Water transport on the neighbourhood of membranes: lessons from molecular dynamics

CHARA OSVALDO; FERRARA, CG; GRIGERA JOSÉ RAÚL

Montevideo

Congreso; 5th Southern Cone Biophysics Congress. 6th International Conference of Biological Physics.; 2007

IUPAP - SAB

The problem of water transport across an aquaporin is fascinating. The aquaporins are proteins working as water channels located on thecellular plasma membrane. From crystallography of the first aquaporin reported (AQP1) three-dimensional structure is available. An hourglass model for the pore as well as single file behaviour for the water transport was then proposed. Although previous molecular dynamicsstudies revealed interesting features of the water movements through aquaporins, more than few questions remain open about the watermovements on the neighbourhood of aquaporins. In this work, this last point is studied. The objective is to investigate if AQP1 located on aplasma membrane could disturb water transport on its boundary. In order to do this, a study of water behaviour across a nanotube located ona membrane was developed. Equilibrium classical molecular dynamics using GROMACS package was here performed. Two kinds ofsimulations were performed: hydrophilic (PL) and hydrophobic (PB). In the first one (PL), membrane was simulated using an ice crystal anda nanotube were developed removing water molecules emulating the aquaporin three-dimensional structure of AQP1. In the second one(PB), water molecules of the ice crystal were replaced by Lennard-Jones particles with oxygen diameter but without any charge. All thesimulations were performed with temperatures ranging from 250 to 350 K.Water diffusion on the boundary of the membrane was calculatedusing g_rms of GROMACS. Results showed that water diffusion increases monotonically with temperature in both, PL and PB simulations.Surprisingly, water diffusion coefficient obtained in PB simulations is higher compared to that obtained in PS simulations. In order to deepinsight these results a density profile (over the coordinate normal to the membrane) was then calculated, using an ad hoc program developedin FORTRAN. Analysis showed that density oscillations decrease with the temperature. Interestingly, this decrease is more important in PLsimulations compared to that observed in PB simulations.The aquaporins are not purely hydrophilic or hydrophobic structures. It remains toelucidate if the simulations here presented could be considered as limit cases of water transport on the boundary of aquaporins located inplasma membranes.