What are Peroxisomes?

Peroxisomes, identified as membrane-bound cell organelles in eukaryotic cells, were first described by J. Rhodin in 1954 and subsequently recognized as cell organelles by Christian de Duve in 1967. These organelles, small vesicles containing digestive and oxidative enzymes, play a pivotal role in cellular metabolism. They are integral in converting hydrogen peroxide (H2O2) into water and oxygen, a reaction facilitated by the catalase enzyme. Additionally, peroxisomes contribute to lipid biosynthesis and fatty acid oxidation.

Unlike mitochondria and chloroplasts, peroxisomes do not possess their own DNA. However, they replicate through division, similar to these organelles. Notably, peroxisomes are characterized by a single unit membrane, distinguishing them from other cellular organelles like mitochondria, which have a double membrane. This unique structural feature underscores their distinct functional role within the cell.

Peroxisomes are abundant in detoxifying organs such as the liver and kidneys. However, their proliferation can be induced in response to metabolic demands. These organelles are responsible for various metabolic processes, including aspects of energy metabolism. They play a crucial role in the catabolism of long-chain fatty acids, branched-chain fatty acids, bile acid intermediates, D-amino acids, and polyamines. Moreover, peroxisomes are involved in the reduction of reactive oxygen species, particularly hydrogen peroxide, and in the biosynthesis of plasmalogens, essential for the proper functioning of mammalian brains and lungs.

Peroxisomes account for about 10% of the total activity of two enzymes (Glucose-6-phosphate dehydrogenase and 6-Phosphogluconate dehydrogenase) in the pentose phosphate pathway, which is significant for energy metabolism. The involvement of peroxisomes in isoprenoid and cholesterol synthesis in animals is a topic of ongoing debate. In addition to these functions, peroxisomes also participate in the glyoxylate cycle in germinating seeds, photorespiration in leaves, glycolysis in trypanosomes, and methanol and/or amine oxidation and assimilation in some yeasts.

Therefore, peroxisomes are multifunctional organelles that contribute significantly to various biochemical pathways and cellular processes, underscoring their importance in both plant and animal cells.

Peroxisome Lecture Video

Definition of Peroxisome

A peroxisome is a small, membrane-bound organelle found in the cytoplasm of eukaryotic cells. It contains enzymes that are primarily involved in the detoxification of harmful substances, the breakdown of fatty acids, and the metabolism of reactive oxygen species, particularly hydrogen peroxide.

Structure of Peroxisomes

• Basic Structure and Size Peroxisomes are membrane-bound spherical bodies, typically ranging from 0.2 to 1.5 μm in diameter. They are fundamental components of eukaryotic cells, found in both plants and animal cells. These organelles are usually circular in cross-section and vary in size and shape according to the specific needs of the cell.
• Location and Association within the Cell These organelles are located freely within the cytoplasm and often found in close association with the endoplasmic reticulum, mitochondria, or chloroplasts. This positioning within the cell underscores their interconnected roles in cellular metabolism.
• Structural Variability Peroxisomes exist either as individual microperoxisomes or interconnected tubules forming a peroxisome reticulum. Their number, size, and protein composition are variable and highly dependent on the cell type and environmental conditions. For instance, in yeast cells, the abundance and size of peroxisomes can vary significantly based on the available carbon source.
• Membrane Composition and Matrix Content The peroxisome is enclosed by a single limiting membrane composed of lipids and proteins. This membrane encases a granular matrix, which may contain fibrils or a crystalloid structure. The matrix is rich in various enzymes, vital for the organelle’s metabolic functions.
• Dynamics According to Cellular Needs The diameter and number of peroxisomes within a cell can vary based on the cell’s energy requirements. In carbohydrate-rich cells, peroxisomes tend to be smaller, while in lipid-rich cells, they are larger and more numerous. The number of peroxisomes also varies between different types of cells, like fibroblasts, which possess fewer peroxisomes.
• Enzymatic Composition The peroxisome matrix hosts around 60 different enzymes, such as urate oxidase, D-amino acid oxidase, and catalase. These enzymes play critical roles in various metabolic processes, including the breakdown of fatty acids and the detoxification of harmful substances.
• Alternate Forms and Specializations Besides existing as individual entities, peroxisomes can also form a peroxisome reticulum, where they interconnect through tubules. This formation can be observed as clusters within certain cells, such as the preputial gland. In specific cases, like in festucoid grasses, the matrix consists of fibrillar structures, indicating functional adaptations to different cellular environments.

