Table 5.

Distribution of transporter families based on substrate specificity

SubstrateNo. of families of indicated type using substratea
ChannelsPorinsPrimary carriersSecondary carriersTotal
I. Inorganic molecules32131644105
 A. Nonselective1390022
 B. Water-selective20002
 C. Cations142153061
 D. Anions4431728
II. Carbon sources2322633
 A. Sugars and polyols12249
 B. Monocarboxylates1101315
 C. Di- and tricarboxylates0011213
 D. Organoanions (noncarboxylic)01124
III. Amino acids and their derivatives3122026
 A. Amino acids and conjugates0011820
 B. Amines, amides, and polyamines312814
 C. Peptides00145
IV. Bases and their derivatives0201012
 A. Nucleobases00022
 B. Nucleosides01067
 C. Nucleotides01023
V. Vitamins, cofactors, and cofactor precursors0211114
 A. Vitamins and vitamin or cofactor precursors01179
 B. Enzyme and redox cofactors01146
 C. Siderophores; siderophore-Fe complexes02114
 D. Signaling molecules00134
VI. Drugs, dyes, sterols, and toxics01168
 A. Multiple drugs01157
 B. Specific drugs00123
 C. Bile salts and conjugates00134
 D. Sterols and conjugates00033
VII. Macromolecules598426
 A. Carbohydrates03126
 B. Proteins566118
 C. Nucleic acids01203
 D. Lipids00213
  • a Totals are boldfaced. Because a family can include members that transport different substrates, it may be found in more than one subcategory. Hence, the number of families in each category is less than or equal to the sum of the numbers of families in the corresponding subcategories.