# WHAT ARE PEROXISOMES?

-> Peroxisomes are small, membrane-bound organelles found ubiquitously in virtually all eukaryotic cells. They are also known as microbodies and are considered single-membrane organelles— unlike mitochondria or chloroplasts, they do not contain DNA. Their name is derived from their primary biochemical activity: the generation and rapid decomposition of hydrogen peroxide (H₂O₂).

-> You will find them in the highest concentration in liver hepatocytes, kidney proximal tubule cells, and macrophages - exactly where detoxification and fatty acid processing demand are greatest.

# KEY CHARACTERISTICS:

1) Size: 0.1 – 1.0 µm in diameter (smaller than mitochondria; visible under electron microscopy).

2) Membrane: Bounded by a single phospholipid bilayer (unlike double-membrane organelles).

3) Genome: No DNA, no ribosomes — all proteins are nuclear-encoded and post-translationally imported.

4) Biogenesis: Arise by division of pre-existing peroxisomes or from the ER membrane.

5) Metabolic hallmark: Contains FAD-linked oxidases and catalase; generates and decomposes H₂O₂.

# STRUCTURAL TOUR:

Peroxisomes exhibit a relatively simple but functionally specialised structure. They consist of a limiting membrane enclosing a granular matrix that houses the enzymatic machinery responsible for oxidative reactions.

A) The Peroxisomal Membrane -

-> Made of a phospholipid bilayer approximately 6–7 nm in thickness.

- Peroxins (PEX proteins): ~35 distinct PEX proteins are embedded in or associated with the membrane; they mediate protein import and organelle biogenesis.

- ABCD transporters: ATP-binding cassette (ABC) transporters import long-chain fatty acids and acyl-CoA esters into the peroxisomal lumen.

- Channel proteins: Porins allow passage of small metabolites (e.g., NADH, pyruvate, acyl-CoA); peroxisomes lack the complex solute carriers found in mitochondria.

- Membrane peroxins: PMPs (peroxisomal membrane proteins) maintain organelle integrity and facilitate signal-mediated matrix protein import via PTS1 and PTS2 targeting sequences.

B) The Peroxisomal Matrix -

-> The matrix is the soluble interior of the peroxisome, where the majority of enzymatic reactions occur. It is characteristically electron-dense under TEM due to its high protein concentration. It contains over 40 known enzyme species responsible for oxidation, lipid catabolism, and biosynthetic reactions. The Matrix pH is approximately neutral (~7.0), unlike the acidic lysosome.

C) Crystalline Core (Nucleoid)

-> In many mammalian liver and kidney peroxisomes, a dense paracrystalline core or nucleoid is visible under electron microscopy. 

# HOW ARE PEROXISOMES MADE?

1) Division model: Most peroxisomes form by division of pre-existing ones. The process involves elongation, constriction, and scission, mediated by DRP1 (dynamin-related protein) and adaptor proteins FIS1/MFF. This is analogous to mitochondrial fission.

2) de novo model: New peroxisomes can also emerge from ER membrane vesicles that acquire peroxisomal membrane proteins, are then loaded with matrix enzymes, and mature into functional peroxisomes.

3) Peroxisome proliferator-activated receptors (PPARs):  Are nuclear receptors that transcriptionally upregulate peroxisome biogenesis in response to fatty acids and fibrate drugs.

# FUNCTIONS:

1) β-Oxidation of Very Long-Chain Fatty Acids (VLCFAs): 

->Peroxisomes are the exclusive site for oxidising very long-chain fatty acids (VLCFAs, C20–C26) and branched-chain fatty acids.

[ Key Difference to Know: Peroxisomal β-oxidation generates H₂O₂ instead of FADH₂, meaning energy is partly released as heat rather than feeding into oxidative phosphorylation. The chain-shortened products are then transferred to mitochondria for complete oxidation.] 

2) α-Oxidation of Phytanic Acid:-

->Phytanic acid (3-methyl branched-chain fatty acid from dietary chlorophyll) cannot undergo direct β-oxidation due to the 3-methyl group. Peroxisomes carry out α-oxidation — removal of one carbon from the carboxyl end — via phytanoyl-CoA hydroxylase (PHYH/PAHX). This converts phytanic acid → pristanic acid, which can then proceed through oxidation.

3)  Ether Lipid / Plasmalogen Biosynthesis:-

->Plasmalogens are a class of ether phospholipids making up roughly 20% of all human phospholipids. They are especially abundant in brain white matter (myelin), the heart, and immune cells. Their biological roles include acting as antioxidants, regulating signal transduction, and maintaining myelin sheath stability. Crucially, their first two biosynthetic steps occur exclusively in peroxisomes.

4) Cholesterol and Isoprenoid Metabolism:-

->Peroxisomes contain key enzymes of the mevalonate pathway and express sterol carrier protein 2 (SCP-2), a major lipid transfer protein facilitating intra-organellar lipid trafficking and cholesterol metabolism.

5) Amino Acid and Glyoxylate Metabolism:-

-> The enzyme pipecolic acid oxidase degrades L-pipecolic acid, a catabolite of lysine. Its accumulation in the blood is a clinical marker of peroxisomal dysfunction — seen in Zellweger syndrome.

6) Purine Catabolism:-

->In some tissues, peroxisomes contain xanthine oxidase, contributing to purine degradation. This reaction also generates reactive oxygen species (ROS), underlining the importance of containing these reactions within the peroxisomal compartment.

# THE HYDROGEN PEROXIDE CYCLE: Detox at Its Finest

-> The defining biochemical feature of peroxisomes is their elegant coupled H₂O₂-generating and H₂O₂-destroying system. FAD-linked oxidases perform substrate oxidation, producing H₂O₂ as a by-product. Catalase — one of the most active enzymes known — then immediately breaks it down into harmless water and oxygen. Catalase also uses H₂O₂ to oxidise other toxic substrates such as ethanol, formaldehyde, and formate — a process called peroxidation. This makes the peroxisome especially vital in liver cells where these compounds must be safely processed.

# PEROXISOMES AND INNATE IMMUNITY SYSTEM:

-> One of the most exciting recent discoveries in cell biology is the role of peroxisomes in antiviral innate immunity. It turns out these organelles do more than just process lipids — they also serve as platforms for immune signalling.

-> MAVS (mitochondrial antiviral signalling protein) is not only found on mitochondria — it is also localised to the peroxisomal membrane. Peroxisomal MAVS triggers a rapid, transient type III interferon response upon viral infection, providing an early wave of antiviral defence.

-> Additionally, peroxisome numbers increase during macrophage activation and other inflammatory conditions — suggesting that peroxisome biogenesis itself is an integrated part of the immune response.

"Ultimately, peroxisomes are not just cellular trash cans; they are specialised, highly dynamic, and essential 'workshops' that keep our cells healthy, energised, and balanced, From breaking down fats to detoxifying harmful substances, these tiny organelles pack a massive punch in keeping our bodies running smoothly".