Clostridial Function

Important members of this toxin family are Clostridium difficile toxins A and B, which are implicated in antibiotics-associated diarrhea and pseudomembranous colitis. The large clostridial cytotoxins are single-chain toxins with molecular masses of 250-308 kDa. The enzyme domain is located at the N terminus. The toxins are taken up from an acidic endosomal compartment. They glucosylate RhoA at Thr37 also, Rac and Cdc42 are substrates. Other members of this toxin family such as Clostridium sordellii lethal toxin possess a different substrate specificity and modify Rac but not Rho. In addition, Ras subfamily proteins (e.g., Ras, Ral, and Rap) are modified. As for C3, they are widely used as tools to study Rho functions [2] [4]. 

Although also iron-sulfur proteins, the rubredoxins do not generate H2S on acidification since in this case the thiol groups are contributed by cysteinyl residues in the polypeptide chain. The function of clostridial rubredoxin is as yet unknown in Pseudomonas sp. a similar protein catalyzes the co-hydroxylation of alkanes, a reaction requiring molecular O2. 

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). 

In analogy to the clostridial system the following reaction sequence describing the functions of Ni/Fe-SP (Ni/Fe sulfur protein) and C/Fe-SP (corrinoid/Fe-sulfur protein) of M. thermophila has been proposed [252,253] ... 

Chaddock, J.A. and Marks, P.M. 2006. Clostridial neurotoxins structure-function led design of new therapeutics. Cell. Mol. Life Sci. 63 540-551. 

Iron-sulfur proteins occur in animal, plant, and bacterial cells. The proteins are characterized by the presence of 1-0, 2-2, 4rA, 6-6, or 8-8 atoms of iroursulfide. Only the structure of clostridial rubredoxin, a 1-0 protein, is knoum. It contains iron ligated to four sulfur atoms of cysteine residues of the polypeptide. With the exception of the ""high potential iron protein, all the proteins show unexpectedly low redox potentials and function in biological oxidationr-reduc-tion reactions. 

The oxidation state of the iron in these proteins is really a diflScult thing to determine. If one simply releases the iron by treatment with acid, one finds all the iron in the ferrous state, even in the so-called oxidized form of clostridial ferredoxin (28). This is because the iron gets reduced by the sulfur and sulfhydryl groups present in these proteins. If one treats the protein with a sulfhydryl reagent and prevents this kind of reduction by sulfhydryl groups, one finds 4 Fe and 4 Fe (34), But even this is misleading, and we think the concept of the oxidation state of the iron in these proteins is not a meaningful one the electron is probably shared by the iron and sulfur atoms in the protein, and they function as electron carriers with the uptake of either one or two electrons by the iron-sulfur chromophore. 

On either enzymatic or chemical reduction, these peaks disappear but the absorption does not go to zero. However, absorption at 280 nm remains unchanged in all cases. This absorption peak at 280 nm must be a function of the chromophore since removal of the iron and sulfide in a way to be described shortly causes this absorption to fall very markedly, whereas reduction has fittle effect on this absorption at 280 nm. I would like to emphasize the difference that exists between the spectra of the chloroplast ferredoxin and the clostridial ferredoxin. Even though they are both called ferredoxins, I think it remains to be determined how closely these compounds are related. 

The reaction catalyzed by the clostridial pyruvate oxidase is a reversible reaction. In the presence of reduced ferredoxin, CO2 may be fixed to form pyruvate, which then can be used in various metabolic reactions and is believed to be a very important reaction in certain of the photosynthetic bacteria where the reaction was first discovered (46), In the Clostridia, however, the enzyme appears to function in the direction of pyruvate oxidation (47). 

This enzyme, as well as nicotinic acid hydroxylase was recently reported by Andreesan to be a selenoenzyme. The discovery of both these enzymes was based on the clever assumption that selenium might well be a component of multisubunit enzymes containing redox centers such as iron-sulfur, flavin, molybdenum, etc. When Clostridium acidiurici was cultured in media with supplemental selenium, an elevated activity of xanthine dehydrogenase was observed. The clostridial enzyme is comparable to mammalian xanthine oxidases in that it contains flavin adeninedinucleotide (FAD), molybdenum and nonheme iron. This enzyme functions in vivo under anaerobic conditions and appears to catalyze the reduction of uric acid to xanthine. Again it will be interesting to learn the form of selenium in this enzyme. 

Habermann E. Clostridial neurotoxins and the central nervous system Functional studies on isolated preparations. In Simpson LL, ed. Botulinum Neurotoxin and Tetanus Toxin. New York, NY Academic Press, Inc 1989 53-67. 

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