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 |
| 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).