Torpedo califomica ACh

The detailed mechanism by which AChE and BChE hydrolyze ACh has been the subject of much research, especially since the crystal structure of the Torpedo califomica AChE was elucidated by Sussman et al. in 1991 [12]. (Reviews of these enzymes and their interactions can be found in Refs. [5,13]). This mechanism will be described here only briefly, as an introduction to the reaction of the enzyme with carbamates. The active site of AChE is located at the bottom of a 20 A-deep gorge, where acetylcholine fits in by attachment of the quaternary ammonium group to the so-called anionic site (mainly through cation interaction with the n electrons of Trp84), and by dipole interactions between the ester group and Ser200 at the esteratic site . 

Millard, C. B., G. Koellner, A. Ordentlich, A. Shafferman, 1. Silman andj. L. Sussman,/. Am. Chem. Soc., 121,1999, 9883-9884. (Crystal structures of products of reaction of VX with Torpedo califomica AChE)... 

The determination of the three-dimensional structure of AChE from Torpedo califomica (Sussman et al, 1991) opened the way to rational redesign of cholinesterases. The possibility of converting a ChE into an OP hydrolase (OPH)... 

To date, 61 cholinesterase structures have been deposited in the Protein Data Bank. Forty are of T. califomica AChE, nine of mouse AChE, three of Drosophila AChE, two of human AChE, and seven of human BuChE. Overlaying of al Torpedo AChE structures reveals exceptional similarity in their protein backbone (Fig. 3A) and even in their side chain conformations (Fig. 3B). The mean value of the root mean square (RMS) deviation of the 40 Torpech AChE structure.s from the alpha carbon trace of the highest resolution (1.8 A), unligandcd Ica5 structure is only 0.26 iO. 10 A. Twelve... 

Insects carrying two AChE genes include mosquitoes and aphids, and their sequences are shown in Table 4 in parallel to T. califomica and D. melanogaster sequences. Clearly, two sequences found in each insect appear significantly different. Intraspecies sequence comparisons yield only 36—40% identity, but interspccics comparison of like sequences within each cluster shows larger (60-99%) identity. Both clusters of sequences have approximately 40% identity with Torpedo AChE. One of the two clusters (the lower one in the Table 4). however, shares significantly... 

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