Mechanism of NH4+ Recruitment and NH3 Transport in Rh Proteins

Summary In human cells, membrane proteins of the rhesus (Rh) family excrete ammonium and play a role in pH regulation. Based on high-resolution structures, Rh proteins are generally understood to act as NH 3 channels. Given that cell membranes are permeable to gases like NH 3 , the role of such proteins remains a paradox. Using molecular and quantum mechanical calculations, we show that a crystallographically identified site in the RhCG pore actually recruits NH 4 + , which is found in higher concentration and binds with higher affinity than NH 3 , increasing the efficiency of the transport mechanism. A proton is transferred from NH 4 + to a signature histidine (the only moiety thermodynamically likely to accept a proton) followed by the diffusion of NH 3 down the pore. The excess proton is circulated back to the extracellular vestibule through a hydrogen bond network, which involves a highly conserved and functionally important aspartic acid, resulting in the net transport of NH 3 .