RESUMO
Small, hydrophilic molecules, including most important antibiotics in clinical use, cross the Gram-negative outer membrane through the water-filled channels provided by porins. We have determined the X-ray crystal structures of the principal general porins from three species of Enterobacteriaceae, namely Enterobacter aerogenes, Enterobacter cloacae, and Klebsiella pneumoniae, and determined their antibiotic permeabilities as well as those of the orthologues from Escherichia coli. Starting from the structure of the porins and molecules, we propose a physical mechanism underlying transport and condense it in a computationally efficient scoring function. The scoring function shows good agreement with in vitro penetration data and will enable the screening of virtual databases to identify molecules with optimal permeability through porins and help to guide the optimization of antibiotics with poor permeation.
Assuntos
Antibacterianos/metabolismo , Enterobacteriaceae/metabolismo , Porinas/química , Porinas/metabolismo , beta-Lactamas/metabolismo , Antibacterianos/química , Cátions/metabolismo , Permeabilidade da Membrana Celular , Cristalografia por Raios X , Difusão Facilitada , Glicina/metabolismo , Bibliotecas Digitais , Lipossomos/metabolismo , Concentração Osmolar , Multimerização Proteica , Eletricidade Estática , beta-Lactamas/químicaRESUMO
The Gram-negative organism Campylobacter jejuni is the major cause of food poisoning. Unlike Escherichia coli, which has two major porins, OmpC and OmpF, C. jejuni has one, termed major outer membrane protein (MOMP) through which nutrients and antibiotics transit. We report the 2.1-Å crystal structure of C. jejuni MOMP expressed in E. coli and a lower resolution but otherwise identical structure purified directly from C. jejuni. The 2.1-Å resolution structure of recombinant MOMP showed that although the protein has timeric arrangement similar to OmpC, it is an 18-stranded, not 16-stranded, ß-barrel. The structure has identified a Ca2+ bound at the constriction zone, which is functionally significant as suggested by molecular dynamics and single-channel experiments. The water-filled channel of MOMP has a narrow constriction zone, and single-molecule studies show a monomeric conductivity of 0.7±0.2 nS and a trimeric conductance of 2.2±0.2 nS. The ion neutralizes negative charges at the constriction zone, reducing the transverse electric field and reversing ion selectivity. Modeling of the transit of ciprofloxacin, an antibiotic of choice for treating Campylobacter infection, through the pore of MOMP reveals a trajectory that is dependent upon the presence metal ion.