Role of hydrogen bond network in the function, stability, and structure of ketosteroid isomerase from Pseudomonas putida

H-bond networks that are formed by polar or charged residues play a critical role in the structure, catalytic activity and stability of proteins. Ketosteroid isomerase (KSI) catalyzes the allylic isomerization of a variety of Δ5-3-ketosteroids by the intramolecular transfer of a proton from the 4β-position to the 6β-position via a dienolate intermediate. An H-bond network in KSI consists of two catalytic residues (Tyr14 and Asp99), Tyr30, Tyr55, and a water molecule in the highly hydrophobic active site. In this review, we summarize our current understanding of the role of the H-bond network in the activity, stability, and structure of KSI. Both mutational and structural studies suggest that the H-bond newtork could contribute to the catalysis and stability of KSI through the maintenance of the active-site geometry and the provision of structural support.