# INTRODUCTION:

-> 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).

# 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.

6) Occurrence: Liver hepatocytes, kidney proximal tubule cells, macrophages, plant leaf cells — highest density in metabolically active tissues.

# STRUCTURE:

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

A) The Peroxisomal Membrane -

> Composition: 

-> 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) transp.orters 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.

-> Contains over 40 known enzyme species responsible for oxidation, lipid catabolism, and biosynthetic reactions.

-> 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. 

# BIOGENESIS:

1) Division model: Most peroxisomes arise by fission of pre-existing peroxisomes; involves elongation, constriction, and scission mediated by DRP1 (dynamin-related protein) and FIS1/MFF.

2) de novo model: New peroxisomes can also form from ER membrane vesicles that acquire peroxisomal membrane proteins and are subsequently imported with matrix enzymes.

3) Peroxisome proliferator-activated receptors (PPARs): Nuclear receptors 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 oxidation of very long-chain fatty acids (VLCFAs, C20–C26) and branched-chain fatty acids. 

Step 1 (Activation): VLCFAs are activated to acyl-CoA by very long-chain acyl-CoA synthetase (ACSL) at the peroxisomal membrane

Step 2 (Import): Acyl-CoA is transported across the membrane via ABCD1 transporter

Step 3 (Oxidation cycle): Successive rounds of β-oxidation: dehydrogenation (by acyl-CoA oxidase) → hydration → dehydrogenation → thiolysis

-> Key difference: Peroxisomal β-oxidation generates H₂O₂ (not FADH₂ for oxidative phosphorylation); energy is partly lost as heat.

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 that constitute ~20% of all phospholipids in humans. They are abundant in brain white matter (myelin), heart, and inflammatory cells (neutrophils, macrophages).

-> Biological significance: Plasmalogens act as antioxidants (preferentially oxidized over membrane phospholipids), regulate signal transduction, and are critical for myelin sheath stability.

4) Cholesterol and Isoprenoid Metabolism:-

-> Peroxisomes contain a subset of cholesterol biosynthesis enzymes (the mevalonate pathway operates partly here in some species).

-> Key enzyme: Sterol carrier protein 2 (SCP-2) is a major peroxisomal lipid transfer protein; facilitates intra-organellar lipid trafficking.

5) Amino Acid and Glyoxylate Metabolism:-

-> Pipecolic acid oxidase: Degrades L-pipecolic acid (a lysine catabolite); peroxisomal dysfunction leads to its accumulation (seen in Zellweger syndrome).

6) Purine Catabolism:-

-> Xanthine oxidase (present in peroxisomes of some tissues) contributes to purine degradation and generates ROS.

7) Importance in Cellular Metabolism & Detoxification:-

-> The defining biochemical feature of peroxisomes is their role as a coupled H₂O₂-generating and H₂O₂-destroying compartment. This compartmentalization prevents cytoplasmic oxidative damage.

-> The H₂O₂ Cycle in Peroxisomes.

 8)Peroxisomes in Lipid Homeostasis and Signaling:-

-> PPARα (peroxisome proliferator-activated receptor α) is activated by fatty acids and fibrate drugs, transcriptionally upregulating peroxisomal and mitochondrial fatty acid oxidation genes.

-> Peroxisomes supply acetyl-CoA and propionyl-CoA (from β-oxidation) which enter mitochondria for energy generation.

-> Plasmalogen levels in cell membranes are regulated by peroxisomal activity; their depletion is associated with Alzheimer's disease pathology.

9) Peroxisomes and Innate Immunity:-

-> Recent research has identified peroxisomes as platforms for antiviral innate immune signaling.

-> MAVS (mitochondrial antiviral signaling protein) is also present on peroxisomal membranes; peroxisomal MAVS triggers a rapid, transient type III interferon response upon viral infection.

-> Peroxisome proliferation is observed during inflammatory conditions (macrophage activation).

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