TABLE 4.
Classification of transport system substrates based on biological significance
| Category and substrate type | Subcategories |
|---|---|
| I. Inorganic molecules | A. Nonselective |
| B. Water | |
| C. Cations | |
| D. Anions | |
| E. Others | |
| II. Carbon compounds | A. Sugars, polyols, and their derivatives |
| B. Monocarboxylates | |
| C. Di- and tricarboxylates | |
| D. Organic anions (noncarboxylates) | |
| E. Others | |
| III. Amino acids and their derivatives | A. Amino acids and conjugates |
| B. Amines, amides, and polyamines | |
| C. Peptides | |
| D. Other related organocations | |
| E. Others | |
| IV. Bases and their derivatives | A. (Nucleo)bases |
| B. Nucleosides | |
| C. Nucleotides | |
| D. Other nucleobase derivatives | |
| E. Others | |
| V. Vitamins, cofactors, and their precursors | A. 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 toxics | A. Multiple drugs and dyes |
| B. Specific drugs | |
| C. Bile salts and conjugates | |
| D. Sterols and conjugates | |
| E. Others | |
| VII. Macromolecules | A. Carbohydrates |
| B. Proteins | |
| C. Nucleic acids | |
| D. Lipids | |
| E. Others | |
| VIII. Miscellaneous compounds |