Requirement, followed by mechanistic reason for “activity”
Excellent aqueous solubility: Provides for rapid movement/transport to the cell via diffusion and mass flow.
Structural simplicity: Direct utilization in major catabolic pathways (such as the citric acid cycle)
Small molecular size (<600 Da): Small compounds do not require time- and resource-intensive extracellular depolymerization.
Compound mix: ’empowers’ oligotrophic organisms that would be idle in a single substrate situation.
Carbon oxidation state near zero: Such substrates are best in satifying both biosynthetic and energetic needs at the same time.
No less than 4 C-atoms: Organic acids containing only 2& 3 C-atoms cannot be used as growth substrates by the citric acid cycle alone, while 4-6 C acids can.
Anomeric C in sugars: Reducing sugars are more likely to become oxidized than nonreducing sugars.
(Content extracted from image of slide by Soil scientist Markus Kleber. Reprinted with permission in blog post)