An enzyme in glycolysis is downregulated by 60% in hepatocytes. Analyze and predict likely metabolic consequences at the cellular level, considering substrate accumulation and flux through alternative pathways.

• A. Downregulation only affects gene transcription of unrelated genes in the nucleus and therefore will not change metabolic flux or substrate accumulation in the cytoplasm.
• B. Downregulation of a glycolytic enzyme will immediately double ATP production because the cell will compensate by running glycolysis in reverse to produce more energy.
• C. A single enzyme downregulation has no metabolic effect because all other enzymes in the pathway will automatically increase expression within seconds, keeping flux constant.
• D. Reduced enzyme activity will slow the specific glycolytic step, causing upstream metabolite accumulation and redirecting flux into alternative pathways such as glycogen synthesis or the pentose phosphate pathway, and potentially reducing ATP yield and altering redox balance.

Answer: (D)