subunits, a heavy chain (100kDas) and a light chain (50kDas), which are held together by a disulfide bond. The action of BoNT follows a 4-step mechanism including binding to the neuronal membrane, endocytosis, membrane translocation, and proteolysis of SNARE proteins. In its mechanism of action, the heavy chain of BoNT is first used to find its neuronal targets and bind to the gangliosides and membrane proteins of presynaptic neurons. Next, the toxin is then endocytosed into the cell membrane. The heavy chain undergoes a conformational change important for translocating the light chain into the cytosol of the neuron. Finally, after the light chain of BoNT is brought into the cytosol of the targeted neuron, it is released from the heavy chain so that it can reach its active cleavage sites on the SNARE proteins. The light chain is released from the heavy chain by the reduction of the disulfide bond holding the two together. The reduction of this disulfide bond is mediated by the NADPH-thioredoxin reductase-thioredoxin system. The light chain of BoNT acts as a metalloprotease on SNARE proteins that is dependent on Zn(II) ions, cleaving them and eliminating their function in exocytosis. There are 8 known isotypes of BoNT, BoNT/A – BoNT/H, each with different specific cleavage sites on SNARE proteins. SNAP25, a member of the SNARE protein family located in the membrane of cells, is cleaved by BoNT isotypes A, C, and E. The cleavage of SNAP-25 by these isotypes of BoNT greatly inhibits their function in forming the SNARE complex for fusion of vesicles to the synaptic membrane. BoNT/C also targets Syntaxin-1, another SNARE protein located in the synaptic membrane. It degenerates these Syntaxin proteins with a similar outcome as with SNAP-25. A third SNARE protein, Synaptobrevin (VAMP), is located on cell vesicles. VAMP2 is targeted and cleaved by BoNT isotypes B, D, and F in synaptic neurons. The targets of these various isotypes of BoNT as well as Tetanus Neurotoxin (TeNT) are shown in the figure to the right. In each of these cases, Botulinum Neurotoxin causes functional damage to SNARE proteins, which has significant physiological and medical implications. By damaging SNARE proteins, the toxin prevents synaptic vesicles from fusing to the synaptic membrane and releasing their neurotransmitters into the synaptic cleft. With the inhibition of neurotransmitter release into the synaptic cleft, action potentials cannot be propagated to stimulate muscle cells. This result in paralysis of