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Structure
Stx is composed of two main subunits labeled A and B. The A subunit bonds noncovalently to the B subunit. The A subunit is 293 amino acids long with its active site being glutamic acid 167. Studies have shown that the first 239 residues are essential for the enzymatic activity of the A subunit. The B subunit is a pentamer that resembles a star and can be divided into five identical subunits. Each of the subunits is 69 amino acids in length. The purpose of the B subunit is to bind to globotriaosylceramide (GB3) which is a glycosphingolipid that resides on the surface of some Eukaryotic cells. Each monomer of the B subunit has three binding sites for GB3 making the protein very potent (Melton-Celsa 2013).
Function
The pathway of stx entering a cell begins with the B subunit’s binding to GB3. Once this occurs, the A subunit disconnects from the B subunit and enters the cell through endocytosis. Using retrograde transport the A subunit passes through the Golgi apparatus and the rough endoplasmic reticulum. In the rough endoplasmic reticulum, the A subunit is cleaved into two parts called A1 and A2. A2 is degraded, but A1 freely enters the cytosol (Sandvig 2000). Once in the cytosol, A1 acts as an N-glycosidase, which is an enzyme that hydrolyzes bonds that link sugars. With this enzymatic activity, A1 removes adenines from the 28S RNA of the 60S ribosomal subunit (Melton-Celsa 2013). This inhibits protein synthesis and ultimately leads to cell death.
Disease
Relevance
Structural highlights
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