In metabolic pathways, what can limit flux?

• A. Membrane thickness
• B. Enzyme capacity
• C. Oxygen concentration
• D. DNA sequence

Answer: (B)

The rate at which a metabolic pathway runs is set by how fast the enzymes can catalyze each step. This is determined by enzyme capacity—the amount of enzyme available and how effectively it converts substrate to product. When enzyme capacity is limited, the overall throughput drops because the pathway can’t push more substrate through each step, even if other steps could proceed faster. In other words, the slowest, least-capable part of the pathway governs the whole flux, making enzyme capacity the decisive factor.

Oxygen concentration can influence flux in pathways that rely on it as the final electron acceptor, so it matters in those contexts but isn’t a universal limiter. DNA sequence shapes which enzymes can be produced, affecting potential capacity, but flux at any moment depends on actual enzyme activity and availability rather than sequence alone. Membrane thickness doesn’t directly set the speed of the catalytic reactions in a pathway.