Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Intracellular Transport
Transmembrane Translocation

Besides Lysosomes, Eukaryotic Cells constantly contain other similar Organelles known as Microbodies. The most common microbodies are Peroxisomes—small vesicles about 0.5 µm in diameter, surrounded by a single membrane (Figure 39).

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Figure 39 - Peroxisome: 1 - peroxisomal Membrane Proteins; 2 - catalase

Peroxisomes bud off from the smooth Endoplasmic reticulum and contain Enzymes that drive oxidation reactions involving molecular oxygen. The main reaction carried out by peroxisomes can be written as

RH₂ +О₂ → R + H₂O₂,

where RH₂ is the substance being oxidized. The resulting hydrogen peroxide is either used by The Cell to oxidize other substances or broken down within the peroxisomes by the enzyme catalase:

2Н₂O₂→ 2Н₂O + O₂.

Oxidation via molecular oxygen often serves a protective function—in this way, peroxisomes convert xenobiotics (foreign low-molecular-weight substances that enter the Organism) into harmless products.

For example, nearly half of ingested ethanol is oxidized to acetaldehyde in Liver peroxisomes. In plant cells, peroxisomes play a special role by catalyzing ENZYMATIC REACTIONS OF a unique metabolic pathway known as Photorespiration.

In addition, the cells of germinating seeds contain a specialized type of microbodies called glyoxysomes. They facilitate The conversion of Fatty acids stored in seed Lipids into sugars, which are essential for biosynthetic reactions in the young plant.

In most single-membrane-bounded organelles, proteins are delivered via Vesicular Transport (Figure 31). Peroxisomes are an exception to this rule.

Peroxisomal proteins are synthesized in the Cytosol and subsequently transported into peroxisomes. Peroxisomal targeting signals within the Amino acid sequences of these proteins bind to import receptor proteins already in the cytosol. These transport protein-receptor complexes then attach to the peroxisomal membrane, cross it, release the cargo protein into the peroxisomal lumen, and the receptors are recycled back to the cytosol.

Protein transport into the mitochondrion. Mitochondria contain their own DNA and synthesize some of the proteins required for their function, but the majority of mitochondrial proteins are encoded by the cell's nuclear genome. Such proteins are synthesized on cytosolic Ribosomes and then transported into the mitochondrion. As noted above, proteins destined for the mitochondrial matrix possess a specific N-terminal targeting sequence. Mitochondrial receptor proteins recognize this sequence and bind to a translocation complex, which unfolds the protein globule and threads the polypeptide chain across both mitochondrial membranes. Following translocation, the targeting sequence is cleaved off, and the protein folds into its native functional conformation.

Chaperone helper proteins play an active role in unfolding the protein globule prior to translocation and in its subsequent refolding, protecting the polypeptide chain from non-specific aggregation.