Molecular docking simulation analysis of alcohol acyltransferases from two related fruit species explains their different substrate selectivities

Abstract

Tropical papaya (Carica papaya) and mountain papaya (Vasconcellea pubescens) fruits are characterised for their strong and particular aroma. The aroma of both fruits is different and dominated by esters, which are synthesised by alcohol acyltransferases (AATs). The ability to produce esters is contrasting, V. pubescens (VpAAT1) being a very active enzyme towards the production of benzyl acetate, whereas C. papaya (CpAAT1) is more active towards the production of ethyl butanoate and methyl butanoate, but not benzyl acetate. In order to understand the mechanism of action at the molecular level, the structural model of CpAAT1 protein was built by comparative modelling. Conformational interaction between the protein and several ligands was carried out by molecular docking. CpAAT1 structure showed two domains connected by a large crossover loop, with a solvent channel in the centre of the structure. CpAAT1 and VpAAT1 proteins showed similar 3D structures, including their catalytic sites, but their solvent channels showed differences in size and shape. CpAAT1 solvent channel is larger, in agreement with its higher selectivity for large acyl-CoA substrates. In addition, the most favourably predicted substrate orientation in CpAAT1 was observed for methanol and butanoyl-CoA, showing a perfect coincidence with the high production rate of methyl butanoate of C. papaya fruit.