Cell Biology

Peroxisome

The cell's hydrogen peroxide factory — making H₂O₂ on purpose, and destroying it just as fast

Peroxisomes are single-membrane organelles that run reactions producing hydrogen peroxide as an intermediate. They oxidize very-long-chain fatty acids, synthesize plasmalogens (the lipids that make up most of brain white matter), detoxify alcohol, and contain catalase to immediately neutralize the H₂O₂ they generate. Unlike mitochondria, they do not couple their oxidations to ATP synthesis — the energy is released as heat. Loss of peroxisome biogenesis (Zellweger syndrome) is among the most severe metabolic diseases of infancy; loss of one specific peroxisomal transporter causes X-linked adrenoleukodystrophy, the disease of Lorenzo's Oil.

Discovered byChristian de Duve, 1965 (named for H₂O₂)
MembraneSingle (vs lysosome single, mitochondrion double)
Defining enzymeCatalase — kcat ~10⁷ s⁻¹
Key reactionsVLCFA β-oxidation, plasmalogen synthesis, alcohol detox
Targeting signalsPTS1 (-SKL C-terminus), PTS2 (N-terminal nonet)
Major diseaseZellweger, X-ALD, Refsum

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A small organelle with dangerous chemistry

A peroxisome is a roughly spherical organelle 0.1-1 µm across, single-membraned, with a finely granular matrix. Some species show a crystalline core where catalase has packed into a paracrystalline lattice. Each cell has a few hundred peroxisomes; liver and kidney have more.

The internal chemistry is dangerous. Most peroxisomal oxidases use O₂ as electron acceptor and produce H₂O₂:

     RH₂  +  O₂   ─[oxidase]→   R  +  H₂O₂
     │ leak → oxidative damage to DNA, lipids, proteins
     ↓
     2 H₂O₂   ─[catalase]→   2 H₂O  +  O₂

Colocation is the trick: every H₂O₂-producing enzyme shares an organelle with catalase, which destroys it. Catalase runs at ~10⁷ events/s per heme, keeping steady-state [H₂O₂] well below damaging levels. Escaping H₂O₂ also functions as a redox second messenger to the cytosol.

What peroxisomes actually do

VLCFA β-oxidation. Fatty acids longer than ~22 carbons (C24 lignoceric, C26 cerotic) cannot enter mitochondria. The ABCD1 transporter imports them as CoA esters; acyl-CoA oxidase / MFP2 / thiolase shorten them, with the first step generating H₂O₂. Once shortened to ~C8 they are exported to mitochondria. ABCD1 loss causes X-linked ALD.
Plasmalogen synthesis. Ether-linked phospholipids — ~20% of human phospholipids, >50% in myelin. The first two steps (DHAP-AT, alkyl-DHAP synthase) are exclusively peroxisomal. PBD patients have drastically reduced plasmalogens.
Alcohol and aldehyde detoxification. Catalase's peroxidatic mode oxidizes ethanol, methanol, formaldehyde, formate (CH₃CH₂OH + H₂O₂ → CH₃CHO + 2 H₂O). Accounts for ~10% of hepatic ethanol metabolism.
Phytanic acid α-oxidation. Phytanic acid (from chlorophyll) has a β-methyl that blocks normal β-oxidation. Peroxisomes shorten it by one carbon first, then β-oxidize. Failure of phytanoyl-CoA hydroxylase causes Refsum disease.

How peroxisomes are made — PEX genes

Peroxisomes have no DNA, no ribosomes, no internal protein synthesis. Every matrix protein is made on free cytosolic ribosomes and imported post-translationally — by a machinery distinct from mitochondrial or ER import.

Two targeting signals direct cargo: PTS1 (a C-terminal -SKL tripeptide), recognized by PEX5; and PTS2 (an N-terminal nonet R-L-X₅-H-L), recognized by PEX7. Receptor-cargo complexes dock at PEX13/PEX14 and translocate through a transient pore — peroxisomes can uniquely import fully-folded, even oligomeric proteins. PEX5/PEX7 are then ubiquitinated and exported back by PEX1/PEX6 ATPases for another round.

