2.A.62 The NhaD Na⁺:H⁺ Antiporter (NhaD) Family

The NhaD Na⁺/H⁺ antiporter has been characterized from two Vibrio species: V. parahaemolyticus and V. cholerae (Nozaki et al., 1998; Ostroumov et al., 2002) and in the haloalkaliphile, Alkalimonas amylolytica (Liu et al., 2005). These proteins and their homologues are 400-500 aas long and exhibit 10-13 TMSs. They catalyze Na⁺/H⁺ and Li⁺/H⁺ antiport. They exhibit activity at basic pH (8-10) with no activity at pH 7.5. The Amylolytica antiporter has low Na⁺ affinity and has optimal activity at 600 mM Na⁺. Homologues are found in proteobacteria of all groups, Flavobacteria and Chlamydia. Distant homologues of the IT superfamily are ubiquitous. The NhaD family is a constituent of the IT superfamily (Prakash et al., 2003).

The generalized reaction catalyzed by NhaD is:

nH⁺ (in) + mNa⁺ (out) ⇌ nH⁺ (out) + mNa⁺ (in).

(n and m are unknown, but are probably 1 or 2)