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The proteasome [1]: a crucial structure of protein degradation

The proteasome

Proteins are constantly being synthesized and at the same time being degraded in each cell of our body.
One of the most known mechanisms of proteolysis (= protein degradation) is the degradation done by lysosomes.

However, an other important mechanism is degradation by the ubiquitin proteasome system (UPS).
First of all, it is needed to understand that a cell maintains its protein concentration by a constant turn-over of proteins: when there is a lot of synthesis of a certain protein, this certain protein will also be degraded by a higher level, and vice versa.

A protein that has to be degraded will be marked by a polyubiquitin chain which consists of 4 or more ubiquitinmolecules.
This protein will be transported to a 26S proteasome. This structure in the cell is built by a 19S part and a 20S part. The 19S part mainly serves as a recognition and binding structure for the polyubiquitinated protein, and the 20S part will destroy the protein.

Once attached to the proteasome, the protein will be defolded and the polyubiquitin chain will be recycled in the cell, done by de-ubiquitinated enzymes (DUB's) - when this is completed this unfolded protein will be translocated inwards the proteasome to the 20S complex.

In the 20S complex there are a lot of proteases, enzymes that will degrade proteins by cleaving its peptide bonds, the protein will be cleaved into oligopeptides.

These peptides will be released back into the cell where they will be used in other cell processes or will be further degraded to amino acids.

Below is a very simplified drawing of the proteasome in work, degrading a protein:









Note: this is a VERY basic explanation of the function of the proteasome in protein degradation in cells.

Reference: Proteasomes and their kin: proteases in the machine age.  2004 Mar;5(3):177-87. Pickart CMCohen RE.

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