What primarily determines which metabolic pathways can occur in a cell?

• A. The DNA sequence
• B. The enzymes present
• C. The amount of water
• D. The temperature

Answer: (B)

Metabolic pathways that a cell can perform are determined by the enzymes that are present to catalyze the reactions. Each step in a pathway requires a specific enzyme, and without that enzyme the reaction cannot proceed. While the DNA sequence provides the blueprint for which enzymes could be made, the actual set of pathways depends on which enzymes are expressed and active in the cell at a given time. Gene regulation, environmental signals, and post-translational modifications shape which enzymes are available. Water availability and temperature influence how fast reactions occur and how well enzymes function, but they don’t define which pathways can exist in the cell. So, the presence and activity of the enzymes essentially decide which metabolic routes can operate.