An accumulated intermediate harms the cell through one or more of three routes: chemical reactivity of its functional groups, osmotic and pH disturbance from its total solute load, and physical disruption of membranes. These routes are independent, so harm cannot be predicted from concentration alone.
Reactivity is a property of the functional group
Aldehydes form Schiff bases with protein lysines; thiols oxidize or exchange with other thiols; reducing sugars glycate proteins; conjugated systems undergo addition. A chemically quiet metabolite can be tolerated at high concentration, while a reactive one damages at low concentration. The decisive comparison is the collision rate with targets versus the removal rate.
Osmotic and pH effects scale with load, not identity
Osmolarity is the sum of all solute particles, so a large backlog draws water into the compartment. An acidic species releases protons on dissociation and lowers pH; a weak base raises it. Because these effects depend on total solute and proton release, they add across every species that accumulates behind the block, not only the most abundant one.
Membrane disruption is structural, not chemical
Amphipathic or detergent-like intermediates insert into lipid bilayers and change packing, permeability, and the activity of embedded proteins. No chemical reaction is required; the molecule only has to occupy the wrong environment.
Three routes compared
Chemical reactivity
- Depends on the functional group
- Damage at low concentration if the group is reactive
- Targets proteins, nucleic acids, other metabolites
Osmotic and pH
- Depends on total solute and proton release
- Additive across all accumulated species
- Affects compartment volume and enzyme activity
Membrane disruption
- Depends on amphipathic structure
- No reaction needed
- Alters bilayer packing and protein function
A single intermediate can act through more than one route at once. Predicting harm therefore requires three separate questions: which property does the molecule have, how concentrated does it become, and how long must the cell tolerate it?