Location of Peroxisomes

1. Ubiquity in Eukaryotic Cells: Peroxisomes are found in most eukaryotic cells, with the notable exception of mature erythrocytes (red blood cells). This widespread presence underscores their essential role in various cellular processes across diverse organisms.
2. Presence in Animal and Plant Cells: In animal cells, peroxisomes play a significant role in detoxification and metabolic processes. Their presence is particularly notable in cells that have high metabolic rates or are involved in detoxification, such as liver and kidney cells. In plant cells, especially in the leaves of higher plants, peroxisomes are crucial for processes like photorespiration.
3. Associations with Other Organelles: In plant cells, peroxisomes often associate with the endoplasmic reticulum, chloroplasts, and mitochondria. This association facilitates efficient metabolic interactions, particularly in processes like photorespiration, where the cooperation of these organelles is essential.
4. Variability in Organelle Structure: The structural form of peroxisomes can vary depending on the cell type. For example, in fibroblasts, peroxisomes typically occur as individual organelles. In contrast, in liver cells, they often exist in the form of interconnected tubules, creating a network known as the peroxisome reticulum. This variation in structure reflects the adaptability of peroxisomes to the specific metabolic needs of different cell types.

Peroxisomal Enzymes

1. Enzymatic Composition of Peroxisomes: The matrix of peroxisomes contains approximately 60 known enzymes. These enzymes are crucial for a variety of biochemical reactions within the organelle, highlighting the peroxisome’s role as a metabolic hub in eukaryotic cells.
2. Primary Function of Peroxisomal Enzymes: A primary function of these enzymes is to catalyze oxidation reactions. During these reactions, hydrogen peroxide (H2O2) is produced as a byproduct. The generation of hydrogen peroxide is an integral aspect of peroxisomal metabolism, linking these organelles to cellular processes that involve reactive oxygen species.
3. Major Enzyme Groups in Peroxisomes: Among the enzymes present in peroxisomes, several key groups stand out due to their critical roles:
   • Urate Oxidase: This enzyme is involved in the purine degradation pathway, where it catalyzes the oxidation of uric acid to allantoin. This reaction is an essential step in the metabolism of nitrogen-containing compounds in certain organisms.
   • D-amino Acid Oxidase: This enzyme is responsible for the oxidation of D-amino acids to their corresponding keto acids. It plays a pivotal role in the metabolism of amino acids, particularly in the catabolism of unusual D-isomers.
   • Catalase: Perhaps the most well-known enzyme within peroxisomes, catalase is crucial for the detoxification of hydrogen peroxide. It catalyzes the decomposition of H2O2 into water and oxygen, thereby protecting the cell from oxidative damage.

Peroxisome Biogenesis

Peroxisomes, essential for various metabolic functions, lack nucleic acids. Proteins in peroxisomes are encoded by nuclear genes and synthesized in the cytoplasm. These proteins are then imported posttranslationally into the peroxisome. This unique mechanism underlines the significance of accurately synthesizing and importing proteins for peroxisome functionality.

