View source for Diphtheria toxin

{{Short description|Exotoxin}}
{{Infobox nonhuman protein
| Name = tox diphtheria toxin precursor
| image = PDB 1xdt EBI.jpg
| width = 
| caption = Cartoon representation of the diphtheria toxin protein
| Organism = ''Corynebacterium diphtheriae''
| TaxID = 257309
| Symbol = tox
| AltSymbols = 
| IUPHAR_id = 
| ATC_prefix = 
| ATC_suffix = 
| ATC_supplemental = 
| CAS_number = 
| CAS_supplemental = 
| DrugBank = 
| EntrezGene = 2650491
| PDB = 
| RefSeqmRNA = 
| RefSeqProtein = NP_938615
| UniProt = P00587
| ECnumber = 2.4.2.36
| Chromosome = genome
| EntrezChromosome = NC_002935
| GenLoc_start = 188726
| GenLoc_end = 190912
}}
{{Infobox protein family
| Symbol = Diphtheria_C
| Name = Diphtheria toxin, C domain
| Pfam = PF02763
| Pfam_clan = CL0084 
| InterPro = IPR022406
| SMART = 
| PROSITE = 
| MEROPS = 
| SCOP = 1ddt
| TCDB = 1.C.7
| OPM family = 
| OPM protein = 
| CAZy = 
| CDD = 
}}
{{Infobox protein family
| Symbol = Diphtheria_T
| Name = Diphtheria toxin, T domain
| Pfam = PF02764
| Pfam_clan =  
| InterPro = IPR022405
| SMART = 
| PROSITE = 
| MEROPS = 
| SCOP = 1ddt
| TCDB = 1.C.7
| OPM family = 
| OPM protein = 
| CAZy = 
| CDD = 
}}
{{Infobox protein family
| Symbol = Diphtheria_R
| Name = Diphtheria toxin, R domain 
| Pfam = PF01324
| Pfam_clan =  
| InterPro = IPR022404
| SMART = 
| PROSITE = 
| MEROPS = 
| SCOP = 1ddt
| TCDB = 1.C.7
| OPM family = 
| OPM protein = 
| CAZy = 
| CDD = 
}}

'''Diphtheria toxin''' is an [[exotoxin]] secreted mainly by ''[[Corynebacterium diphtheriae]]'' but also by ''[[Corynebacterium ulcerans]]'' and ''[[Corynebacterium pseudotuberculosis]]'',  the [[pathogen]]ic [[bacteria|bacterium]] that causes [[diphtheria]]. The [[toxin]] [[gene]] is encoded by a [[prophage]]<ref group="annotation">A prophage is a [[bacteriophage|virus]] that has inserted itself into the [[genome]] of the host bacterium.</ref> called [[Corynebacteriophage|corynephage β]].<ref>[http://iai.asm.org/cgi/content/full/70/8/3985/T1 TABLE 1. Bacterial virulence properties altered by bacteriophages] from {{cite journal | vauthors = Wagner PL, Waldor MK | title = Bacteriophage control of bacterial virulence | journal = Infection and Immunity | volume = 70 | issue = 8 | pages = 3985–93 | date = August 2002 | pmid = 12117903 | pmc = 128183 | doi = 10.1128/IAI.70.8.3985-3993.2002 }}</ref><ref name="Johnson1986">{{cite journal | vauthors = Johnson LP, Tomai MA, Schlievert PM | title = Bacteriophage Involvement in Group A Streptococcal Pyrogenic Exotoxin A Production | journal = Journal of Bacteriology | volume = 166 | issue = 2 | pages = 623–7 | date = May 1986 | pmid = 3009415 | pmc = 214650 | doi = 10.1128/jb.166.2.623-627.1986 }}</ref>
The toxin causes the disease in humans by gaining entry into the cell [[cytoplasm]] and inhibiting [[protein]] synthesis.<ref name="pmid8573568">{{cite journal | vauthors = Bell CE, Eisenberg D | title = Crystal structure of diphtheria toxin bound to nicotinamide adenine dinucleotide | journal = Biochemistry | volume = 35 | issue = 4 | pages = 1137–49 | date = January 1996 | pmid = 8573568 | doi = 10.1021/bi9520848 }}</ref>

