Anthrax Lethal Factor

Introduction

Lethal Factor (LF) is one of the enzyme components belonging to the Anthrax Toxin . Anthrax toxin is a three protein exotoxin secreted by the bacterium Bacillus anthracis made up of a binding protein known as the protective antigen (PA) and two enzyme components known as edema factor (EF) and lethal factor (LF).

Anthrax Toxin is considered an AB toxin, with two A domains (EF and LF) and one B domain (PA). On their own, these three domains are nontoxic, but any combination involving PA with EF and/or LF is what causes the physiological effects. Toxic effects begin when PA binds to host Anthrax toxin Receptor (ATR). Initially PA is a 83 KDa protein but is cleaved into two fragments by a furin proteases to become a 63 kDa protein bound to the ATR. Cleavage of PA allows it self associate to form a ring shaped heptamer know as the pore precursor (prepore). The prepore is now able to bind up to three molecules of EF and/or LF. This complex is then endocytosed. In the endosome, the prepore converts to a transmembrane pore, allowing translocation of EF and LF to the cytosol through a mechanism that is not will understood. EF and LF are now able to carry out their enzymatic activity on the host cell.

Human Interaction

Anthrax is primarily a disease of domesticated and wild animals. Herbivores, such as cattle, sheep, horses, mules and goats are primarily affected because these animals maybe grazing on soils contaminated with Bacillus anthracis endospores.^[1][2] The blood of an animal that dies of anthrax can contain upward of 109 vegetative bacteria per milliliter. As the carcass decays, the bacteria form highly infectious endospores, which contaminate the local environment and can remain viable for long time periods. ^[1] The endosopres produced by Bacillus Anthracis remains viable for lengthy periods do to the poly-D- glutamic acid capsule, which itself is nontoxic. This capsule functions to protect the endospore from complement and other bactericidal components found in serum against phagocytic engulfment and destruction. This capsule plays an important role during the infection of anthrax, but is not important during the disease phase, which is primarily caused by PA, EF, LF. Genes encoding this plasmid are located on plasmid pX02.^[1][3]

Anthrax is not common is for humans. Humans become infect by be exposed to farm animals or contaminated animal products such as wool, hides, flesh and blood. Anthrax is not human to human transmittable. There are three ways in which Anthrax can be transmitted to humans:

Cutaneous Anthrax which is the most common form of the disease. This usually occurs when the endospores enter the body through injured skin and germinate. In most cases the bacteria remain contain at the site of infection and present as a lesion. A main characteristic of this type of infection is gelatinous edema at the site infection. As the infection advances, the site of infection goes through multiple stages. Next a papule (solid elevation of the skin) develops which turns into a vesicle (a small fluid-filled blister). The vesicle then develops into a pustule (pus-filled blister). The finally stage is the formation of a necrotic ulcer. In rare case, the infection could spread to the blood stream and cause septicemia. Treatment is often not needed.

Gastrointestinal Anthrax occurs through the ingestion of spores-contaminated meat. The spores then invade the mucosa through a preexisting lesion. After germination, spores spread from the mucosal lesion into the lymphatic system. The form of Anthrax is associated with a high mortality rate but is considered the rarest of the three types of infection.

Inhalation Anthrax is the most fatal of the three infections. Also know as woolsorters' disease, this form involves the inhalation of spore usually contained in animal hair and hides. The spores colonize the Alveolar macrophages and it is believed the believed the macrophages serve both as the sites of germination and as vehicles for transporting the bacteria. At this point the bacteria can rapidly spread throughout the body. If left untreated death is certain. Even with antibiotics mortality rates are high.

Treatments

Antibiotics are used to treat Cutaneous and Inhalation Anthrax infections. The primary antibiotics used are Ciprofloxacin and Doxycycline. Antibiotics should to be administered before symptoms arise, which will likely decrease the fatality rate. If administered after symptoms arise, there is a high chance of fatality. The duration of treatment is 60 days of antibiotics to ensure all spores have germinated. In so cases more than two antibiotics are administered. If systemic spread is seen, this reduces survival rate.^[4].

Structure of Lethal Factor

Anthrax Lethal Factor is composed of four domains:

Domain I functions in the binding of Lethal Factor to Protective Antigen 63 (PA63), which is the membrane translocation component of Anthrax Toxin.^[1] The actual location where interacts with PA is unknown. This domain, made up of amino acids residues 1-263, is perched above the other three domains and is connected to the rest of the domains through an abrupt turn at the end of the last helix.^[1] Domain I consist of 12-helix bundle, packs against one face of a mixed four-stranded beta-sheet.^{[1] [5]}

Domain II consist of residues 263-297 and 385-550; also it has a similar structure with that of B. Cereus toxin catalytic domain VIP2, which contains a NAD binding pocket. Lethal Factor domain II lacks ADP-ribosylating activity due to the lack of conserved residues.^{[1] [6]}

Domain III residues 303-383, Sequence analysis had revealed the presence of a 101-residue segment comprising five tandem repeats residues 282-382, and suggested that repeats 2-5 arose from a duplication of repeat 1. The crystal structure reveals that repeat 1 actually forms the second helix-turn element of domain II, whereas repeats 2-5 form the four helix-turn elements of the helical bundle. Required for LF activity, shares same hydrophobic surface as domain IV and its location restricts access to the active site. Also, it contributes to substrate specificity by making interactions with the substrate.^[1][7]

Domain IV residues 552-776, consists of a nine-helix bundle packed against a four-stranded beta-sheet and contains a HExxH motif. The first six helices and the beta-sheet can be superposed with those of the metalloprotease. A zinc ion (Zn2+) is coordinated tetrahedrally by a water molecule and three protein side chains (Fig. 3), in an arrangement typical of the thermolysin family. Two coordinating residues are the histidines from the HExxH motif (His 686 and His 690) and Glu 735. ^[1][8]

Domains II, III, and IV for the binding pocket for the substrate.

Function of Lethal Factor

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Biowarfare

1J7N

References