1. Biogenesis of Peroxisomal Membrane Proteins (PMPs): PMPs are key components of the peroxisome membrane, identified by specific peroxisomal targeting signals called mPTSs. These signals consist of at least one transmembrane domain and a binding site for PEX19, a chaperone and import receptor for PMPs. PEX19 serves two roles: maintaining solubility of new PMPs in the cytoplasm and transporting them to the peroxisome surface by interacting with PEX3. The integration of PMPs into the peroxisome membrane involves their release from PEX19, although the precise mechanism remains partially understood.
2. Biogenesis of Peroxisomal Matrix Proteins: Peroxisomal matrix proteins, which include most peroxisomal enzymes, need to pass through the peroxisome membrane. These proteins have peroxisomal targeting signals (PTSs) – PTS1 at the C-terminus or PTS2 near the N-terminus. PTS1 and PTS2 are recognized by PEX5 and PEX7 receptors, respectively. These receptors transport enzymes to the peroxisome, a process facilitated by the docking factor PEX14 and assisted by other peroxins for enzyme translocation and receptor recycling.
3. Diseases Associated with Peroxisome Biogenesis Disorders: Defects in genes responsible for importing PMPs or matrix proteins can lead to diseases like Zellweger syndrome, characterized by the loss of peroxisomal functions and various physiological abnormalities. Mutations in PEX7 specifically cause rhizomelic chondrodysplasia punctata (RCDP) type 1, leading to specific peroxisomal dysfunction.
4. Peroxisome Division and Formation: Peroxisomes predominantly multiply through budding or scission from existing peroxisomes, involving PEX11 family peroxins and the GTPase DLP1. Under certain conditions, peroxisomes can also form de novo, originating from ER-derived vesicles that mature into functional peroxisomes. This alternate formation pathway is particularly evident under specific experimental conditions.

Division and Proliferation of Peroxisomes

Two primary models are proposed for peroxisome division. The first suggests peroxisomes originate by budding and fission from pre-existing peroxisomes, a likely mechanism in cells undergoing mitotic division. The second model posits that peroxisomes can form de novo or from a distinct membrane source, such as the endoplasmic reticulum (ER). This process involves a series of biogenesis intermediates that culminate in mature peroxisomes, potentially significant in proliferative peroxisomes or the regeneration of peroxisomes in pex mutants.

1. Initiation of Peroxisome Division in Yeast: In Yarrowia lipolytica, a yeast species, peroxisome division is initiated internally. This signal could arise from peroxisomal membrane lipid remodeling or from metabolites generated by peroxisomal fatty-acid β-oxidation.
2. Proteins Facilitating Peroxisome Formation from the ER:
   • Pex3p: An essential integral peroxisomal membrane protein for the assembly and stability of the RING-domain subcomplex.
   • Pex16p: A protein in Y. lipolytica that aids in importing specific peroxisomal proteins and inhibits peroxisome division.
   • Pex19p: A farnesylated protein acting as a chaperone and import factor for new integral PMPs.
3. Matrix Protein Import Proteins:
   • Pex1p: An ATPase involved in peroxisome targeting signal (PTS) receptor recycling during import.
   • Pex2p: An integral PMP with a RING domain, partaking in receptor recycling.
   • Pex5p: The primary receptor for PTS1, interacting with multiple peroxins.
   • Pex6p: Collaborates with Pex1p for PTS receptor recycling.
4. Regulation of Peroxisome Size and Number:
   • Pex11p: Crucial for normal peroxisome abundance and division.
   • Pex25p, Pex27p, Pex28p, Pex29p: These proteins influence peroxisome size, number, and distribution.
5. Peroxisome Inheritance Proteins:
   • Inp1p and Inp2p: Fungal PMPs that regulate peroxisome retention and movement.
   • Myo2p: An actin-based motor protein that facilitates peroxisome movement.

