Heart attacks and hard ons

Would you believe me if I told you that the enzyme that can prevent heart attacks is almost identical to the enzyme that gives men erections? So much so, that in 1985 during clinical trials for a drug designed to relieve high blood pressure, male patients displayed one particularly surprising side effect, and this drug went on to be one of the world’s highest grossing drugs as a cure for erectile dysfunction. That’s right – I’m talking about the little blue pill – Viagra.

Molecular structure of sildenafil


Viagra (or sildenafil) was designed to inhibit (block) an enzyme called phosphodiesterase (PDE for short). There are several strains of this enzyme, each located in different areas of the body, and this drug was designed to inhibit the PDE enzyme in the walls of blood vessels resulting in their vasodilation (dilation of vessels), lowering blood pressure, and reducing strain on the heart.

Instead, the drug targeted the PDE enzyme located in the smooth muscle of the penis (PDE-5). To get to grips with how the drug works, you first need to know what an erection is (chemically speaking). Erections arise due to the build up of the molecule called cGMP in the smooth muscle of the penis. PDE-5 catalyses the conversion of cGMP to GMP to terminate the erection. Men with erectile dysfunction have over active PDE-5 enzymes and Viagra competitively inhibits PDE-5 so that the erection can be maintained.

The chemistry of an erection

The chemistry of an erection. Instead of PDE-5 converting cGMP back to GMP, sildenafil occupies the enzyme’s active site so that it can no longer do its job. Guanylate cystase is the enzyme that can catalyse the conversion of GMP to cGMP in the presence of nitric oxide.

Interestingly, there is a phosphodiesterase enzyme in the retina of the eye (PDE-6), which sildenafil can also inhibit, and This is potentially why some men observe visual side effects after taking Viagra.

There are some difficult concepts in this post that non-chemists might not understand. Please let me know and I will try and explain if you don’t get it. Thanks for reading x


About ChemistDan

A second year PhD student based in Sheffield researching the reasons behind the molecular blinking phenomenon that allowed for the development of super-resolution optical microscopy.
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One Response to Heart attacks and hard ons

  1. Pingback: The study of Viagra Chemisty | Biochemmarie

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