== Structure ==
Diphtheria toxin is a single [[polypeptide]] chain of 535 amino acids consisting of two [[Protein subunit|subunits]] linked by [[disulfide bridge]]s, known as an [[AB toxin|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 (biology)|host cell]].<ref name=Baron>{{cite book | vauthors = Murphy JR | veditors = Baron S  | title = Medical microbiology | year = 1996 | publisher = Univ. of Texas Medical Branch | location = Galveston, Texas | isbn = 978-0-9631172-1-2 | edition = 4th | chapter = ''Corynebacterium Diphtheriae'': Diphtheria Toxin Production | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.1752 |display-editors=etal| pmid = 21413281  }}</ref>

The [[crystal structure]] of the diphtheria toxin [[homodimer]] has been determined to 2.5 [[Angstrom|Ångstrom]] resolution. The [[secondary structure|structure]] reveals a Y-shaped [[molecule]] consisting of three [[protein domains|domains]]. Fragment A contains the [[catalytic]] C domain, and fragment B consists of the T and R domains:<ref name="pmid1589020">{{cite journal | vauthors = Choe S, Bennett MJ, Fujii G, Curmi PM, Kantardjieff KA, Collier RJ, Eisenberg D | title = The crystal structure of diphtheria toxin | journal = Nature | volume = 357 | issue = 6375 | pages = 216–22 | date = May 1992 | pmid = 1589020 | doi = 10.1038/357216a0 | bibcode = 1992Natur.357..216C | s2cid = 4264277 }}</ref>

*The [[N-terminal|amino-terminal]] catalytic domain, known as the C domain, has an unusual beta+alpha [[protein folding|fold]].<ref name="pmid9012663">{{cite journal | vauthors = Bell CE, Eisenberg D | title = Crystal structure of nucleotide-free diphtheria toxin | journal = Biochemistry | volume = 36 | issue = 3 | pages = 481–8 | date = January 1997 | pmid = 9012663 | doi = 10.1021/bi962214s | citeseerx = 10.1.1.432.7047 }}</ref> The C domain blocks [[protein biosynthesis|protein synthesis]] by transfer of [[ADP-ribose]] from [[Nicotinamide adenine dinucleotide|NAD]] to a [[diphthamide]] residue of [[EEF2|eukaryotic elongation factor 2]] (eEF-2).<ref name="pmid7833808">{{cite journal | vauthors = Bennett MJ, Eisenberg D | title = Refined structure of monomeric diphtheria toxin at 2.3 A resolution | journal = Protein Science | volume = 3 | issue = 9 | pages = 1464–75 | date = September 1994 | pmid = 7833808 | pmc = 2142954 | doi = 10.1002/pro.5560030912 }}</ref><ref name="pmid8573568"/>
*A central translocation domain, known as the T domain or TM domain, has a multi-helical [[globin]]-like [[protein folding|fold]] with two additional [[Alpha helix|helices]] at the amino terminus but no counterpart to the first [[globin]] [[helix]]. This domain is thought to unfold in the [[Cell membrane|membrane]].<ref name="pmid7833807">{{cite journal | vauthors = Bennett MJ, Choe S, Eisenberg D | title = Refined structure of dimeric diphtheria toxin at 2.0 A resolution | journal = Protein Science | volume = 3 | issue = 9 | pages = 1444–63 | date = September 1994 | pmid = 7833807 | pmc = 2142933 | doi = 10.1002/pro.5560030911 }}</ref> 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]].<ref name="pmid7833808"/><ref name="pmid8573568"/>
*A [[C-terminal|carboxy-terminal]] receptor-binding domain, known as the R domain, has a [[beta-sandwich]] [[protein folding|fold]] consisting of nine strands in two sheets with Greek-key topology; it is a subclass of [[Immunoglobulin domain|immunoglobulin-like fold]].<ref name="pmid9012663"/> The R domain binds to a [[cell surface receptors|cell surface receptor]], permitting the toxin to enter the [[cell (biology)|cell]] by [[receptor-mediated endocytosis]].<ref name="pmid7833808"/><ref name="pmid8573568"/>

