Diphtheria toxin is an
exotoxin secreted by ''
Corynebacterium diphtheriae'', the
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic
bacterium
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...
that causes
diphtheria
Diphtheria is an infection caused by the bacterium '' Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild clinical course, but in some outbreaks more than 10% of those diagnosed with the disease may die. Signs and s ...
. The
toxin
A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
is encoded by a
prophage[A prophage is a ]virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
that has inserted itself into the genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
of the host bacterium. called
corynephage β.
The toxin causes the disease in humans by gaining entry into the cell
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
and inhibiting
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
synthesis.
Structure
Diphtheria toxin is a single
polypeptide chain of 535 amino acids consisting of two
subunits linked by
disulfide bridge
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
s, known as an
A-B toxin. Binding to the cell surface of the B subunit (the less stable of the two subunits) allows the A subunit (the more stable part of the protein) to penetrate the
host cell.
The
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
of the diphtheria toxin
homodimer
In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...
has been determined to 2.5
Ångstrom resolution. The
structure reveals a Y-shaped
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
consisting of three
domains. Fragment A contains the
catalytic
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
C domain, and fragment B consists of the T and R domains:
*The
amino-terminal catalytic domain, known as the C domain, has an unusual beta+alpha
fold.
The C domain blocks
protein synthesis by transfer of
ADP-ribose
Adenosine diphosphate ribose (ADPR) is an ester molecule formed into chains by the enzyme poly ADP ribose polymerase. ADPR is created from cyclic ADP-ribose (cADPR) by the CD38 enzyme using nicotinamide adenine dinucleotide (NAD+) as a cofactor.
...
from
NAD to a
diphthamide residue of
eukaryotic elongation factor 2 (eEF-2).
*A central translocation domain, known as the T domain or TM domain, has a multi-helical
globin
The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myogl ...
-like
fold with two additional
helices at the amino terminus but no counterpart to the first
globin
The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myogl ...
helix
A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helic ...
. This domain is thought to unfold in the
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...
.
A
pH-induced
conformational change in the T domain triggers insertion into the
endosomal membrane and facilitates the transfer of the C domain into the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
.
*A
carboxy-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
receptor-binding domain, known as the R domain, has a
beta-sandwich fold consisting of nine strands in two sheets with Greek-key topology; it is a subclass of
immunoglobulin-like fold.
The R domain binds to a
cell surface receptor
Cell surface receptors (membrane receptors, transmembrane receptors) are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving (binding to) extracellular molecules. They are specialized integral m ...
, permitting the toxin to enter the
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
by
receptor-mediated
endocytosis.
Mechanism
# Processing
## The leader region is cleaved during secretion.
## Proteolytic nicking separates A and B subunits, which remain joined by disulfide bonds until they reach the cytosol.
# The toxin binds to heparin-binding epidermal growth factor precursor (HB-EGF).
# The complex undergoes endocytosis by the host cell.
# Acidification inside the endosome induces translocation of the A subunit into the cytosol.
## Disulfide bonds are broken.
## The B subunit remains in the endosome as a pore.
# The A subunit
ADP-ribosylates host
eEF-2
Eukaryotic elongation factor 2 is a protein that in humans is encoded by the ''EEF2'' gene. It is the archaeal and eukaryotic counterpart of bacterial EF-G.
This gene encodes a member of the GTP-binding translation elongation factor family. This ...
, which is required for protein synthesis; when it is inactivated, the host cannot make protein and thus dies.
The diphtheria toxin has the same mechanism of action as the enzyme
NAD(+)—diphthamide ADP-ribosyltransferase
In enzymology, a NAD+-diphthamide ADP-ribosyltransferase () is an enzyme that catalyzes the chemical reaction
:NAD+ + peptide diphthamide \rightleftharpoons nicotinamide + peptide N-(ADP-D-ribosyl)diphthamide
Thus, the two substrates of this en ...
(). It catalyzes the transfer of NAD
+ to a diphthamide residue in eEF-2, inactivating this protein. It does so by
ADP-ribosylating the unusual
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
diphthamide. In this way, it acts as an
RNA translational inhibitor. The catalysed reaction is as follows:
: NAD
+ + peptide diphthamide
nicotinamide + peptide ''N''-(ADP-D-ribosyl)diphthamide.
