5.13: Peroxisomes

Peroxisomes are specialized organelles present in fungi, plant, and animal cells. It can vary in number, size, morphology, and activity depending on the type of tissue and the nutritional state of the cell. For example, cells with active lipid metabolism, such as adipocytes, neurons, and hepatocytes, have more peroxisomes than other cells in the body. Besides their primary role in breaking down complex organic molecules, peroxisomes can also synthesize specific macromolecules and participate in redox signaling.

Hydrogen peroxide: recycle and reuse

Peroxisomes act as a source as well as a sink for hydrogen peroxide. Oxidation of fatty acids releases hydrogen peroxide, which can either be degraded by catalase or oxidize another organic molecule such as ethanol. Furthermore, through specialized channel proteins present on their membrane, peroxisomes release hydrogen peroxide at low levels for participation in intracellular signaling pathways.

Peroxisomes in biosynthesis

Peroxisomes in the brain and heart cells synthesize plasmalogens, a class of glycerophospholipids present in myelin sheaths.

Peroxisomes in plant cells

In addition to β-oxidation of fatty acids, peroxisomes perform many diverse functions in plants. In leaves, they are involved in photorespiration and link chloroplast and mitochondria to recover any carbon lost during photosynthesis. Germinating seedlings contain specialized peroxisomes called glyoxysomes that convert lipids to sugars using the glyoxylate cycle and generate energy for the growing plant.

Eukaryotic cells have multiple, membrane-bound structures called peroxisomes containing digestive enzymes that help oxidize organic molecules.

The shape, number,  and size of peroxisomes vary according to cell type and environmental signals. For example, fat cells have numerous small peroxisomes, while kidney cells have fewer and larger ones.

New peroxisomes can be formed from the growth and fission of existing peroxisomes. Or, they can be synthesized in the ER, where peroxisomal membrane proteins are packaged into vesicles that fuse to form peroxisomes. The enzymes are imported from the cytosol into the matrix through membrane-bound translocons.

These enzymes use molecular oxygen to oxidize substrates such as fatty acids and produce hydrogen peroxide as a by-product.

Because hydrogen peroxide is cytotoxic, Catalase, a peroxisomal enzyme, rapidly converts it into water and oxygen.

Besides lipid metabolism, peroxisomes are also involved in synthesizing lipids such as dolichol and cholesterol and contain enzymes needed for cholesterol-derived bile acid synthesis.