Sildenafil citrate (colored molecule) is bound to the enzyme cGMP phosphodiesterase - Type 5 [EC 3.1.4.17], on the purple stick figure, 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 that results in continued vasodilation of smooth muscle resulting in increased blood flow.  
Viagra bound to PDE type 5
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 by cGMP phosphodiesterase keeping the increased blood flow rates high.
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