Peroxisomal Fatty Acid b-Oxidation

1. Conservation Across Organisms Peroxisomal fatty acid β-oxidation is a metabolic pathway conserved across various organisms, including yeast, plants, invertebrates, and vertebrates. Yeast and plants exclusively rely on this peroxisomal pathway, whereas vertebrates utilize both peroxisomal and mitochondrial β-oxidation pathways.
2. Acyl-CoA Oxidation The initial step in peroxisomal fatty acid β-oxidation is the oxidation of fatty acyl-CoA to form 2-enoyl-CoA. Acyl-CoA oxidase catalyzes this reaction, which uniquely produces hydrogen peroxide (H₂O₂) as a byproduct. This aspect distinguishes the peroxisomal pathway from its mitochondrial counterpart, which conserves energy for ATP synthesis.
3. Hydration of 2-enoyl-CoAs Following acyl-CoA oxidation, 2-enoyl-CoAs are hydrated to form 3-hydroxyacyl-CoAs. Enoyl-CoA hydratase, often part of multifunctional proteins like the D-bifunctional protein, facilitates this conversion within peroxisomes.
4. Dehydrogenation of 3-Hydroxyacyl-CoA This stage involves the NAD⁺-dependent dehydrogenation of 3-hydroxyacyl-CoAs, yielding 3-ketoacyl-CoA and NADH. This reaction is integral to the β-oxidation cycle’s progressive breakdown of fatty acids.
5. Thiolase-Catalyzed Cleavage The final step is the cleavage of 3-ketoacyl-CoAs by thiolase. This reaction produces acetyl-CoA or propionyl-CoA (in the case of certain branched-chain fatty acids), essential for subsequent metabolic processes.
6. Auxiliary Proteins and Functions
   • Metabolite Transporters: These proteins transport acyl-CoA molecules into the peroxisome.
   • Acyl-CoA Synthetases: Enzymes that activate free fatty acids to acyl-CoA derivatives.
   • ATP/ADP Carriers: Support the activation of specific fatty acids within peroxisomes.
   • Enzymes for Unsaturated Fatty Acid Oxidation: Aid in the oxidation of unsaturated fatty acids.
   • Racemase Activities: Convert specific branched-chain fatty acids for degradation.
   • Acetyl-CoA Export Mechanisms: Facilitate the transport of acetyl groups to the cytoplasm.
   • Metabolite Shuttles: Regenerate NAD⁺ and transport reduced electron carriers out of the peroxisome.
7. Functional Significance in Different Organisms In mammals, peroxisomal β-oxidation primarily targets very long-chain fatty acids, bile acid precursors, and some branched-chain fatty acids. Conversely, in yeast and plants, this pathway is essential for converting fatty acids into acetyl-CoA, playing a critical bioenergetic role, especially under certain growth conditions.

Therefore, peroxisomal fatty acid β-oxidation is a vital metabolic process with unique characteristics and significant implications across various organisms. Its role in energy metabolism and fatty acid processing is crucial for maintaining cellular and physiological health.

Peroxisomal Contributions to Bile Acid Synthesis

• Role in Metabolism of Bile Acid Precursors Peroxisomes are crucial in metabolizing cholesterol-derived bile acid precursors, specifically dihydroxycholestanoic acid (DHCA) and trihydroxycholestanoic acid (THCA). These substances are exclusively processed by peroxisomal β-oxidation enzymes, highlighting the organelle’s specialized metabolic capabilities.
• Transformation of DHCA and THCA DHCA and THCA, characterized as large, lipophilic, 2-methyl branched-chain fatty acids, undergo initial esterification with Coenzyme A (CoA). Post-esterification, they are transported into the peroxisomal matrix. Within the matrix, these acids undergo β-oxidation, leading to the production of significant metabolites like propionyl-CoA and cholyl-CoA.
• Enzymatic Action in Peroxisomes Peroxisomes contain a specific enzyme, bile acyl-CoA: amino acid acyltransferase. This enzyme is responsible for transferring the cholic acid moiety onto amino acids, such as taurine or glycine. This process is critical in the formation of bile acids.
• Synthesis and Transport of Bile Acids The enzymatic action within peroxisomes results in the synthesis of bile acids. These bile acids are then transported from the peroxisome to the cytoplasm. From the cytoplasm, they are directed towards the bile duct and eventually reach the digestive tract.
• Significance in Cholesterol Metabolism and Lipid Homeostasis The peroxisomal contribution to bile acid synthesis is vital for cholesterol metabolism and overall lipid homeostasis. By processing specific substrates and facilitating the formation and transport of bile acids, peroxisomes play a significant role in maintaining the balance of lipids within the body.

