Toxins subunits, synthesis

Transforming growth factor-a-Pseudomonas aeruginosa exotoxin A 40 (TGFa-PE40, Mr = 44,960) is a recombinant fusion protein synthesized in E. co/z.127,128 The growth factor moiety binds to surface epidermal growth factor (EGF) receptors on cancerous cells and is internalized where it releases the exotoxin domain into the cytosol.128 The toxin subunit catalytically inactivates the protein synthesis machinery of the cancer cell and the cell subsequently dies.129 It is a recombinant protein intended to be an anticancer therapeutic. 

The synthesis of diol 56 from L-lyxose is reported. Compound 56 constitutes a subunit of the toxin erythroskyrine <96JCS(P1)1323>. 

Diphtheria causes a demyelinative neuropathy. Coryne-bacterium diphtheriae colonizes the pharynx or open wounds, and secretes a protein exotoxin. The B subunit of this exotoxin binds to plasma membranes and facilitates entry into cytosol of the A subunit, which catalyzes ADP-ribosylation, and inactivation of an elongation factor required for protein synthesis. Cardiac muscle and Schwann cells are particularly susceptible to this toxin, and hence patients with diphtheria develop cardiomyopathy and demyelinative polyneuropathy [20]. While diphtheria is now uncommon because of childhood immunization against C. diphtheriae, the disruption in preventative medicine programs caused by disintegration of the Soviet Union was followed by a substantial incidence of diphtheritic polyneuropathy in Russia. 

Several other inhibitors of protein synthesis are notable because of their toxicity to humans and other mammals. Diphtheria toxin (Mr 58,330) catalyzes the ADP-ribosylation of a diphthamide (a modified histidine) residue of eukaryotic elongation factor eEF2, thereby inactivating it. Ricin (Afr 29,895), an extremely toxic protein of the castor bean, inactivates the 60S subunit of eukaryotic ribosomes by depurinating a specific adenosine in 23S rRNA. 

The total synthesis of pederin, a potent insect toxin was achieved by T. Takemura and co-workers. One of the key steps of the synthesis was an intramolecular samarium diiodide induced Reformatsky reaction to construct the lactone subunit of the molecule. The transformation was carried out in tetrahydrofuran at 0 °C without the use of additives or catalysts. 

The first total synthesis of the marine polycyclic ether toxin (-)-gambierol was accomplished in the laboratory of M. Sasaki. The introduction of the a,(3-unsaturation into the seven-membered H ring of the FGH tricyclic subunit proved to be problematic, because both the conventional selenium-based method and the Nicolaou oxidation with IBX failed. However, when the seven-membered ketone was treated with LiHMDS in the presence of TMSCI and EtsN, the corresponding silyl enol ether was formed, which was oxidized under Saegusa conditions to give the desired cyclic enone in high yield. Because of the small scale of the reaction, a large excess of Pd(OAc)2 was used in acetonitrile so the presence of a co-oxidant was not necessary. 

Shiga toxin produced by Shigella dysenteriae has similar structural features. The toxin binds to a glycolipid (Gb3), undergoes endocytosis, and the enzymatie Ai fragment, which is a specific N-glycosidase, removes adenine from one particular adenosine residue in the 28S RNA of the 60S ribosomal subunit. Removal of the adenine inactivates the 60S ribosome, blocking protein synthesis. Ricin, abrin, and a number of related plant proteins inhibit eukaryotic protein synthesis in a similar manner (Chapter 25). 

The actions of diphtheria and pertussis toxins are also mediated by ADP-ribosylation. Diphtheria toxin inhibits eukaryotic protein synthesis by ADP ribosylation of elongation factor II (Chapter 23). Pertussis toxin inactivates Gi by ADP ribosylation of its A-subunit and causes an increase in cAMP production. Unlike cholera toxin, pertussis and diphtheria toxins gain access to many tissues to produce diverse biological effects. Severe watery diarrhea... 

A group of plant lectins, such as abrin, ricin, and mod-eccin, are highly toxic to eukaryotic cells. Their mode of action consists of inhibition of protein synthesis by enzymatically inactivating the EF-2 binding region of the 60S ribosomal subunit, whereas the diphtheria toxin inactivates the EF-2 protein itself. Ricin is isolated from castor beans and has a molecular weight of 66,000. Like most plant and bacterial toxic proteins, ricin contains two... 

Sperti, S., Montanaro, L., Mattioli, A. and Stirpe, F. (1973) Inhibition by ricin of protein synthesis in vitro 60 S ribosomal subunit as the target of the toxin. Biochem J, 136, 813-815. 

A. Diphtheria toxin has two subunits. The B subunit binds to a cell surface receptor and facilitates the entry of the A subunit into the cell. The A snbnnit then catalyzes the ADP-ribosylation of elongation factor 2 (EF2). EF2 is thus inhibited from participating in the translocation process of protein synthesis hence, protein synthesis stops. 

Many toxins interfere with intracellular functions. The best-characterized of these are diphtheria toxin and cholera toxin, produced by the bacteria Corynebacterium diptheriae and Vibrio cholerae, respectively. Both of these toxins contain two subunits, called A and B. The A subunit is responsible for the toxic effect, whereas the B subunit binds to the target cell. Once diphtheria toxin has entered the target cell, the A and B subunits split apart. The A subunit, which is an enzyme, catalyzes a reaction that prevents protein synthesis. The cell dies because it cannot synthesize proteins. The host organisms dies because cardiac, kidney, and nervous tissue are destroyed. 

Periodate promoted cleavage of vicinal diols has also been used to prepare monocyclic products. Oxabicyclo[4.2.1]nonadiene 116 derived from diiodoke-tone 77 was subjected to sodium periodate and sodium borohydride reduction to generate 117, Eq. 87. Subsequent elaborations resulted in the stereocontrolled synthesis of oxepine 118, a subunit designed for the assembly of polyether toxins such as ciguatoxin [135]. 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

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