Theoretical Model of the Protochlorophyllide Oxidoreductase from a Hierarchy of Protocols

The enzyme protochlorophyllide oxidoreductase (LPOR) catalyzes the light-driven reduction of protochlorophyllide (Pchlide), a crucial step in chlorophyll biosynthesis. Molecular understanding of the photocatalytic mechanism of LPOR is essential for harnessing light energy to mediate enzymatic reactions. The absence of X-ray crystal structure has promoted the development of LPOR homology models that lack a catalytically competent active site and could not explain the variously reported spectroscopic evidence, including time-resolved optical spectroscopy data. We have refined previous structural models to account for the catalytic active site and the characteristic experimental spectral features of Pchlide binding, including the 26 cm^-1 red shift of the C₁₃₍₁₎ carbonyl stretch vibration in the mid-infrared (IR) and the 12 nm red shift of the Q _x electronic band. A hierarchy of theoretical methods, including homology modeling, molecular dynamics simulations, hybrid quantum mechanics [(TD-)DFT]/molecular mechanics [AMBER] calculations, and computational vibrational and electronic spectroscopies, have been combined in an iterative protocol to reproduce experimental evidence and to predict ultrafast transient IR spectroscopic fingerprints associated with the catalytic process. The successful application to the LPOR enzyme indicates that the presented hierarchical protocol provides a general workflow to protein structure refinement.