34. Lysosomes, peroxisomes

Lysosomes

The signal sequence of proteins targeted for the lysosomes is a mannose 6-phosphate attached to the protein. The enzyme that attaches mannose 6-phosphate to proteins is called N-acetylglucosamine-1-phosphate transferase and is found in Golgi. The deficiency of this enzyme causes I-cell disease.

Peroxisomes

The peroxisomes have many functions. It’s involved in the metabolism of very long chain fatty acids and branched chained fatty acids. It performs β-oxidation on these very long chain fatty acids until they are 8 carbons long, at which point they’re transported to the mitochondria.

The peroxisomes also degrade a special fatty acid called phytanic acid, which cannot be broken down by β-oxidation. Phytanic acid can only be broken down by α-oxidation in the peroxisome. The defect of phytanic acid metabolism causes Refsum disease.

The β-oxidation that takes place in the peroxisomes produce less energy than the β-oxidation in the mitochondria. This is because the FADH2 cannot be utilized to produce ATP there.

Recall from biochem 1 that the signal sequences for peroxisomal proteins are PTS1 and PTS2. They’re recognized by two receptors on the surface of the peroxisomes called PEX5 and PEX7 respectively. When proteins when one of these signaling sequences bind to its receptor will the protein be transported into the peroxisome.