CitMHS family

The Citrate-Mg²⁺:H⁺ / Citrate-Ca²⁺:H⁺ Symporter Family is a family of transport proteins belonging to the Ion transporter superfamily. 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 have reviewed these systems, classifying the porters with three superfamilies, according to ion-preference:
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 .

CitM and CitH

Two of the characterized members of the CitMHS family, both citrate uptake permeases from Bacillus subtilis, are CitM and CitH ''.''

Function

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. 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. For CitH, the cation specificity is: Ca²⁺, Ba²⁺ and Sr²⁺. 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⁺.
The transport reactions catalyzed by CitM and CitH, respectively, are:
Citrate • Mg + nH⁺ ⇌ Citrate • Mg + nH⁺
Citrate + nH⁺ ⇌ Citrate + nH⁺
Citrate • Ca²⁺ + nH⁺ ⇌ Citrate • Ca²⁺ + nH⁺