Peroxisomal Fatty Acid a-Oxidation

1. Collaborative Metabolism of Dietary Fatty Acids Peroxisomal and mitochondrial β-oxidation systems work together in animals to metabolize a diverse range of dietary fatty acids. This collaboration is particularly crucial for the metabolism of 3-methyl substituted fatty acids like phytanic acid, which are resistant to β-oxidation.
2. Peroxisomal Fatty Acid α-Oxidation Pathway The peroxisomal fatty acid α-oxidation pathway specifically addresses the metabolism of phytanic acid. This pathway shortens phytanic acid by removing one carbon atom, leading to the formation of pristanic acid, a 2-methyl branched-chain fatty acid.
3. Sequential Enzymatic Steps in α-Oxidation
   • Activation of Phytanic Acid: Phytanic acid is activated into phytanoyl-CoA by phytanoyl-CoA synthetase, an enzyme potentially located on the peroxisome’s outer surface. This step might involve a metabolite transporter from the ABCD1–4 family.
   • Conversion by Dioxygenase: Phytanoyl-CoA is transformed into 2-hydroxyphytanoyl-CoA by phytanoyl-CoA 2-hydroxylase, a dioxygenase requiring 2-oxoglutarate and oxygen.
   • Degradation by Lyase: The enzyme 2-hydroxyphytanoyl-CoA lyase breaks down 2-hydroxyphytanoyl-CoA, yielding pristanal and formyl-CoA. Formyl-CoA subsequently breaks down into free CoA and formate, which is released as CO2.
   • Oxidation of Pristanal: Pristanal undergoes NAD⁺-dependent oxidation by pristanal dehydrogenase to form pristanic acid, which is then activated to pristanoyl-CoA.
4. Interplay with Peroxisomal β-Oxidation Following α-oxidation, pristanoyl-CoA enters the peroxisomal β-oxidation pathway. Here, it undergoes three rounds of β-oxidation, producing acetyl-CoA, two molecules of propionyl-CoA, and the branched-chain fatty acid 4,8-dimethylnonanoyl-CoA.
5. Export and Further Metabolism The resulting 4,8-dimethylnonanoyl-CoA is exported from the peroxisome for further metabolism. This export and subsequent processing are crucial steps in the comprehensive degradation of phytanic acid and its derivatives.

Ether–Phospholipid Biosynthesis

Ether–phospholipids, making up a significant portion of human phospholipids, are distinguished by a fatty alcohol linked via an ether bond to the 1 carbon. Their synthesis involves a series of enzymes across peroxisomes, cytoplasm, and the endoplasmic reticulum (ER).

1. Initial Steps in the Peroxisome The foundational steps occur in the peroxisome, crucial for producing alkyl-dihydroxyacetone phosphate (alkyl-DHAP), the key precursor for ether–phospholipid synthesis. These steps include:
   • Glycerol-3-Phosphate Oxidation: The enzyme glycerol-3-phosphate dehydrogenase converts glycerol-3-phosphate into dihydroxyacetone phosphate (DHAP).
   • Formation of 1-Acyl-DHAP: DHAP acyltransferase in the peroxisome synthesizes 1-acyl-DHAP from DHAP and a fatty acyl-CoA.
   • Reduction of Acyl-CoAs: Fatty acyl-CoA reductase, located on the peroxisome’s exterior, reduces acyl-CoAs to fatty alcohols, essential for alkyl-DHAP production.
   • Synthesis of 1-Alkyl-DHAP: Alkyldihydroxyacetone phosphate synthase within the peroxisome facilitates the formation of 1-alkyl-DHAP, incorporating a fatty alcohol into 1-acyl-DHAP.
2. Formation of 1-Alkyl-G3P The conversion of alkyl-DHAP to 1-alkyl-glycerol-3-phosphate (1-alkyl-G3P) is overseen by alkyl-DHAP reductase. This enzyme is active on the cytoplasmic side of both peroxisomes and the ER.
3. Transformation in the Endoplasmic Reticulum Within the ER, 1-alkyl-G3P undergoes transformation into 1-alkyl-2-acyl-G3P. Through several complex steps, this intermediate is further processed into final products like 1-alkyl-2-acyl-glycerophosphatidylcholine and 1-alkyl-2-acyl-glycerophosphatidylethanolamine.
4. Significance in Mammalian Cells These ether-linked phospholipids, particularly 1-alkyl-2-acyl-glycerophosphatidylcholine and 1-alkyl-2-acyl-glycerophosphatidylethanolamine, are major components of mammalian cell membranes.

