The Citrate-Mg2+:H+ (CitM) / Citrate-Ca2+:H+ (CitH) Symporter (CitMHS) Family ( TC# 2.A.11) is a family of transport proteins belonging to the Ion transporter superfamily. [1] Members of this family are found in Gram-positive and Gram-negative bacteria, archaea and possibly eukaryotes. These proteins all probably arose by an internal gene duplication event. Lensbouer & Doyle (2010) have reviewed these systems, classifying the porters with three superfamilies, according to ion-preference: [2]
1) Mg2+-preferring,
2) Ca2+-preferring, and
3) Fe2+-preferring.
A representative list of proteins belonging to the CitMHS family can be found in the Transporter Classification Database. [3]
Two of the characterized members of the CitMHS family, both citrate uptake permeases from Bacillus subtilis, are CitM ( TC# 2.A.11.1.1) and CitH ( TC# 2.A.11.1.2).
CitM is believed to transport a citrate2−-Mg2+complex in symport with one H+ per Mg2+-citrate while CitH apparently transports a citrate2−-Ca2+ complex in symport with protons. [4] [5] The cation specificity of CitM is: Mg2+, Mn2+, Ba2+, Ni2+, Co2+, Ca2+ and Zn2+, in this preferential order. CitM is highly specific for citrate and D-isocitrate and does not transport other di- and tri-carboxylates including succinate, L-isocitrate, cis-aconitate and tricarballylate. [6] [7] For CitH, the cation specificity (in order of preference) is: Ca2+, Ba2+ and Sr2+. [5] The two proteins are 60% identical, contain about 400 amino acyl residues and possess twelve putative transmembrane spanners. A CitM homologue in S. mutans transports citrate conjugated to Fe2+ or Mn2+ but not Ca2+, Mg2+ or Ni2+. [8]
The transport reactions catalyzed by (1) CitM and (2) CitH, respectively, are:
(1) Citrate • Mg (out) + nH+ (out) ⇌ Citrate • Mg (in) + nH+ (in)
(2) Citrate (out) + nH+ (out) ⇌ Citrate (in) + nH+ (in)
(3) Citrate • Ca2+ (out) + nH+ (out) ⇌ Citrate • Ca2+ (in) + nH+ (in)
As of 9 March 2016, this article is derived in whole or in part from Transporter Classification Database. The copyright holder has licensed the content in a manner that permits reuse under CC BY-SA 3.0 and GFDL. All relevant terms must be followed. The original text was at "The Citrate-Mg2+:H+ (CitM) Citrate-Ca2+:H+ (CitH) Symporter (CitMHS) Family"