Multifaceted ammonia transporters

Figures & data

Table 1. Types of nitrogenous waste products.

#/#	Type of ammonia excretion	Main ammonia derivatives	Endproduct solubility in water	Endproduct toxicity	Examples
1	Ammonotelic	Ammonia and nitrites	highly soluble	highly toxic	Poriferans, cnidarians, crustaceans, platyhelminths, echinoderms, fishes, tadpoles of amphibians, some plants and microorganisms
2	Ureotelic	Urea	less soluble	less toxic	Mammals, some marine fishes and amphibians
3	Uricotelic	Uric acid	quasi-insoluble	low toxic	Birds, reptiles, and a majority of insects
4	Guanotelic	Guanine	insoluble	least toxic	Spiders, scorpions, mites

Figure 1. Conservativity of the molecular structure of ammonium transporters, based on the example of the trimeric forms of AmtB from Escherichia coli (A); Mep2 from Saccharomyces cerevisiae (B), and RhCG from Homo sapiens (C).The average radius of the trimeric systems ranges from 80 to 120 Å. The minimum radius of the ammonia-conducting pore is 0.6 Å, which is almost three times smaller than the average unhydrated ion radius for NH₄⁺(partly adapted from (Khademi et al. 2004; Zheng et al. 2004; Fischer et al. 2009; Gruswitz et al. 2010; van den Berg et al. 2016; Brito et al. 2020)).

Table 2. Taxonomic distribution of ammonia transporters.

#/#	Type of ammonia transporter	Ammonium affinity (Km, M)	Dominant transport processes	Kingdom	Domain
1	Amt	10^−6	quasi-electrogenic	Archaea	Prokaryota
2	Amt	10^−6	quasi-electrogenic	Bacteria
3	Amt, MEP1-3	10^−6–10^−3	electroneutral	Fungi	Eukaryota
4	Amt	?	unknown	Protozoa
5	Amt1,2	10^−6–10^−3	electrogenic, electroneutral	Plantae
6	Amt/Mep-like, Rh glycoproteins	10^−3	electroneutral	Animalia

Figure 2. A simplified diagram illustrating the general mechanisms underlying the functionality of Rh-like ammonia transporters in mammalian renal cells and in gill ionocytes.

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Data availability statement

Data sharing is not applicable to this article as this is a review and no cardinally new data were created or analyzed in this study.