Human Peroxisome-Related Disorders

Human peroxisomal disorders, resulting from defects in peroxisomal function, encompass a wide range of conditions. Approximately 20 such disorders are currently known, broadly classified into peroxisomal metabolic disorders and peroxisome biogenesis disorders.

• Peroxisomal Metabolic Disorders These disorders are typically characterized by deficiencies in specific enzymes or transporters involved in peroxisomal metabolism:
  • Pseudoneonatal Adrenoleukodystrophy: Linked to a deficiency in Acyl-CoA oxidase (Acox1).
  • Multifunctional Protein 2 (MFP2) Deficiency: Affects β-oxidation of specific fatty acids.
  • Peroxisomal Thiolase Deficiency: Due to a lack of 3-Ketoacyl-CoA thiolase enzyme.
  • X-linked Adrenoleukodystrophy: Caused by a defect in the ALDp transporter.
  • Rhizomelic Chondrodysplasia Punctata Types 2 & 3: Result from deficiencies in specific enzymes involved in ether–phospholipid biosynthesis.
  • Refsum’s Disease (Classical): Arises from a deficiency in Phytanoyl-CoA hydroxylase.
  • Glutaric Aciduria Type 3: Linked to a defect in Glutaryl-CoA oxidase.
  • Hyperoxaluria Type I: Caused by a deficiency in Alanine:glyoxylate aminotransferase.
  • Acatalasaemia: Due to a lack of Catalase enzyme.
  • Mevalonic Aciduria: Arises from a deficiency in Mevalonate kinase.
  • Di/Trihydroxycholestanoic Acidaemia: Linked to a defect in Trihydroxycholestanoyl-CoA oxidase (Acox2).
  • Adult-onset Sensory Motor Neuropathy: Caused by a deficiency in 2-Methylacyl-CoA racemase.
• Peroxisome Biogenesis Disorders These disorders are related to defects in genes responsible for peroxisome formation and maintenance:
  • Zellweger Syndrome: Attributed to defects in multiple PEX genes.
  • Neonatal Adrenoleukodystrophy: Associated with defects in various PEX genes.
  • Infantile Refsum’s Disease: Linked to defects in multiple PEX genes.
  • Rhizomelic Chondrodysplasia Punctata Type I: Arises from a defect in the PEX7 gene.
  • Neonatal Microcephaly: Caused by a defect in the DLP1 gene.
• Research and Insights Insights into the functions of PEX genes in simpler organisms, such as yeasts, contribute to our understanding of human peroxisomal disorders. Additionally, mouse models have become instrumental in advancing our knowledge and potential therapeutic approaches for these conditions.

