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)
NhaD Na+(Li+)/H+ antiporter (Liu et al., 2005). AaNhaD is from the soda lake alkaliphile, Alkalimonas amylolytica, is crucial for the bacterium's resistance to salt/alkali stresses (Zhong et al. 2012).
NhaD of Alkalimonas amylolytica (AAX63482)
Chloroplast Na+/H+ antiporter of 582 aas, NhaD (Barrero-Gil et al. 2007). Mediates ionic homeostasis in chloroplasts from red algae to flowering plants ((Barrero-Gil et al. 2007).
NhaD of Physcomitrella patens (Moss)
Na+/H+ antiporter, NhaD of 495 aas; involved in the response to salt stress and adaptation to a marine environment (Kurz et al. 2006).
NhaD of the halophilic eubacterium, Halomonas elongata
Sodium/proton antiporter, NhaD1, of 488 aas and 14 TMSs. Two NhaD homologues that catalyze Na+/H+ antiport were identified from this halotolerant and alkaliphilic strain isolated from sodium enriched black liquor. They exhibit 72% identity and have similar binding affinities for Na+ and Li+, while having different pH profiles. Ha-NhaD1 was active at pH 6.0 and most active at pH 8.0-8.5, whereas Ha-NhaD2 lacked activity at pH 6.0 but exhibited maximal activity at pH 9.5 or higher. Residues involved in transport were identified (Cui et al. 2016).
NhaD1 of Halomonas sp. Y2