TABLE 5.

Distribution of transporter families based on substrate specificity

Substrate	No. of families of indicated type using substratea
	Channels	Porins	Primary carriers	Secondary carriers	Total
I. Inorganic molecules	32	13	16	44	105
A. Nonselective	13	9	0	0	22
B. Water-selective	2	0	0	0	2
C. Cations	14	2	15	30	61
D. Anions	4	4	3	17	28
II. Carbon sources	2	3	2	26	33
A. Sugars and polyols	1	2	2	4	9
B. Monocarboxylates	1	1	0	13	15
C. Di- and tricarboxylates	0	0	1	12	13
D. Organoanions (noncarboxylic)	0	1	1	2	4
III. Amino acids and their derivatives	3	1	2	20	26
A. Amino acids and conjugates	0	0	1	18	20
B. Amines, amides, and polyamines	3	1	2	8	14
C. Peptides	0	0	1	4	5
IV. Bases and their derivatives	0	2	0	10	12
A. Nucleobases	0	0	0	2	2
B. Nucleosides	0	1	0	6	7
C. Nucleotides	0	1	0	2	3
V. Vitamins, cofactors, and cofactor precursors	0	2	1	11	14
A. Vitamins and vitamin or cofactor precursors	0	1	1	7	9
B. Enzyme and redox cofactors	0	1	1	4	6
C. Siderophores; siderophore-Fe complexes	0	2	1	1	4
D. Signaling molecules	0	0	1	3	4
VI. Drugs, dyes, sterols, and toxics	0	1	1	6	8
A. Multiple drugs	0	1	1	5	7
B. Specific drugs	0	0	1	2	3
C. Bile salts and conjugates	0	0	1	3	4
D. Sterols and conjugates	0	0	0	3	3
VII. Macromolecules	5	9	8	4	26
A. Carbohydrates	0	3	1	2	6
B. Proteins	5	6	6	1	18
C. Nucleic acids	0	1	2	0	3
D. Lipids	0	0	2	1	3

^aTotals 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.