About 14 PEX genes are required; mutations cause Zellweger-spectrum disorders. Biogenesis occurs de novo from the ER (PEX3-dependent budding) or by growth and division (PEX11-mediated elongation, DRP1/MFF fission — same fission machinery as mitochondria).

Peroxisome vs lysosome vs glyoxysome vs mitochondrion

	Peroxisome	Lysosome	Glyoxysome	Mitochondrion
Membrane	Single	Single	Single (specialized peroxisome)	Double
Internal pH	~7	4.5-5.0 (acidic)	~7	~8 (matrix)
Defining enzyme	Catalase	Acid hydrolases (~50)	Isocitrate lyase, malate synthase	ATP synthase, electron transport
Main reactions	VLCFA β-ox, plasmalogen, H₂O₂ detox	Hydrolytic degradation, autophagy	Glyoxylate cycle, fat → carbohydrate	Krebs cycle, oxidative phosphorylation
Has DNA	No	No	No	Yes (~16.5 kb circular)
Origin	From ER + division (PEX1/6)	From Golgi (M6P pathway)	Specialized peroxisome (plant seeds)	Endosymbiotic α-proteobacterium
Major disease	Zellweger, X-ALD, Refsum	Tay-Sachs, Gaucher, Pompe	None (plants only)	Leigh, MELAS, MERRF
Couples to ATP synthesis	No (heat as waste)	No	No	Yes — primary cellular ATP source

Peroxisomal diseases

Zellweger syndrome. The most severe peroxisome biogenesis disorder (PEX1, PEX6 most often). Characteristic facies (high forehead, large fontanelles), severe hypotonia, seizures, hepatic dysfunction, cortical malformations. Death usually in the first year. Elevated plasma VLCFAs, reduced plasmalogens.
Neonatal ALD and infantile Refsum. Less severe PBDs — same genes, partial function preserved. Survive into childhood with developmental delay and progressive neurodegeneration.
X-linked adrenoleukodystrophy (X-ALD). ABCD1 mutations (the VLCFA importer). About one in 17,000 males. Cerebral form: rapid demyelination from age ~7 plus adrenal insufficiency. Adult adrenomyeloneuropathy: later spastic paraparesis. Subject of the 1992 film Lorenzo's Oil.
Adult Refsum disease. PHYH or PEX7 mutations; failed phytanic α-oxidation. Phytanic acid accumulates in nerve, retina, skin — peripheral neuropathy, retinitis pigmentosa, ichthyosis. Treated by dietary restriction of dairy, ruminant fat, fish.
Acatalasemia. CAT mutations. Most carriers asymptomatic but show oral ulcers with H₂O₂-rich infections. Described in Japan by Takahara in 1948 — sores that did not foam with hydrogen peroxide.
Rhizomelic chondrodysplasia punctata. PEX7 or plasmalogen-synthesis defects. Short proximal limbs, long-bone stippling, severe intellectual disability.

Why peroxisomes matter

Brain development. Plasmalogens dominate myelin lipid; peroxisome dysfunction causes leukodystrophy.
Adrenal function. VLCFAs accumulate in steroid-producing cells; X-ALD presents with adrenal insufficiency.
Liver detox. Hepatic peroxisomes handle ~10% of ethanol metabolism plus phytanic and pristanic acid.
Plant germination. Glyoxysomes let seedlings convert stored fat into sugar.
Drug pharmacology. Fibrates (gemfibrozil, fenofibrate) act via PPAR-α, inducing peroxisome proliferation.
Bile-acid synthesis. Peroxisomes shorten cholesterol side chains to bile acids; PBD disrupts bile-salt synthesis.

Common misconceptions

Peroxisomes are smaller lysosomes. Unrelated — peroxisomes do oxidations; lysosomes do hydrolyses at acidic pH.
Peroxisomes generate ATP. They oxidize substrates but do not couple to ATP synthesis; energy is heat.
Catalase is a generic antioxidant. Specific for H₂O₂ — superoxide is handled by SOD, organic peroxides by glutathione peroxidase.
Peroxisomes have their own DNA. Unlike mitochondria and chloroplasts, no genome.
Peroxisomes import unfolded proteins. They can import fully folded, even oligomeric proteins — uniquely.

