Difference in Catalytic Loop Repositioning Leads to GMP Variation between Two Human GBP Homologues.

Interferon-gamma-inducible human large GTPases, hGBP1 and hGBP2, have a distinctive feature of hydrolyzing GTP to GDP and GMP through successive phosphate cleavages. In hGBP1, GMP is the major product, which is essential for its anti-pathogenic activities. However, its close homologue hGBP2 produces significantly less GMP, despite having a similar active site architecture. The molecular basis for less GMP formation and catalytic residue(s) in hGBP2 are not fully explored. To address these issues, we performed systematic biochemical, biophysical, and microsecond simulation studies. Our data suggest that the less GMP formation in hGBP2 is due to the lack of H-bond formation between the W79 side-chain (located near the active site) and main-chain carbonyl of K76 (present in the catalytic loop) in the substrate-bound hGBP2. The absence of this H-bond could not redirect the catalytic loop toward the beta phosphate after the cleavage of gamma-phosphate, a step essential for enhanced GMP formation. Furthermore, based on the mutational and structural analyses, this study for the first time indicates that the same residue, T75, mediates both phosphate cleavages in hGBP2 and hGBP1. This suggests the conservation of the catalytic residue in hGBP homologues. These findings emphasize the indispensable role of correct catalytic loop repositioning for efficient beta phosphate cleavage. This led us to propose a new substrate hydrolysis mechanism by hGBP1 and hGBP2, which may also be helpful to understand the GTP hydrolysis in other hGBP homologues. Overall, the study could provide insight into how these two close homologues play crucial roles in host-mediated immunity through different mechanisms.