Parsa, P. M.; Sankararamakrishnan, R.

The protozoan parasite Trypanosoma brucei is implicated in deadly African sleeping sickness. Experimental studies show that T. brucei codes for three aquaporin homologs (TbAQP1 to TbAQP3). TbAQP2 has been established as the high affinity drug transporter of drugs pentamidine and melarsoprol. Mutation in TbAQP2 or its loss result in pentamidine-melarsoprol cross-resistance. TbAQP2 is also shown to transport water, glycerol and other solutes to respond to osmoregulation in the infected hosts or glycerol metabolism. Experimentally determined structures of TbAQP2 shows that it adopts the same aquaporin-like hourglass helical fold. However, the so called aromatic/arginine selectivity filter (Ar/R SF) in TbAQP2 has neither arginine nor aromatic residue and all four residues are hydrophobic. Mutation and functional studies have demonstrated the role of Ar/R SF residues in the transport and selectivity of solutes in aquaporin homologs. The intriguing question is how the completely hydrophobic Ar/R SF region enables the transport of water and glycerol molecules. In this study, we used computational approach to elucidate the molecular mechanism of water and glycerol transport. Our equilibrium molecular dynamics simulations showed that the number of water molecules transported by TbAQP2 is almost one order of magnitude higher than that of prototype water channel AQP1. Moreover, the residence time within TbAQP2 channel is much less compared to that found in AQP1. The relatively wider constriction, interactions of water molecules with the selectivity filter residues and the contact duration, all contribute to a large number of water molecules transported through TbAQP2 channel. Our umbrella sampling studies show that when glycerol is transported through TbAQP2, it participates in interactions with channel residues that can be considered as complimentary to that observed in prototype glycerol transporter GlpF. Our studies reveal the molecular mechanism of water and glycerol transport in TbAQP2 and establish that TbAQP2 is an efficient water transporter.