A drug binds only to the enzyme-substrate (ES) complex and not to free enzyme. On a Lineweaver-Burk plot, this produces parallel lines with both V_max and K_m decreased proportionally. What type of inhibition is this?

Explanation

## Uncompetitive Inhibition **Key Point:** Uncompetitive inhibitors bind **exclusively to the ES complex** and cannot bind to free enzyme. This is a unique mechanism where the inhibitor has no affinity for the uncomplexed enzyme. ### Defining Characteristics | Feature | Uncompetitive | |---------|---------------| | Binding site | ES complex only (not free enzyme) | | V_max | Decreased | | K_m | Decreased by same factor as V_max | | V_max/K_m ratio | Unchanged | | Lineweaver-Burk plot | Parallel lines (slope unchanged) | | Substrate can overcome? | No | **High-Yield:** The **parallel lines on Lineweaver-Burk plot** distinguish uncompetitive from competitive inhibition. Both produce parallel lines, but uncompetitive has **both parameters decreased equally**, while competitive has **only K_m increased**. ### Mechanism 1. Enzyme binds substrate normally → ES complex forms 2. Inhibitor binds to ES complex → ESI complex (inactive) 3. Free enzyme and substrate concentration unaffected 4. Only the ES complex is sequestered **Clinical Pearl:** Cephalosporins and penicillins act as uncompetitive inhibitors of bacterial transpeptidase by binding to the acyl-enzyme intermediate (analogous to ES complex). **Mnemonic:** **UE** — **U**ncompetitive: binds only to **E**S complex, **E**qual decrease in both V_max and K_m.