Modelling of the Citrus CCD4 Family Members: In Silico Analysis of Membrane Binding and Substrate Preference

Coloring is one of the most important characteristics in commercial flowers and fruits, generally due to the accumulation of carotenoid pigments. Enzymes of the CCD4 family in citrus intervene in the generation of β-citraurin, an apocarotenoid responsible for the reddish-orange color of mandarins. Citrus CCD4s enzymes could be capable of interacting with the thylakoid membrane inside chloroplasts. However, to date, this interaction has not been studied in detail. In this work, we present three new complete models of the CCD4 family members (CCD4a, CCD4b, and CCD4c), modeled with a lipid membrane. To identify the preference for substrates, typical carotenoids were inserted in the active site of the receptors and the protein-ligand interaction energy was evaluated. The results show a clear preference of CCD4s for xanthophylls over aliphatic carotenes. Our findings indicate the ability to penetrate the membrane and maintain a stable interaction through the N-terminal α-helical domain, spanning a contact surface of 2250 to 3250 Ų. The orientation and depth of penetration at the membrane surface suggest that CCD4s have the ability to extract carotenoids directly from the membrane through a tunnel consisting mainly of hydrophobic residues that extends up to the catalytic center of the enzyme.