producing protoporphyrinogen IX, consuming 2\,\text{O}2 & releasing 2\,\text{H}2\text{O}_2 (Fig 12-17).
  • Dimeric FAD-independent; gene 3q12.

Hereditary Coproporphyria (HCP)

  • Autosomal dominant, low penetrance; rarest acute porphyria.
  • Clinical: AIP-like neurovisceral attacks ± PCT-like skin lesions; homozygous/compound heterozygous children → severe photosensitivity, growth delay.
  • Special variant: Harderoporphyria (homozygous K404E) → neonatal jaundice, haemolytic anaemia, harderoporphyrin in faeces.
  • Labs: huge ↑ faecal coproporphyrin III, ↑ urinary coproporphyrin during attacks, plasma scan – no 626 nm peak.
  • Therapy: treat attacks with hemin; chronic photosensitivity ⇒ avoidance.

Process 4 – Oxidation of protoporphyrinogen & metal insertion

Protoporphyrinogen Oxidase (PPO)

  • \text{Protoporphyrinogen IX}+3\,\text{O}2 \xrightarrow{\text{PPO}} \text{Protoporphyrin IX}+3\,\text{H}2\text{O}_2 (Fig 12-18).
  • FAD-dependent homodimer; inner membrane (matrix side); gene 1q22.

Variegate Porphyria (VP)

  • Autosomal dominant; founder R59W in South Africa (prevalence 1:300).
  • Phenotypes:
    • Cutaneous only (≈ 60 %)
    • Acute attacks only (≈ 20 %)
    • Both (≈ 20 %).
  • Homozygous/compound heterozygous children: severe photosensitivity, neuropathy, growth failure, but no acute crises.
  • Labs: plasma fluorescence peak 626\,\text{nm} pathognomonic; ↑ faecal proto- & coproporphyrin; ALA/PBG ↑ during attacks.
  • Management: as AIP; liver Tx anecdotal.

Ferrochelatase (FECH)

  • Inserts Fe²⁺ into protoporphyrin IX: \text{Proto IX} + \text{Fe}^{2+} \xrightarrow{\text{FECH}} \text{Heme} + 2\,H^+ (Fig 12-19).
  • Homodimer each with 2Fe-2S cluster; gene 18q21.3; hypoxia/iron regulate transcription.

Erythropoietic Protoporphyria (EPP)

  • Genetics
    • ≥ 90 %: trans combination of loss-of-function FECH mutation + low-expression IVS3-48C hypomorphic allele.
    • 2–4 %: biallelic FECH nulls (true AR).
  • Prevalence 7–17 per million in N. Europe; childhood onset.
  • Clinical: immediate painful phototoxicity (burning, oedema, erythema) within minutes of sun; minimal blister/scarring; leathery skin; microcytic anaemia; 10 % early gall-stones; 1–5 % cholestatic liver failure.
  • Labs: free erythrocyte protoporphyrin ↑↑ (6–100×), % Zn-proto low (≈ 8 %). Plasma porphyrin ↑ (protect from light), faecal proto ↑.
  • Therapy:
    • Sun avoidance, opaque sunscreens.
    • \beta-carotene 120–300 mg/d → carotenemia improves tolerance.
    • Cholestyramine, plasmapheresis for liver danger; liver ± marrow Tx if failure.

X-Linked Protoporphyria (XLPP)

  • Gain-of-function ALA-S2 C-terminal deletions (e.g., ΔAGTG 1706-1709) ⇒ hyperactive enzyme.
  • Dominant X-linked; M = F clinically; higher RBC proto & Zn-proto (≈ 40 %); liver disease more frequent.
  • Pathophysiology tied to altered C-terminal loop relieving auto-inhibition of ALA-S2.
  • Management like EPP; consider early marrow ± liver Tx for progressive disease.

Cross-cutting diagnostic & therapeutic pearls

  • Urgent screen in neurovisceral crisis: spot urine PBG (+ALA) → if ↑ start IV hemin immediately.
  • Plasma fluorescence scanning differentiates VP (626 nm) from HCP/AIP.
  • Lead poisoning, tyrosinaemia, drugs may mimic porphyrias biochemically; always measure blood lead & review meds.
  • Iron status pivotal: overload drives PCT; deficiency worsens anaemia in EPP but iron therapy can raise proto levels.
  • Heme/hemin arginate ↓ALA-S1 via negative feedback; glucose infusion works by ↓PGC-1α thus ↓ALA-S1 transcription.
  • Gene sequencing now preferred for definitive sub-typing, family screening, prenatal Dx.

Key equations & numerical facts to remember

  • Soret absorption band: \lambda_{max} \approx 400\,\text{nm}
  • ALA-S reaction ∆G driven by thio-ester cleavage of succinyl-CoA.
  • Phototoxic stoichiometry: 1\,\text{porphyrin} + h\nu \rightarrow ^1!O_2 + \text{ROS} \rightarrow \text{lipid/ protein damage}
  • PPO oxidation: \text{Proto’gen IX}+3\,\text{O}2 → \text{Proto IX}+3\,\text{H}2\text{O}_2 (6-electron total transfer).
  • FECH: \text{Proto IX} + \text{Fe}^{2+} → \text{Heme} + 2H^+

Ethical & practical considerations

  • Misdiagnosis => unnecessary surgery; awareness critical.
  • Drug safety databases (www.porphyriafoundation.com; www.porphyria-europe.com) indispensable for prescribing.
  • Genetic counselling essential: many carriers remain asymptomatic yet transmit disease.
  • Liver-directed gene/ mRNA therapies and stem-cell gene editing represent emerging curative avenues.