Peroxisome
Peroxisoma
Also known as: microbody, peroxisomes
A small single-membrane organelle that uses oxidases to break down very-long-chain and branched fatty acids, producing hydrogen peroxide that its catalase then destroys; it also makes plasmalogens for myelin.
Overview
Peroxisomes are spherical organelles ≈0.2–1 µm in diameter bounded by a single membrane. They lack DNA and ribosomes: their matrix proteins are made on free ribosomes and imported post-translationally in a folded state, guided by a C-terminal targeting signal (PTS1, classically Ser-Lys-Leu) or an N-terminal PTS2 and by import proteins called peroxins (PEX). New peroxisomes form by growth and fission of existing ones and by budding from the ER.
Their oxidases transfer hydrogen from substrates directly to O₂, forming H₂O₂, which catalase converts to water and O₂ (or uses to oxidise other substrates such as ethanol, formaldehyde and phenols). Key functions are β-oxidation of very-long-chain fatty acids (≥ C22) and branched-chain fatty acids, shortening them for completion in mitochondria; α-oxidation of phytanic acid; the first steps of plasmalogen (ether phospholipid) synthesis, essential for myelin and cardiac membranes; and steps in bile-acid synthesis.
Human peroxisomes lack urate oxidase (uricase), so they have no crystalline core and humans excrete uric acid — the reason we are susceptible to gout. Peroxisomes are most numerous in hepatocytes and proximal tubule cells.
Function
- β-oxidation of very-long-chain and branched-chain fatty acids.
- α-oxidation of phytanic acid.
- Plasmalogen synthesis (myelin).
- H₂O₂ production and its detoxification by catalase.
- Bile-acid synthesis and detoxification of ethanol and other compounds (liver).
Clinical significance
Zellweger spectrum disorders
Autosomal recessive PEX gene mutations block import of matrix enzymes ("ghost" peroxisomes): hypotonia, seizures, craniofacial dysmorphism, hepatomegaly, renal cysts; raised very-long-chain fatty acids; death often in infancy.
X-linked adrenoleukodystrophy
ABCD1 mutation impairs transport of VLCFA into peroxisomes; VLCFA accumulate in adrenal cortex and white matter → adrenal insufficiency and progressive demyelination in boys.
Refsum disease
Deficient α-oxidation of dietary phytanic acid (from dairy and ruminant fat): retinitis pigmentosa, peripheral neuropathy, ataxia, ichthyosis; managed by dietary restriction.
Test yourself
An infant with hypotonia, seizures, hepatomegaly and accumulation of very-long-chain fatty acids most likely has a defect in:
- Lysosomes
- Peroxisome biogenesis
- Mitochondrial DNA
- Proteasomes
Answer: Peroxisome biogenesis
Zellweger syndrome — PEX gene mutations prevent import of peroxisomal matrix enzymes, so very-long-chain fatty acids are not β-oxidised and plasmalogens (myelin lipids) are not made.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.