Frequently asked questions

Why is a peroxisome called a peroxisome?

Christian de Duve named the organelle in 1965 because nearly every reaction it carries out uses molecular oxygen and produces hydrogen peroxide (H₂O₂) as an intermediate. The same lab had discovered lysosomes a decade earlier; both came out of cell-fractionation experiments where novel enzymatic activities settled at intermediate sucrose-gradient densities. The defining enzyme is catalase, which destroys the H₂O₂ produced by the other peroxisomal oxidases — turning a toxic by-product back into water and oxygen.

How does catalase neutralize hydrogen peroxide?

Catalase is one of the fastest enzymes known — kcat around 10⁷ per second per heme. It runs the dismutation reaction 2 H₂O₂ → 2 H₂O + O₂, using one H₂O₂ as substrate and another as electron donor. Catalase concentrations inside a peroxisome can reach 30 mg/mL, so the H₂O₂ generated by oxidases is destroyed locally before it can leak out and oxidize cytosolic targets. At low H₂O₂ concentrations, catalase can also use H₂O₂ to oxidize ethanol, methanol, formate, and other short-chain alcohols — its peroxidatic activity, which contributes about 10% of ethanol metabolism in liver.

What does peroxisomal beta-oxidation do that mitochondrial does not?

Mitochondrial β-oxidation handles short, medium, and long-chain fatty acids — it produces ATP through coupled electron transport. Peroxisomal β-oxidation handles very-long-chain fatty acids (VLCFA, more than 22 carbons), branched-chain fatty acids like phytanic acid, and bile-acid intermediates. It does not produce ATP; the first dehydrogenation step transfers electrons to FAD and then directly to O₂, generating H₂O₂ that catalase destroys. Once the substrate is shortened to about 8 carbons, it is exported to mitochondria for the rest of the work. X-linked adrenoleukodystrophy is caused by failure to import VLCFAs into peroxisomes; they accumulate in plasma and the central nervous system.

What is a glyoxysome?

A specialized peroxisome found in plant seeds and some fungi. Glyoxysomes contain the glyoxylate cycle — a metabolic shortcut that lets seedlings convert stored fat directly into carbohydrate during germination. They have all the standard peroxisomal β-oxidation enzymes plus two extra enzymes (isocitrate lyase and malate synthase) that bypass the decarboxylating steps of the Krebs cycle. The product, succinate, exits to mitochondria and ends up as glucose. Animals lack glyoxysomes — and therefore cannot net-synthesize sugars from fat, only from amino acids and lactate.

What is Zellweger syndrome?

The most severe peroxisome biogenesis disorder. Mutations in any of about a dozen PEX genes (most often PEX1) prevent peroxisome assembly altogether — patients' cells contain only empty peroxisomal membrane ghosts with no matrix enzymes. Clinical features: severe hypotonia, seizures, characteristic high forehead and large fontanelles, hepatic dysfunction, cortical malformations, and death typically within the first year. Plasma shows accumulated very-long-chain fatty acids and reduced plasmalogens. Less severe alleles produce neonatal adrenoleukodystrophy or infantile Refsum disease — the same biochemical problem at lower severity.

How are peroxisomes related to the ER?

More closely than once thought. Genuine de novo biogenesis of peroxisomes — when a cell starts with none — happens by budding of pre-peroxisomal vesicles from the ER, dependent on PEX3 and PEX19. In a cell that already has peroxisomes, most new ones come from growth and division of existing ones. The ER continues to supply membrane phospholipids to mature peroxisomes through membrane contact sites. Some plant lineages have lost peroxisomes entirely and rebuild them de novo from the ER each generation.

Is bone marrow transplant a treatment for X-ALD?

Yes, for the cerebral form, if done early. Allogeneic bone marrow transplant or autologous gene therapy (Skysona, the elivaldogene autotemcel from Bluebird Bio, FDA-approved 2022) introduces functional ABCD1-expressing microglial precursors. Transplant arrests cerebral progression if started before significant white-matter damage. It does not reverse existing damage and does not help adrenomyeloneuropathy, the spinal-cord form. Adrenal insufficiency is treated with steroid replacement.