Journal:CHEMBIOINT:1

The four-helix bundle in cholinesterase dimers: structural and energetic determinants of stability

Dana A. Novichkova, Sofya V. Lushchekina, Orly Dym, Patrick Masson, Israel Silman and Joel Sussman ^[1]

Molecular Tour
The crystal structures of truncated forms of cholinesterases provide good models for assessing the role of non-covalent interactions in dimer assembly in the absence of cross-linking disulfide bonds. These structures identify the four-helix bundle that serves as the interface for formation of acetylcholinesterase and butyrylcholinesterase dimers. Here we performed a theoretical comparison of the structural and energetic factors governing dimerization. This included identification of inter-subunit and intra-subunit hydrogen bonds and hydrophobic interactions, evaluation of solvent-accessible surfaces, and estimation of electrostatic contributions to dimerization. To reveal the contribution to dimerization of individual amino acids within the contact area, free energy perturbation alanine screening was performed. Markov state modelling shows that the loop between the α13 and α14 helices in BChE is unstable, and occupies 4 macro-states. The order of magnitude of mean first passage times between these macrostates is ~10-8 seconds. Replica exchange molecular dynamics umbrella sampling calculations revealed that the free energy of human BChE dimerization is -15.5 kcal/mol, while that for human AChE is -26.4 kcal/mol. Thus, the C-terminally truncated human butyrylcholinesterase dimer is substantially less stable than that of human acetylcholinesterase.

: pink, hAChE (PDB ID 4ey4); gray, mAChE (PDB ID 1j06); blue, TcAChE (PDB ID 1w76); green, DmAChE (PDB ID 1qo9). .

(PDB ID 4aqd). (PDB ID 1p0i).

All crystal structures of AChE deposited in the PDB (https://www.rcsb.org) display dimers in which the monomers associate through an antiparallel four-helix bundle. With respect to BChE, crystal structures have been obtained only for monomeric forms of hBChE. In the first hBChE crystal structure obtained (PDB ID 1p0i ^[2]), of partially glycosylated enzyme expressed in CHO cells, dimers assembling via a ^[3][17]. Subsequently, however, a (PDB ID 4aqd) was obtained for fully glycosylated hBChE expressed in Drosophila S2 cells [18]. This dissimilarity of the two crystal structures reveals the high sensitivity of hBChE dimerization to the expression system and crystallization conditions.

References

1. ↑ REF
2. ↑ Nicolet Y, Lockridge O, Masson P, Fontecilla-Camps JC, Nachon F. Crystal structure of human butyrylcholinesterase and of its complexes with substrate and products. J Biol Chem. 2003 Oct 17;278(42):41141-7. Epub 2003 Jul 17. PMID:12869558 doi:http://dx.doi.org/10.1074/jbc.M210241200
3. ↑ doi: https://dx.doi.org/10.1046/j.0014-2956.2001.02692