Table 2.

Clusters of soluble SBPs

Cluster	Subcluster	Types of ligands	Main feature
A	I	Metal ions	Single rigid α helix connects the two domains
	II	Siderophores
B		Carbohydrates, Leu, Ile, Val, autoinducer-2	Three interconnecting segments between the two domains
C		Di- and oligopeptides, Arg, cellubiose, nickel	An additional domain. These SBPs are significantly larger.
D	I	Carbohydrates	Two relative short hinges between the two domains
	II	Putrescine, thiamine
	III	Tetrahedral oxyanions
E		Sialic acid, 2-keto acids, ectoine, pyroglutamic acid	Large flexible helix in between the two domains. Only associated with TRAP transporters.
F	I	Trigonal planar anions, unknown ligands	Two hinges in between the two domains as in cluster D, but these hinges are almost twice as long, giving the SBP more flexibility
	II	Methionine
	III	Compatible solutes
	IV	Amino acids

Data taken from Berntsson et al. (2010).