If all active sites of an enzyme are occupied by substrate, the enzyme is in what state?

• A. Active
• B. Saturated
• C. Inhibited
• D. Unoccupied

Answer: (B)

In enzyme kinetics, saturation occurs when substrate concentration is so high that every active site on every enzyme molecule is bound. When all active sites are occupied, the enzyme is saturated with substrate, and the reaction runs at its maximum rate (Vmax) because no additional substrate can bind to increase the rate further. This is distinct from an enzyme that still has free (unoccupied) sites, which could respond to more substrate, and from an inhibited state where activity is reduced or blocked. The idea of saturation also ties to Km: at very high substrate levels, essentially all sites are occupied, regardless of Km, and you approach Vmax.