Multiple conformations of phosphodiesterase-5: implications for enzyme function and drug development

Area of Science:

• Biochemistry
• Structural Biology
• Pharmacology

Background:

• Phosphodiesterase-5 (PDE5) is a key therapeutic target for erectile dysfunction and pulmonary hypertension.
• Sildenafil, vardenafil, and tadalafil are established PDE5 inhibitor drugs.
• Understanding PDE5 structure-activity relationships is vital for drug development.

Purpose of the Study:

• To elucidate the structural basis of PDE5 inhibition by sildenafil and icarisid II.
• To characterize the dynamic behavior of the PDE5 active site, particularly the H-loop.
• To investigate the role of the H-loop and adjacent residues in substrate binding and catalysis.

Main Methods:

• X-ray crystallography was used to determine the structures of unliganded PDE5A1 and its complexes with inhibitors.
• Mutagenesis studies were performed to assess the functional importance of specific residues.
• Kinetic analyses were conducted to evaluate substrate affinity and catalytic activity.

Main Results:

• Crystal structures revealed four distinct conformations of the PDE5A1 H-loop (residues 660-683), which undergoes significant migration upon inhibitor binding.
• The conformation of sildenafil in the PDE5A1 complex differs from previously reported structures.
• Mutagenesis and kinetic data confirmed the H-loop's critical role in substrate recognition and Gly(659)'s importance for affinity and activity.

Conclusions:

• The dynamic H-loop is a key determinant of substrate specificity and inhibitor binding in PDE5.
• Structural plasticity of the H-loop is essential for the efficacy of PDE5 inhibitors.
• Invariant Gly(659) plays a crucial role in optimizing PDE5 substrate binding and catalytic function.