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Function

PDE5 is an phosphodiesterase : this enzyme catalyzes the hydrolysis of cGMP into 5'GMP. CGMP is an usual second messager in cell transduction. PDE5 regulates its concentration by hydrolysis. Human PDE5 is a PDE of class I. PDE5 function affects the smooth muscle, blood vessels or penis, uterus and intestines.

Structural highlights

PDE5 is a homodimeric protein of 875 amino acids long.

This protein is composed of several important domains:

• • pol-GLY (10-24 amino acids) in Ntermini (structural importance).
  • GAFA (164-314 amino acids)
  • GAFB (346-503 amino acids)
  • Catalytic domain (588-853 amino acids) in Ctermini involved in cGMP hydrolysis.

The full structure of PDE5 has not been crystallized contrary to this of isolated domains of the protein. GAFA and GAFB are homologous domains and allosteric binding site of cGMP. The of the catalytic domain allows to see that this domain is mostly constituted by alpha helix and turns.

Catalytic activity

PDE5 is a phosphodiesterase. cGMP could bind the catalytic domain thanks to hydrogen bonds made with Gln775 and Gln817 and coordination bonds made with Zn2+ and Mg2+ in the catalytic site. To see these interactions, report to Sildenafil in Inhibitors and medical application. cGMP is hydrolyzed in GMP. The catalytic chemical reaction deals with breaking a phosphodiester bound in cGMP between the 3'O of the guanoside and the phosphate of cGMP: cGMP + H20 => GMP.

The catalytic domain is able to bind ligands thanks to an (hydrophylic domain. Moreover with this it appears the cGMP binding site in the catalytic domain is more conservedthan the the rest of the domain that could be variable.

Some residues could be affected by post traductional modifications :

phosphorylation : S60, S86, S92, S102, S104, S108, T111, T127, T137, S869. acylation : K364. ubiquitinylation : K714. The ubiquitinylation of (in fushia) and the phosphorylation of (in yellow) are present in the catalytic site and the phosphorylation of S819 is involved in cGMP binding.

Inhibitors and medical application

Inhibitors of PDE5 prevent cGMP from binding the catalytic site by competititve inhibition. They have more affinity for this domain than cGMP. Sildenafil (active substance of Viagra) binds to PDE5 catalytic domain through:

• • hydrogen bonds made between Gln775 and Gln817 of PDE5 (green) and (red)
  • Mg2+: coordination bond made with in catalytic site
  • Zn2+: coordination bond made with in catalytic site

Allosteric activation

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