How can a blocked metabolic pathway lead to toxic substance buildup?

A blocked metabolic pathway occurs when an enzyme—a protein responsible for catalyzing the conversion of one substrate into another—is missing or has diminished activity. This disruption leads to toxic substance buildup through two primary mechanisms:

• Substrate Accumulation: Because the pathway is blocked, the specific substrate that the dysfunctional enzyme would normally process begins to accumulate. When present in excess concentrations, these otherwise normal metabolites can become toxic to the body, particularly causing damage to the developing brain.12
• Activation of Alternative Pathways: When a primary substrate accumulates, minor metabolic pathways that are typically dormant may be activated. These alternative routes can produce atypical, potentially toxic substances that would not be formed under normal metabolic conditions.1

For example, in phenylketonuria, a defect in the enzyme phenylalanine hydroxylase leads to the extreme elevation of the amino acid phenylalanine, which is toxic to the brain. Similarly, in organic acidemias, the accumulation of organic acids in tissues and fluids can cause severe systemic complications.23