Sildenafil citrate (colored molecule) is an inhibitor bound to the enzyme cGMP phosphodiesterase - Type 5 [EC 3.1.4.17], on the purple stick figure], and it inhibits its activity preventing the enzyme from hydrolyzing cGMP to  5'-GMP.

     Viagra is a potent  competitive inhibitor, via competitive binding, of this PDE-Type 5 enzyme which results in continued vasodilation of smooth muscle resulting in increased blood flow.  
Viagra bound to cGMP phosphodiesterase inhibiting it
     The enzyme guanylate cyclase [EC 4.6.1.2] in response to increase nitric oxide (NO) converts GTP into cGMP, which leads to smooth muscle relaxation (vasodilation) of the internal cushions of the helicine arteries resulting in creased blood flow. The action of Viagra prevents the degradation of this cGMP keeping the increased blood flow rates high.
   Viagra by Pfizer