Functions of Peroxisomes

1. Hydrogen Peroxide Metabolism Peroxisomes play a crucial role in hydrogen peroxide (H₂O₂) metabolism. Enzymes within peroxisomes produce H₂O₂, a reactive oxygen species, and are also involved in its elimination. Catalase, a key peroxisomal enzyme, decomposes H₂O₂ into water and oxygen, neutralizing its potential toxicity.
2. Fatty Acid Oxidation In animal cells, fatty acid oxidation occurs both in peroxisomes and mitochondria, while in yeasts and plants, it is confined to peroxisomes. This process is significant for energy production and involves the generation of H₂O₂, which is then decomposed by catalase.
3. Lipid Biosynthesis Peroxisomes are involved in the synthesis of cholesterol, dolichol, and bile acids. Bile acid synthesis, crucial in liver cells, originates from cholesterol. Moreover, peroxisomes contain enzymes for synthesizing plasmalogens, essential phospholipids in heart and brain tissues.
4. Germination of Seeds During seed germination, peroxisomes convert stored fatty acids into carbohydrates. This conversion is vital for providing energy and materials necessary for the growth of germinating plants.
5. Photorespiration In green leaves, peroxisomes participate in photorespiration, a process conducted in conjunction with chloroplasts. This process protects plants from photooxidative damage and involves the recycling of carbon from phosphoglycolate.
6. Degradation of Purines and Bioluminescence Peroxisomes are involved in catabolizing purines, polyamines, and amino acids, particularly through uric acid oxidase. Additionally, peroxisomes in fireflies contain luciferase enzymes, crucial for bioluminescence, aiding in mate attraction or predation.
7. Peroxisomal Respiration Peroxisomes are essential for metabolizing H₂O₂, a process mediated by enzymes that both produce and degrade H₂O₂. Catalase plays a critical role in converting H₂O₂ into less harmful substances.
8. Biosynthesis of Lipids Alongside the endoplasmic reticulum, peroxisomes contribute to the biosynthesis of lipids like cholesterol and dolichol. Additionally, they are involved in forming plasmalogens, which are fundamental to the structural integrity of nerve fibers and are crucial in brain and heart tissues.
9. Fatty Acid β-Oxidation In plants and yeast, peroxisomes are the exclusive sites of fatty acid oxidation. In animal cells, this process occurs in both peroxisomes and mitochondria, contributing to the production of ATP, a high-energy molecule.
10. Role in Immune Response and Cell Signaling Peroxisomes contribute to phagocytosis and innate immune signaling, enhancing the cell’s ability to respond to microbial challenges.

Practice Quiz

What is the primary function of peroxisomes in cells?
a) Protein synthesis
b) DNA replication
c) Breakdown of fatty acids
d) Cell division

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Which enzyme is responsible for breaking down hydrogen peroxide in peroxisomes?
a) Catalase
b) Peroxidase
c) Lipase
d) Amylase

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Which of the following is NOT synthesized in peroxisomes?
a) Bile acids
b) Plasmalogens
c) Steroid hormones
d) Phospholipids

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Peroxisomes are primarily involved in the metabolism of:
a) Glucose
b) Amino acids
c) Fatty acids
d) Nucleic acids

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Which organelle is closely related to peroxisomes in terms of origin?
a) Mitochondria
b) Endoplasmic reticulum
c) Golgi apparatus
d) Lysosomes

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Peroxisomes are most abundant in which type of cells?
a) Muscle cells
b) Nerve cells
c) Liver cells
d) Red blood cells

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Which of the following diseases is associated with a defect in peroxisome function?
a) Alzheimer’s disease
b) Zellweger syndrome
c) Parkinson’s disease
d) Cystic fibrosis

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Which enzyme is responsible for the initial steps in the synthesis of plasmalogens in peroxisomes?
a) Acyl-CoA oxidase
b) Dihydroxyacetone phosphate acyltransferase
c) Catalase
d) Peroxidase

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Peroxisomes are involved in the detoxification of which of the following compounds?
a) Alcohol
b) Glucose
c) Amino acids
d) Nucleotides

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Which of the following is a unique feature of peroxisomes compared to other cellular organelles?
a) They contain DNA
b) They can self-replicate
c) They lack a membrane
d) They are involved in lipid synthesis

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