2.A.35 The NhaC Na+:H+ Antiporter (NhaC) Family
Two members of the NhaC family have been functionally characterized. One is believed to be a Na+:H+ antiporter (Ito et al., 1997); the other is a malate · H+:lactate · Na+ antiporter (Wei et al., 2000). Several paralogues are found in Vibrio cholerae, and two paralogues each are found encoded in the completely sequenced genomes of Haemophilus influenzae and Bacillus subtilis. E. coli lacks such a homologue. Pyrococcus species also have at least one homologue each. Thus, members of the NhaC family are found in both Gram-negative bacteria and Gram-positive bacteria as well as archaea. NhaC of B. firmus is 462 amino acyl residues long and possesses 12 putative transmembrane α-helical segments. MleN of B. subtilis (468 aas) also exhibits 12 putative TMSs.
The transport reaction catalyzed by NhaC is probably:
Na+ (in) + nH+ (out) ⇌ Na+ (out) + nH+ (in). (n > 1)
That catalyzed by MleN is probably:
Malate (out) + H+ (out) + Lactate (in) + Na+ (in) ⇌ Malate (in) + H+ (in) + Lactate (out) + Na+ (out)