The
exotoxin A
The Pseudomonas exotoxin (or exotoxin A) is an exotoxin produced by ''Pseudomonas aeruginosa''. ''Vibrio cholerae'' produces a similar protein called the Cholix toxin ().
It inhibits elongation factor-2. It does so by ADP-ribosylation of EF2 u ...
of ''
Pseudomonas aeruginosa
''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aerug ...
'' uses a similar mechanism of action.
Lethal dose and effects
Diphtheria toxin is extraordinarily potent.
The
lethal dose
In toxicology, the lethal dose (LD) is an indication of the lethal toxicity of a given substance or type of radiation. Because resistance varies from one individual to another, the "lethal dose" represents a dose (usually recorded as dose per kilog ...
for humans is about 0.1 μg of toxin per kg of body weight. Death occurs through
necrosis of the
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
and
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
.
Diphtheria toxin has also been associated with the development of myocarditis. Myocarditis secondary to diphtheria toxin is considered one of the biggest risks to unimmunized children.
History
Diphtheria toxin was discovered in 1888 by
Émile Roux and
Alexandre Yersin
Alexandre Emile Jean Yersin (22 September 1863 – 1 March 1943) was a Swiss- French physician and bacteriologist. He is remembered as the co-discoverer of the bacillus responsible for the bubonic plague or pest, which was later named in hi ...
. In 1890,
Emil Adolf von Behring
Emil von Behring (; Emil Adolf von Behring), born Emil Adolf Behring (15 March 1854 – 31 March 1917), was a German physiologist who received the 1901 Nobel Prize in Physiology or Medicine, the first one awarded in that field, for his discovery ...
developed an anti-toxin based on the blood of horses immunized with attenuated bacteria. In 1951, Freeman found that the toxin gene was not encoded on the bacterial chromosome, but by a
lysogenic
Lysogeny, or the lysogenic cycle, is one of two cycles of viral reproduction (the lytic cycle being the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome or formation of a circu ...
phage
A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacter ...
(
corynephage β)
infecting all toxigenic strains.
Clinical use
The drug
denileukin diftitox
Denileukin diftitox (trade name Ontak) was an antineoplastic agent, an engineered protein combining interleukin-2 and diphtheria toxin. Denileukin diftitox could bind to interleukin-2 receptors and introduce the diphtheria toxin into cells that ...
uses diphtheria toxin as an
antineoplastic agent.
Resimmune
Resimmune or A-dmDT390-bisFv(UCHT1) is an experimental drug — an anti-T cell immunotoxin — that is being investigated for treatment of blood cancers such as cutaneous T cell lymphoma (CTCL). It was developed by Doctors Neville, Woo, ...
is an
immunotoxin
An immunotoxin is an artificial protein consisting of a targeting portion linked to a toxin. When the protein binds to that cell, it is taken in through endocytosis, and the toxin kills the cell. They are used for the treatment of some kinds of c ...
that is in clinical trials in
cutaneous T cell lymphoma
Cutaneous T-cell lymphoma (CTCL) is a class of non-Hodgkin lymphoma, which is a type of cancer of the immune system. Unlike most non-Hodgkin lymphomas (which are generally B-cell-related), CTCL is caused by a mutation of T cells. The cancerous ...
patients. It uses diphtheria toxin (truncated by the cell binding domain) coupled to an antibody to
CD3ε (UCHT1).
Research
Similar to other A-B toxins, diphtheria toxin is adept at transporting exogenous proteins across mammalian cell membranes, which are usually impermeable to large proteins. This unique ability can be repurposed to deliver therapeutic proteins, instead of the catalytic domain of the toxin.
This toxin has also been used in neuroscientific and cancer research to ablate specific populations of cells which express the diphtheria toxin receptor (
heparin-binding EGF-like growth factor). Administration of the toxin into the organism which does not naturally express this receptor (e.g. mice) will result in the selective ablation of the cell population which do express it.
Annotations
References
External links
*
How Diphtheria Toxin Works - Animation
{{InterPro content, IPR022404
Hepatotoxins
Protein domains
Peripheral membrane proteins
Bacterial toxins
Toxin
A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...
Protein synthesis inhibitors