Atlas of plant and animal histology

Peroxisomes are membrane-bound organelles, more of less rounded, and between 0.1 to 1 µm in diameter. They can be found in all eukaryote cells, and have a salient metabolic role. Sometimes the concentration of enzymes they contain is so high that enzymes become crystals.

1. Biogenesis

Peroxisomes are adaptable organelles. They can increase in number and size according to the cell needs and be restored to normal levels after they complete their role. They can also modify their repertory of enzymes. Peroxisomes acquire proteins synthesized by free ribosomes, which can be targeted to the peroxisome membrane or the interior of the organelle. At the peroxisome membrane, there are transporters known as peroxins, which recognize and transport molecules into peroxisomes. There are 12 peroxin types. Proteins targeted to peroxisomes have amino acid sequences known as PTS1 and PTS2 (peroxisome target sequence), which are recognized by peroxins. Metabolic enzymes are translocated across the peroxisome membrane, but some proteins are targeted to the membrane itself. Unlike other organelles where protein are acquired in an unfolded manner, folded and even aggregates of proteins can be incorporated by peroxisomes. Addition of new molecules makes peroxisomes mature and grow.

Biogenesis or formation of new peroxisomes can be done in two ways: a) growing and division from preexisting peroxisomes, and b) by emerging from the endoplasmic reticulum and mitochondria if there are no peroxisomes in the cell (Figure 1).

1) Peroxisomes may proliferate by growing and strangulation. The fission molecular machinery is similar to that used during the division of mitochondria and chloroplasts, despite their different evolutionary origins. Peroxisome division process begins when peroxisome membrane contacts endoplasmic reticulum membranes, which allows lipids transfer from the endoplasmic reticulum membranes to peroxisome membranes. Thus, peroxisome can enlarge its membrane and get bigger. Two new peroxisomes are formed by strangulation of the growing peroxisome. These new peroxisomes maturate by incorporating proteins from the cytosol.

2) Cells can generate peroxisomes de novo without preexisting peroxisomes. Vesicles from the endoplasmic reticulum and mitochondria can form new peroxisomes. These vesicles fuse between each other and mature by incorporating proteins from the cytosol. The hypothesis about the process is as follows. The synthesis of peroxins is started in the cytosol and their membrane sequences "search" for a membrane similar to that of peroxisomes. Since there are no peroxisomes, they get inserted in the membranes of the endoplasmic reticulum or mitochondria. The presence of peroxins in a membrane lead to form vesicles that are released to the cytosol as pre-peroxisomes, with peroxins in their membranes. Physical contacts between pre-peroxisomes and endoplasmic reticulum increase the membrane surface of pre-peroxisomes by transferring lipids from the endoplasmic reticulum, and maturation involves proteins coming from cytosol. It has also been suggested that peroxisomes may grow by gathering vesicles from the endoplasmic reticulum, but there are not convincing evidences.

Peroxisomes are distributed through the cytoplasm thanks to interactions with microtubules and actin filaments. These interactions also allow them to change the morphology and move away the two new peroxisomes from each other after the division.

2. Functions

Peroxidases were the first type of enzyme discovered in this organelle, that is why the name peroxisome. After that, more than 50 different types of enzymes have been found in peroxisomes. However, particular repertories of enzymes depend on the cell type and functional state of the cell. Peroxisomes carry out two main functions: lipid metabolism and protection against peroxides and oxidative radicals. In mammals, peroxisomes degrade lipids with long fatty acid chains, branched lipids, D-amino acid, polyamines and participate in plasminogenesis. In some yeasts, they facilitated the assimilation of alcohol. Catalase and urate oxidase are common enzymes in peroxisomes. Catalase removes hydrogen peroxide (H₂O₂), which is a product of the oxidative reactions. Oxidative reactions can be generally described as follows:

Hydrogen peroxide is a highly reactive molecule and therefore very toxic. Catalase is able to inactivate hydrogen peroxide:

Peroxisomes cooperate with other organelles in many metabolic pathways (see table below). In plants and fungi, β-oxidation is confined to peroxisomes, whereas, in animal cells, β-oxidation is also carried out by mitochondria. In the liver, they are important for synthesizing biliary acids. In plants, peroxisomes can reduce products from CO₂ fixation by a process known as photorespiration, where oxygen is consumed and CO₂ is released. In seeds, however, peroxisomes store fatty acids that are transformed in carbohydrates during germination. These peroxisomes are known as glyoxysomes, which also can be found in filamentous fungi. It is noteworthy that when photosynthesis starts, after the first leaves have developed, glyoxysomes become normal peroxisomes in the mature cells of leaves. In trypanosomes, malaria parasite, glycosomes are a type of peroxisome where glycolysis happens. The different types of peroxisomes are known altogether as microbodies.

Recently, peroxisomes have been proposed as intracellular signaling platforms in mammalian cells.

Different metabolic functions of peroxisomes and the eukaryotic type of cell where they are preformed (from Smith and Aitchison, 2013).

Bibliography

Costello JL, Schrader M. 2018. Unloosing the gordian knot of peroxisome formation. Current opinion in cell biology. 50. 50-56.

Ma C, Agrawal G, Subramani S. 2011. Peroxisome assembly: matrix and membrane protein biogenesis. Journal of cell biology. 193: 7-16.

Smith JJ, Aitchison JD. 2013. Peroxisomes take shape. Nature reviews in molecular and cell biology. 14. 803-817.