Anthrax toxins

In the aftermath of the 9/11 incident (2001), the anthrax scare killed several people and shut down the major portion of the parliamentary building and several post offices on the Capitol Hill and in other places in the United States. Anthrax is considered to be one of the most popnlar biological weapons. This toxin is qnite different from the toxins talked about above. 

The third protein, called the edema factor, is an enzyme catalyzing the conversion of adenyl triphosphate (ATP) to cyclic adenyl monophosphate (cyclic-AMP). The enzyme is called adenyl cyclase. The ordinary adenyl cyclase is an endogenous enzyme involved in the signal transduction. The edema factor s catalytic portion sequesters calmodulin, another protein involved in the signal transduction, and hence makes it unavailable for a proper function. The edema factor is not catalyti-cally active until it binds calmodulin. Once it has bound cahnodulin, it starts making cyclic-AMP. In fact it makes cyclic-AMP in excess, and hence causes the death of the cell. The chemical details of these phenomena are yet to be studied. 

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Protein toxins acting intracellularly are often composed of two subunits (A/B model). One subunit is catalytic (A-subunit) and the other is responsible for binding and cell entry (B-subunit). Following binding to an extracellular membrane receptor, the toxins are endocytosed. From the endosomes, the A-subunit is directly (pH dqDendent) transferred into the cytosol (e.g., diphtheria toxin and anthrax toxin) or the toxin is transported in a retrograde manner via the golgi to the ER (e.g., cholera toxin), where translocation into the cytosol occurs [1]. 

Another subfamily of ADP-iibosylating toxins modifies G-actin (at Argl77), thereby inhibiting actin polymerization. Members of this family are, for example, C. botulinum C2 toxin and Clostridium perfringens iota toxin. These toxins are binary in structure. They consist of an enzyme component and a separate binding component, which is structurally related to the binding component of anthrax toxin [3]. 

Anthrax toxin Lethal factor Lethal factor MEKs Endoprotease Increase in intracellular cAMP Inhibition of MAP-kinase pathways Calmodulin dependent adenylylcyclase... 

The anthrax toxin is a tripartite toxin and consists ofthe binding component protective antigen (PA), the lethal factor (LF), which is a metalloprotease, and the edema factor (EF), which is a calmodulin-dependent adenylyl-cyclase. Both enzyme components are translocated via PA into target cells. PA is activated by furin-induced cleavage and forms heptamers, which are similar to the binding components of C2 toxin and iota toxin. In the low pH compartment of endosomes, the heptamers form pores to allow translocation of LF and EF. LF cleaves six of the seven MEKs (MAPK-kinases) thereby inhibiting these enzymes. The functional consequence is the blockade of the MAPK pathways that control cell proliferation, differentiation, inflammation, stress response, and survival. Whether this is the reason for the LT-induced cell death of macrophages is not clear [1]. 

MA M34 M34.001 Anthrax lethal factor Component of anthrax toxin... 

Anthrax Medium from cuitures of B. anthracis 1 Separation of protective antigen from medium 2 Adsorption 3 + 3 quantal assay in guinea-pigs using challenge with B. anthracis Exclusion of live 6. anthracis and of anthrax toxin... 

The anthrax bioterrorist attacks that followed the events of September 11th 2001 resulted in a renewed interest BadUus anthracis, the causative agent of this disease. Research has focused on the development of better vaccines than the one currently available. It has been estimated that the aerosolized release of 100 kg of anthrax spores upwind of Washington DC would cause mortalities of 130,000-3,000,000 [63]. Nonetheless, wild-type Bacillus anthracis is susceptible to conventional antibiotics, including penicillin, oxyfloxacin and ciprofloxacin. The problem lies not with the bacterial infection itself, but with three proteins released by the bacteria - protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa) and edema factor (EF, 89 kDa) -known as anthrax toxins [63]. 

Classical bacterial exotoxins, such as diphtheria toxin, cholera toxin, clostridial neurotoxins, and the anthrax toxins are enzymes that modify their substrates within the cytosol of mammalian cells. To reach the cytosol, these toxins must first bind to different cell-surface receptors and become subsequently internalized by the cells. To this end, many bacterial exotoxins contain two functionally different domains. The binding (B-) domain binds to a cellular receptor and mediates uptake of the enzymatically active (A-) domain into the cytosol, where the A-domain modifies its specific substrate (see Figure 1). Thus, three important properties characterize the mode of action for any AB-type toxin selectivity, specificity, and potency. Because of their selectivity toward certain cell types and their specificity for cellular substrate molecules, most of the individual exotoxins are associated with a distinct disease. Because of their enzymatic nature, placement of very few A-domain molecules in the cytosol will normally cause a cytopathic effect. Therefore, bacterial AB-type exotoxins which include the potent neurotoxins from Clostridium tetani and C. botulinum are the most toxic substances known today. However, the individual AB-type toxins can greatly vary in terms of subunit composition and enzyme activity (see Table 2). 

