Biochemical characterisation of MdCXE1, a carboxylesterase from apple that is expressed during fruit ripening

• Published in: Phytochemistry (Oxford) 2011-05, Vol.72 (7), p.564-571
• Main Authors: Souleyre, Edwige J.F., Marshall, Sean D.G., Oakeshott, John G., Russell, Robyn J., Plummer, Kim M., Newcomb, Richard D.
• Format: Article
• Language: English
• Subjects: Apple; apples; Biological and medical sciences; biosynthesis; carbon; Carboxylesterase; Carboxylesterase - genetics; Carboxylesterase - metabolism; Chemical constitution; cultivars; Esters; ethylene; flavor; Fruit - enzymology; Fruit - genetics; Fruit - physiology; fruiting; fruits; Fundamental and applied biological sciences. Psychology; gene expression; Gene Expression Regulation, Plant; hormones; Kinetics; Malus - classification; Malus - enzymology; Malus - genetics; Malus - physiology; Malus × domestica; multigene family; organophosphorus compounds; Phylogeny; Plant physiology and development; ripening
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2011.01.020

The carboxylesterase from apple, MdCXE1, reaches a peak of expression at harvest maturity. We demonstrate its ability to hydrolyse esters and discuss its potential impact on apple flavour. [Display omitted] ► Apple contains at least 16 carboxylesterases. ► MdCXE1 is differentially regulated in apple fruit during ripening. ► Recombinant MdCXE1 is inhibited by organophosphates. ► MdCXE1 can hydrolyse esters, suggesting a role of flavour metabolism. Esters are an important component of apple ( Malus × domestica) flavour. Their biosynthesis increases in response to the ripening hormone ethylene, but their metabolism by carboxylesterases (CXEs) is poorly understood. We have identified 16 members of the CXE multigene family from the commercial apple cultivar, ‘Royal Gala’, that contain all the conserved features associated with CXE members of the α/β hydrolase fold superfamily. The expression of two genes, MdCXE1 and MdCXE16 was characterised in an apple fruit development series and in a transgenic line of ‘Royal Gala’ (AO3) that is unable to synthesise ethylene in fruit. In wild-type MdCXE1 is expressed at low levels during early stages of fruit development, rising to a peak of expression in apple fruit at harvest maturity. It is not significantly up-regulated by ethylene in the skin of AO3 fruit. MdCXE16 is expressed constitutively in wild-type throughout fruit development, and is up-regulated by ethylene in skin of AO3 fruit. Semi-purified recombinant MdCXE1 was able to hydrolyse a range of 4-methyl umbelliferyl ester substrates that included those containing acyl moieties that are found in esters produced by apple fruit. Kinetic characterisation of MdCXE1 revealed that the enzyme could be inhibited by organophosphates and that its ability to hydrolyse esters showed increasing affinity ( K m) but decreasing turnover ( k cat) as substrate acyl carbon length increases from C2 to C16. Our results suggest that MdCXE1 may have an impact on apple flavour through its ability to hydrolyse relevant flavour esters in ripe apple fruit.