Ferreira M. M. C., Kiralj, R., “Quantitative relationships between b-lactam molecular properties and b-lactam/AcrAB-TolC complex geometry as determinants of MDR efflux”. São Pedro, SP, 20-23/11/2005: XIII Simpósio Brasileiro de Química Teórica (XIII SBQT) [13th Brazilian Symposium of Theoretical Chemistry], Livro de Resumos [Book of Abstracts], 198. Poster 198.

198
Quantitative relationships between b-lactam molecular properties and b-lactam/AcrAB-TolC complex
geometry as determinants of MDR efflux

Márcia M. C. Ferreira (PQ), Rudolf Kiralj (PQ). rudolf@iqm.unicamp.br

Instituto de Química, Universidade Estadual de Campinas, 13083-970 Campinas, SP, Brazil

Palavras-Chaves: mutidrug resistance, semi-empirical methods, molecular descriptors

INTRODUCTION
Multidrug resistance  (MDR)  of  microbes  and  parasites  as well as of  cancer  cells  to  currently  used drugs
is becoming one of the major problems in combating infectious and parasitic diseases and cancer, respectively.
Among  major  mechanisms  of  multidrug resistance  in  cellular microbes  and  cancer cells  are  efflux  pumps,
macromolecular  systems  that  extrude drugs and a large  variety of structurally dissimilar substances from cell
into the outside medium. Bacterial MDR efflux pump AcrAB-TolC exists in E. coli,  S. typhimurium  and several
other  Gram-negative  bacteria  as  their  major  efflux  system,   being  effective  against  b-lactams  and  other
antibiotics,  organic dyes,  detergents  and  many  xenobiotics.   It is a proton-motive  device  that  connects the
inner and outer cell membranes.   The pump consists of the tube-like  TolC trimer,  the jellyfish-like AcrB trimer,
and   the    AcrA    oligomer.   AcrB   is   responsible   for   attraction   of   substrates    that   are   coming    from
periplasm/cytoplasm,  their accumulation in its central cavity,  and  their  expulsion through its channel  and the
TolC  channel.   Proton influx induces a series of allosteric changes in the pump and its components,  enabling
opening of the channels.   Substrates  in  the central cavity must be placed and oriented  in appropriate way to
be extruded.
METHODS
In this work,   the relationships between  drug  molecular  properties  and  drug-receptor  interaction  geometry
have  been   studied   at   quantitative   level.    Experimental   3D   structures  of   four  AcrB-drug   complexes
(dequalinium,   ethidium,   ciprofloxacin   and   ethidium [1])   were   used.   The   drug-protein   geometry   and
stereoelectronic molecular properties of these drugs and of 16  b-lactams  enabled prediction of positional and
orientational parameters of these  b-lactams  placed  inside  AcrB  [2]  via  linear regression equations with the
best variable selection.   Prior to this analysis,  the geometry  of  all drugs was modeled according to  available
experimental and modeled structural data,  and  then optimized at  PM3  semi-empirical level after  Montecarlo
conformational search,  taking  into account  the  ionic state  of  the  drugs at  neutral  pH  and  preserving  the
bioactive conformation of the  four  drugs  as in the complexes with  AcrB.   Programs  Titan  and  MOPAC 6.0
were used for all quantum-chemical computations.  Crystallographc  C₃  symmetry of AcrB  (space group R32)
facilitated definition of  AcrB crucial axes and points,  as  well  as  drug-receptor  geometry  parameters  (drug-
central  cavity  distance  and  angle  parameters).   After  predicting  the  drug-AcrB  geometry  parameters  for
b-lactams,   all  drugs  were   superimposed  in  the  common  coordinate  system  of   the  uncomplexed  AcrB.
Molecular graphics was performed by using  Titan,  PLATON  and  WebLab Viewer  programs.   Experimental
geometries of AcrB (complexed and uncomplexed) were from E. coli.
RESULTS AND DISCUSSION
Molecular descriptors of the four drugs (dequalinium, ethidium, ciprofloxacin and ethidium)  that  quantitatively
correlated  with  these  parameters  (correlation coefficients  above 0.82)  were  principal  moments  of  inertia,
molecular box parameters, dipole moment and its components,  polarizability  an  hyperpolarizabilities.  These
molecular  properties  showed  similar  behavior  as those  for  b-lactams   in  terms  of  intercorrelations  and
correlations with the efflux activity of AcrAB-TolC pump in three strains of S. typhimurium  (negative logarithm
of Minimal Inhibitory Concentration) [3]. The new modeled AcrB/b-lactam complexes show  that drugs interact
with the vestibule by  electrostatic  interactions,   before  binding  in  the central  cavity and turning  with  their
positive ends  toward  the  opening of  the  AcrB  channel.   These  results are consistent  with  known  pump-
mediated  drug  efflux  mechanism   and  our  previous  quantitative  structure-activity  studies [4].   Elongated
cylinder-like b-lactam antibiotics with lipophylic side chains,  significantly negative  Y  component of the dipole
moment  and  low hydrogen bonding capacity seem  to  be  good  substrates of AcrAB-TolC MDR efflux pump
CONCLUSIONS
Stereoelectronic molecular properties  of  b-lactams  and  structurally dissimilar compounds are quantitatively
related to AcB-drug complex geometry.  This explains the crucial point of the drug efflux, the orientation of an
amphiphilic drug with respect to the inner membrane  and the AcrAB-TolC pump in a Gram-negative bacteria.
ACKNOWLEDGEMENT: FAPESP
LITERATURE
[1] E. W. Yu et al., Science300 (2003) 976-980.
[2] M. M. C. Ferreira, R. Kiralj, J. Mol. Graph. Mod., submitted.
[3] H. Nikaido, M. Basina, V. Nguyen, J. Bacteriol. 180 (1998) 4886-4692.
[4] M. M. C. Ferreira, R. Kiralj, J. Chemometr. 18 (2004) 242-252.