C. botulinum toxins belong to the AB group of toxins, which also includes diphtheria toxin, pseudomonas exotoxin A, anthrax toxin, Shiga(like) toxin, cholera toxin, pertussis toxin, and plant toxins, e.g., ricin. Moiety A has an enzymatic activity and usually modified cellular-target entering cytosol. Moiety B consists of one or more components and binds the toxin to surface receptors, and is responsible for translocation of the A component into cells. AB toxins are produced in a non-active form and are activated by a split between two cysteine residues within a region (Falnes and Sandvig, 2000). 

Anthrax toxin is a bacterial toxin from Bacillus anthracis consisting of three parts protective antigen (PA), lethal factor (LF) and edema factor (EF). Both LF and EF compete for binding sites on the PA protein. The PA protein binds with high affinity to an as yet unknown receptor on macrophages and related cell types. When PA is internalized by the target cells, it functions as a shuttle protein for either EF or LF. Intracellularly, in the acidic environment of the endosome, EF and LF are capable of entering the cytosol by pH-dependent pore formation [139]. 

The same amidation reaction was employed by Joshi et al. to covalently functionalize MWNTs with a specific peptide sequence, which specifically binds to the heptameric receptor-binding subunit of anthrax toxin.33 These nanotube-peptide conjugates were found to selectively destroy anthrax toxin with the reactive oxygen species generated by the nanotubes upon near-IR radiation.33... 

Identification of Novel Anthrax Toxin Countermeasures Using In Silico Methods... 

Chiu T, Solberg J, Patil S et al (2009) Identification of novel non-hydroxamate anthrax toxin lethal factor inhibitors by topo-meric searching, docking and scoring, and in vitro screening. J Chem Inf Model 49 2726-2734... 

Chiu TL, Amin EA (2012) Development of a comprehensive, validated pharmacophore hypothesis for anthrax toxin lethal factor (LF) inhibitors using genetic algorithms, Pareto scoring, and structural biology. J Chem Inf Model, 52 1886-1897... 

Moayeri M, Leppla SH (2004) The roles of anthrax toxin in pathogenesis. Curr Opin Microbiol 7 19-24... 

Liu S, Bugge TH, Leppla SH. Targeting of tumor cells by cell surface urokinase plasminogen activator-dependent anthrax toxin. J Biol Chem 2001 276(21) 17976-17984. 

Bacillus anthracis seeretes three plasmid-eneoded soluble toxin proteins colleetively referred to as anthrax toxin these are PA, LF, and EF. LF and EF funetion individually and in combination as catalytic enzymes in suseeptible host eells. In an unusual twist of nature, they both have evolved to share PA as a eommon reeeptor binding moiety for translocation into the eytosol of the host. PA is a dominant component of the three-part protein toxin seereted by B. anthracis (Liddington et ah, 1999 Petosa et ah, 1997). The mature form of PA, a seereted 735 amino aeid protein, has a molecular weight of 83 kDa. The ribbon strueture is illustrated in Figure 31.5 along with a detailed deseription of... 

Carboxy (C)-terminal end involved in recognition and binding to the anthrax toxin receptor of susceptible host cells as well as pore formation... 

Abalakin, V.A., Sirina, E.G., Cherkasova, T.D. (1990). The effect of lethal anthrax toxin on the functional activity of peritoneal mononuclear phagocytes and polymorphonuclear neutrophils in mice. Zh. Mikrobiol. Epidemiol. Immunobiol. 2 62-1. (In Russian)... 

Arora, N., Leppla, S.H. (1993). Fusions of anthrax toxin lethal factor with shiga toxin and diphtheria toxin enzymatic domains are toxic to mammalian cells. Infect. Immun. 62 4955-61. 

Baldari, T., Tonello, F., Rossi-Paccani, S., Montecucco, C. (2006). Anthrax toxins a paradigm of bacterial immune suppression. Trends Immunol. 27 434-40. 

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