==Mechanism==
[[Image:Diphthamide.png|thumb|[[Diphthamide]]]]
The diphtheria toxin has the same mechanism of action as the enzyme [[NAD(+)—diphthamide ADP-ribosyltransferase]] ({{EC number|2.4.2.36}}). It catalyzes the [[ADP ribosylation]] of the unusual [[amino acid]] [[diphthamide]] in [[EEF2|eEF-2]] by transferring the ADP-ribosyl group from NAD<sup>+</sup>. The ADP ribosylation of diphthamide inactivates the eEF-2 protein, thus, inhibiting the [[Translation (genetics)|translation]] of mRNA. The catalysed reaction is as follows:
: NAD<sup>+</sup> + peptide diphthamide <math>\rightleftharpoons</math> [[nicotinamide]] + peptide ''N''-(ADP-D-ribosyl)diphthamide.

The [[pseudomonas exotoxin|exotoxin A]] of ''[[Pseudomonas aeruginosa]]'' uses a similar mechanism of action.

The steps involved in generating toxicity are as follows:{{citation needed|date=December 2023}}

# 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]]).<ref>{{cite book|last1=Gillet|first1=Daniel|last2=Barbier|first2=Julien|chapter=Chapter 4: Diphtheria toxin|editor-last1=Alouf|editor-first1=Joseph|editor-last2=Ladant|editor-first2=Daniel|editor-last3=Popoff|editor-first3=Michel R.|title=The Comprehensive Sourcebook of Bacterial Protein Toxins|edition=Fourth|pages=111–132|year=2015|publisher=Elsevier|isbn=978-0-12-800188-2}}</ref>{{rp|116}}
# 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-ribosylation|ADP-ribosylates]] host eEF-2, which is required for protein synthesis; when it is inactivated, the host cannot make protein and thus dies.

==Lethal dose and effects==
Diphtheria toxin is extraordinarily potent.<ref name=Baron /> The [[lethal dose]] for humans is about 0.1&nbsp;μg of toxin per kg of body weight. Death occurs through [[necrosis]] of the [[heart]] and [[liver]].<ref name=Pappenheimer_1977>{{cite journal | vauthors = Pappenheimer AM | title = Diphtheria toxin | journal = Annual Review of Biochemistry | volume = 46 | issue = 1 | pages = 69–94 | year = 1977 | pmid = 20040 | doi = 10.1146/annurev.bi.46.070177.000441 }}</ref> 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]]. In 1890, [[Von Behring|Emil Adolf von Behring]] developed an anti-toxin based on the blood of horses immunized with attenuated bacteria.<ref>{{cite journal | vauthors = Enke U | title = 125 Jahre Diphtherieheilserum: Das Behring'sche Gold | trans-title = 125 years of diphtheria healing serum: Behring’s gold | language = German | journal = Deutsches Ärzteblatt | date = 2015 | volume = 112 | issue = 49 | pages = A-2088 | url = https://www.aerzteblatt.de/archiv/173215/125-Jahre-Diphtherieheilserum-Das-Behring-sche-Gold }}</ref> In 1951, Freeman found that the toxin gene was not encoded on the bacterial chromosome, but by a [[Lysogenic cycle|lysogenic]] [[phage]] ([[Corynebacteriophage|corynephage β]])<ref name="Johnson1986" /> infecting all toxigenic strains.<ref name="pmid14850426">{{cite journal | vauthors = Freeman VJ | title = Studies on the virulence of bacteriophage-infected strains of Corynebacterium diphtheriae | journal = Journal of Bacteriology | volume = 61 | issue = 6 | pages = 675–88 | date = June 1951 | pmid = 14850426 | pmc = 386063 | doi = 10.1128/JB.61.6.675-688.1951}}</ref><ref name="pmid14927573">{{cite journal | vauthors = Freeman VJ, Morse IU | title = Further observations on the change to virulence of bacteriophage-infected a virulent strains of Corynebacterium diphtheria | journal = Journal of Bacteriology | volume = 63 | issue = 3 | pages = 407–14 | date = March 1952 | pmid = 14927573 | pmc = 169283 | doi =  10.1128/JB.63.3.407-414.1952}}</ref><ref>{{cite book | chapter-url = http://www.textbookofbacteriology.net/diphtheria.html | chapter = Diphtheria | title = Todar's Online Textbook of Bacteriology | vauthors = Todar K | date = 2009 | publisher = University of Wisconsin }}</ref>

