Cellular retinaldehyde binding protein-different binding modes and micro-solvation patterns for high-affinity 9-cis- and 11-cis-retinal substrates.

We use molecular dynamics (MD) simulations to determine the binding properties of different retinoid species to cellular retinaldehyde binding protein (CRALBP). The complexes formed by 9-cis-retinal or 11-cis-retinal bound to both the native protein and the R234W mutant, associated to Bothnia-retina dystrophy, are investigated. The presented studies are also complemented by analysis of the binding structures of the CRALBP/9-cis-retinol and CRALBP/9,13-dicis-retinal complexes. We find that the poor X-ray scattering properties of the polyene tail of the ligand in all wild-type complexes can be attributed to a high mobility of this region, which does not localize in a single binding conformation even at very low temperatures. Our simulations report a clear difference in the residual solvation pattern in CRALBP complexes with either 9-cis- or 9,13-dicis-retinal. The reported structures indicate that the microsolvation properties of the ligand are the key structural element triggering the very recently discovered isomerase activity of this protein. The binding geometries obtained by MD simulations are validated by calculation of the respective optical spectra by the ZINDO/S semiempirical method, which can reproduce with good qualitative agreement the different red-shifts of the first absorption band of the different complexes.