Viagra

   This conceptual "mode of action" animation is of the drug Viagra developed by Pfizer pharmaceuticals.

   Viagra functions by inhibiting the enzyme phosphodiesterase-5 (PDE-5). Under normal conditions PDE-5 converts cyclic-GMP (cGMP) into GMP (shown as green and red spheres).

   Viagra selectively inhibits PDE-5 by blocking the site where cGMP binds. As a result, the cellular concentration of cGMP increases, causing an increase in blood flow to the corpus cavernosum.