Why can a defect in one enzyme cause a metabolic disease?

A metabolic disease can arise from a single enzyme defect because cellular metabolism relies on highly organized, sequential metabolic pathways. In these pathways, enzymes act as catalysts, converting substrates into specific products that then serve as the substrate for the next step in the sequence.12

When an enzyme is missing or has diminished activity, the pathway becomes blocked, leading to several pathological consequences:

• Deficiency of essential products: If a final product required for a vital function is not formed, disease results.12
• Toxic accumulation: The substrate of the dysfunctional enzyme may accumulate to toxic levels.1
• Activation of atypical pathways: Substrate buildup can trigger minor, normally dormant metabolic pathways, potentially forming atypical and toxic products.1

Because thousands of metabolic pathways in the body are interlinked, a single blockage can disrupt numerous biochemical processes, leading to severe outcomes such as organ failure, seizures, or intellectual disability, depending on the specific enzyme involved. These conditions are often referred to as inborn errors of metabolism.1