Table 4.

Classification of transport system substrates based on biological significance

Category and substrate typeSubcategories
I. Inorganic moleculesA. Nonselective
B. Water
C. Cations
D. Anions
E. Others
II. Carbon compoundsA.  Sugars, polyols, and their derivatives
B. Monocarboxylates
C. Di- and tricarboxylates
D.  Organic anions (noncarboxylates)
E. Others
III. Amino acids and their derivativesA. Amino acids and conjugates
B. Amines, amides, and polyamines
C. Peptides
D. Other related organocations
E. Others
IV. Bases and their derivativesA. (Nucleo)bases
B. Nucleosides
C. Nucleotides
D. Other nucleobase derivatives
E. Others
V. Vitamins, cofactors, and their precursorsA.  Vitamins and vitamin or cofactor precursors
B. Enzyme and redox cofactors
C.  Siderophores; siderophore-Fe complexes
D. Signaling molecules
E. Others
VI. Drugs, dyes, sterols, and toxicsA. Multiple drugs and dyes
B. Specific drugs
C. Bile salts and conjugates
D. Sterols and conjugates
E. Others
VII. MacromoleculesA. Carbohydrates
B. Proteins
C. Nucleic acids
D. Lipids
E. Others
VIII. Miscellaneous compounds