The Ammonia Transport, Retention and Futile Cycling Problem in Cyanobacteria
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PHYSIOLOGY AND BIOTECHNOLOGY
The Ammonia Transport, Retention and Futile Cycling Problem in Cyanobacteria Raymond J. Ritchie
Received: 28 June 2012 / Accepted: 10 August 2012 / Published online: 1 September 2012 # Springer Science+Business Media, LLC 2012
Abstract Ammonia is the preferred nitrogen source for many algae including the cyanobacterium Synechococcus elongatis (Synechococcus R-2; PCC 7942). Modelling ammonia uptake by cells is not straightforward because it exists in solution as NH3 and NH4+. NH3 is readily diffusible not only via the lipid bilayer but also through aquaporins and other more specific porins. On the other hand, NH 4 + requires cationic transporters to cross a membrane. Significant intracellular ammonia pools (≈1–10 molm−3) are essential for the synthesis of amino acids from ammonia. The most common model envisaged for how cells take up ammonia and use it as a nitrogen source is the “pump–leak model” where uptake occurs through a simple diffusion of NH3 or through an energy-driven NH4+ pump balancing a leak of NH3 out of the cell. The flaw in such models is that cells maintain intracellular pools of ammonia much higher than predicted by such models. With caution, [14C]-methylamine can be used as an analogue tracer for ammonia and has been used to test various models of ammonia transport and metabolism. In this study, simple “proton trapping” accumulation by the diffusion of uncharged CH3NH2 has been compared to systems where CH3NH3+ is taken up through channels, driven by the membrane potential (ΔUi,o) or the electrochemical potential for Na+ (ΔμNai,o+). No model can be reconciled with experimental data unless the permeability of CH3NH2 across the cell membrane is asymmetric: permeability into the cell is very high through gated porins, whereas permeability out of the cell is very low (≈40 nms−1) and independent of the extracellular pH. The best model is a Nain+/CH3NH3+in co-porter driven by ΔμNai,o+ balancing
R. J. Ritchie (*) Faculty of Technology & Environment, Prince of Songkla University—Phuket Campus, Kathu, Phuket 83120, Thailand e-mail: [email protected]
synthesis of methylglutamine and a slow leak governed by Ficks law, and so there is significant futile cycling of methylamine across the cell membrane to maintain intracellular methylamine pools high enough for fixation by glutamine synthetase. The modified pump–leak model with asymmetric permeability of the uncharged form is a viable model for understanding ammonia uptake and retention in plants, freeliving microbes and organisms in symbiotic relationships.
Abbreviations A An amine of general formula R–NH2 in the uncharged form BIS-TRIS- 1,3-Bis[tris(hydroxymethyl)methylamino] PROPANE propane CAPS 3-(Cyclohexylamino)-1-propanesulfonic acid HA+ An amine (R-NH2) in the charged form R–NH3+ MES 4-Morpholineethanesulphonic acid P A Permeability of an uncharged amine (R–NH2) P HA+ Permeability of an amine cation (R–NH3+) P NH3 Permeability of NH3 across cell membrane at a specified pH P MA Permeability of CH3NH2 across cell
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