The controversy resolved: how ammonium crosses biological membranes

Scientists appear to have cracked the conundrum of how the highly toxic positively charged ammonium ion (NH4+) manages to be transported across a hydrophobic membrane into and out of cells.

Researchers have shown that ammonium transport is achieved by separation from its proton. Separately NH3 and H+ travel across the

membrane – NH3 in a hydrophobic channel and H+ in a water channel. After crossing, they recombine to become NH4+. It's all

about breaking up, dropping the proton, crossing the membrane separately, and then reforming on the other side.

Ammonium is a highly important substance for plants, fungi and bacteria, and for animals, but for very different reasons.

In non-animals, it is a nutrient to be imported; but in animals and humans, it has to be tightly controlled to maintain correct acid-base, or PH, levels. The import process of ammonium in bacteria, fungi and plants is carried out by Amt/Mep proteins while the pH maintenance in animals is carried out by the Rhesus, Rh, antigen.

In humans, the expelling of ammonium is significant as any failure can lead to a variety of diseases and death.

Research and debate have centered on how the ammonium makes its way through the membrane because hydrophobic membranes are highly unfriendly to charged particles from the cells. The need for special transportation was recognized, but the mechanism has remained a topic for discussion until now.

In an article published in elifesciences.org, authors William J. Allen and Ian Collinson of the School of Biochemistry of the United Kingdom's University of Bristol trace the history of the study of this phenomenon.

They highlight the most recent work of a joint team out of the University of Strathclyde, the University of Dundee and the Université Libre de Bruxelle. This team used computer modeling and traditional empirical research to reach their conclusion.

Building on previous research that identified how ammonium is broken up ahead of the journey through the membrane, the researchers add that they believe the charged proton is dropped – deprotonated – with the constituent parts, H+ via the water channel and NH3 the hydrophonic path. They then rejoin as NH4+, ammonium.

Researchers studied the protein Amt from the bacterium E. coli to gain insights into the mechanism. They found also that water molecules in the Amt protein play a role.

Gordon Williamson, a Ph.D. student at Strathclyde and first lead author of the paper, said:  “Prior to our work, there was a huge controversy in the field. Based on the structure of the E. coli Amt, it shouldn’t be able to transport ammonium, yet transport could be measured."

Williamson told the Dundee university website: "Through molecular simulations carried out by Giulia Tamburrino in the lab of our partner Ulrich Zachariae in Dundee, we discovered that the Amt protein has two chains of water molecules that run through the protein, connecting both ends.

"The experiments we carried out, along with further computational work, demonstrate that these water molecules act as an expressway for the proton, allowing it to bypass parts of the Amt protein that would ordinarily act as a barrier.”

Allen and Collinson concluded in their analysis that the researchers "used detailed computer simulations to generate predictions for how transport is achieved, which were then validated in functional and electrophysiological analyses."

"As such, this work is an excellent example of how integrating computational and empirical observations can produce insights that would otherwise be very difficult to attain," they wrote.