Diphtheria toxin (DT) is produced by the bacterium Corynebacterium diphtheriae, specifically by strains infected by a virus called the beta-phage. The gene responsible for toxin production (tox) is carried by this phage and integrated into the bacterial DNA. This toxin is solely responsible for the disease diphtheria. Its mechanism involves a precise series of steps to invade a host cell and halt its fundamental processes.
Toxin Structure and Activation
Diphtheria toxin is classified as an A-B toxin, composed of two functional components. The ‘B’ component is the binding subunit, which attaches to the host cell surface. The ‘A’ component is the catalytic subunit, which performs the toxic enzymatic function inside the cell.
The toxin is initially manufactured as a single, inactive polypeptide chain. To become functional, it undergoes proteolytic cleavage, known as “nicking.” This separates the A and B domains but leaves them connected by a disulfide bond, preparing the toxin for cellular entry.
Cellular Entry and Translocation
Cellular entry begins when the toxin binds to a specific receptor on the host cell surface. The receptor-binding domain (R-domain) of the B subunit attaches to the heparin-binding epidermal growth factor-like growth factor (HB-EGF) precursor. This binding determines host cell susceptibility.
The toxin-receptor complex is internalized via receptor-mediated endocytosis, forming a membrane-bound sac called an endosome. The internal environment of the endosome begins to acidify.
The drop in pH triggers a conformational change in the B subunit’s translocation domain (T-domain). The T-domain inserts itself into the endosomal membrane, forming a pore. This channel allows the A domain to be threaded across the membrane and released into the cytoplasm. The disulfide bond linking the A and B domains is then broken, freeing the catalytic A domain.
Molecular Action: Inhibiting Protein Synthesis
Once the catalytic A domain is released into the cytoplasm, it targets the cell’s protein-making machinery. The A domain functions as an enzyme, specifically an ADP-ribosyltransferase. This enzymatic activity involves transferring an ADP-ribose group from the cellular molecule Nicotinamide Adenine Dinucleotide (NAD+) to a specific target protein.
The sole target of diphtheria toxin’s ADP-ribosylation is Eukaryotic Elongation Factor 2 (EF-2), a protein required for building new proteins. The toxin covalently attaches the ADP-ribose group to a modified histidine residue on EF-2 called diphthamide. This chemical modification permanently inactivates EF-2.
The role of EF-2 is to facilitate translocation, which involves moving the growing protein chain and its associated messenger RNA within the ribosome. Without a functional EF-2, the ribosome stalls, and the entire process of protein synthesis comes to a halt. Since cells require a constant supply of new proteins to live, the inhibition of all protein production quickly leads to cell death. This mechanism is profoundly toxic because the A domain acts catalytically, meaning a single molecule of the toxin can inactivate thousands of EF-2 molecules.
Clinical Consequences of Toxin Activity
The inhibition of protein synthesis translates directly into the symptoms of diphtheria. Cell death in the throat and nasal passages, where the bacteria initially colonize, results in inflammation and tissue necrosis. This localized destruction forms the characteristic gray, tough layer known as the pseudomembrane, which is composed of dead cells, bacteria, and inflammatory material.
Once the toxin is absorbed into the bloodstream, it circulates and attacks cells throughout the body. The toxin has a particular affinity for cells that have a high rate of protein turnover, such as those in the heart, nervous system, and kidneys. The most common cause of death in diphtheria is myocarditis, or damage to the heart muscle, which can lead to heart failure and dangerous heart rhythm abnormalities.
Damage to the nervous system, known as neuropathy, is another systemic complication caused by protein synthesis inhibition in nerve cells. This can result in paralysis of the throat and palate muscles, followed by more widespread peripheral nerve damage and weakness.