==Clinical use==
The drug [[denileukin diftitox]] uses diphtheria toxin as an [[antineoplastic]] agent.

[[Resimmune]] is an [[immunotoxin]] that is in clinical trials in [[cutaneous T cell lymphoma]] patients. It uses diphtheria toxin (truncated by the cell binding domain) coupled to an antibody to [[T-cell surface glycoprotein CD3 epsilon chain|CD3ε]] (UCHT1).<ref>{{cite book | vauthors = Woo JH, Lee YJ, Neville DM, Frankel AE | title = Immunotherapy of Cancer | chapter = Pharmacology of anti-CD3 diphtheria immunotoxin in CD3 positive T-cell lymphoma trials | series = Methods in Molecular Biology | volume = 651 | issue = 651 | pages = 157–75 | year = 2010 | pmid = 20686966 | doi = 10.1007/978-1-60761-786-0_10 | isbn = 978-1-60761-785-3 }}</ref>

== 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.<ref>{{cite journal | vauthors = Auger A, Park M, Nitschke F, Minassian LM, Beilhartz GL, Minassian BA, Melnyk RA | title = Efficient Delivery of Structurally Diverse Protein Cargo into Mammalian Cells by a Bacterial Toxin | journal = Molecular Pharmaceutics | volume = 12 | issue = 8 | pages = 2962–71 | date = August 2015 | pmid = 26103531 | doi = 10.1021/acs.molpharmaceut.5b00233 }}</ref><ref>{{cite journal | vauthors = Beilhartz GL, Sugiman-Marangos SN, Melnyk RA | s2cid = 6212879 | title = Repurposing bacterial toxins for intracellular delivery of therapeutic proteins | journal = Biochemical Pharmacology | volume = 142 | pages = 13–20 | date = October 2017 | pmid = 28408344 | doi = 10.1016/j.bcp.2017.04.009 }}</ref>

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.<ref>{{cite journal | vauthors = Han JH, Kushner SA, Yiu AP, Hsiang HL, Buch T, Waisman A, Bontempi B, Neve RL, Frankland PW, Josselyn SA | display-authors = 6 | title = Selective erasure of a fear memory | journal = Science | volume = 323 | issue = 5920 | pages = 1492–6 | date = March 2009 | pmid = 19286560 | doi = 10.1126/science.1164139 | s2cid = 1257448 | bibcode = 2009Sci...323.1492H }}</ref><ref>{{cite journal | vauthors = Tammela T, Sage J |doi=10.1146/annurev-cancerbio-030419-033413|doi-access=free|title=Investigating Tumor Heterogeneity in Mouse Models|year=2020 |journal=Annual Review of Cancer Biology|volume=4|issue=1|pages=99–119|pmid=34164589|pmc=8218894}}</ref>

== Annotations ==
<references group="annotation" />

== References ==
{{Reflist|32em}}

== External links ==
* {{MeshName|Diphtheria+Toxin}}

{{Glycosyltransferases}}
{{Toxins}}
{{Pore-forming toxins}}
{{InterPro content|IPR022406}}

{{InterPro content|IPR022405}}

{{InterPro content|IPR022404}}

[[Category:Hepatotoxins]]
[[Category:Protein domains]]
[[Category:Peripheral membrane proteins]]
[[Category:Bacterial toxins]]
[[Category:Diphtheria|Toxin]]
[[Category:Protein synthesis inhibitors]]