Calcium-associated Pro367 ring puckering supports conformational selection in a thermophilic lipase

Thermophilic bacterial lipases must combine structural stability with the conformational flexibility required for interfacial activation, yet the local organization of their metal-binding sites remains incompletely understood. Here, we determined two X-ray crystal structures of the family I.5 lipase from Geobacillus kaustophilus (GkLip). The 2.47-[A] orthorhombic model contains a Ca2+ ion at the conserved metal-binding site, whereas the 2.30-[A] anisotropic triclinic model contains four non-equivalent protein chains in which no Ca2+ ion was modeled. GkLip retains the canonical /{beta}-hydrolase fold, the Ser114-Asp318-His359 catalytic triad, the Phe17/Gln115 oxyanion-hole region, an extended lid, and a conserved Zn2+-binding site. In the orthorhombic structure, Gly287, Glu361, Asp366, and Pro367 contribute to the Ca2+ coordination environment. Pro367 adopts a C{gamma}-endo pucker in the Ca2+-containing structure; three Ca2+-free chains also adopt the endo pucker, whereas one adopts a C{gamma}-exo pucker. The presence of both Pro367 pucker states in the Ca2+-free structure is consistent with calcium preferentially stabilizing a pre-existing, coordination-compatible conformation. This local organization is further supported by an Arg35-centered network of direct and water-mediated contacts. Comparison with the 94%-identical open G. thermocatenulatus lipase reveals substantial lid rearrangement despite strong conservation of the protein core. Together, these structures highlight localized conformational variability at the calcium-binding region within an otherwise highly conserved thermophilic lipase scaffold.