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)