Glycosyltransferases in grapevine secondary metabolism: Understanding and modifying their activity and selectivity

Summary

Background

The Australian wine industry currently generates ~$6 billion in revenue per annum and contributes ~$46 billion to the national economy each year. However, the industry seeks to grow revenue to $15 billion/year and to contribute $100 billion to the economy by 2050. To remain successful and achieve this ambitious goal, industry will not only need to increase production volume, but enhance wine quality, since a more profitable industry will result arise from more sought-after, high value wines.

Premium quality wines are recognised by their pronounced sensory profiles, and the absence of faults or taint. An array of secondary metabolites are biosynthesised in grapes, often in glycoconjugate forms (i.e., with one or more sugar moieties attached), which impart pleasant fruit aromas and flavours to wine. During winemaking, glycosylated aroma precursors can be hydrolysed by yeast and/or enzymes, releasing volatiles that contribute desirable sensory characteristics. However, grapes can also take up volatiles from their environment (e.g., smoke-derived volatile phenols), which can also accumulate as glycoconjugates.

Since 2003, the Australian wine industry is estimated to have incurred financial losses in excess of $1 billion. Despite significant progress towards understanding the chemical and sensory consequences of vineyard exposure to smoke, the biochemical response of grapevines to smoke has not been comprehensively studied.

Indeed, few studies have investigated the enzymes (glucosyltransferases) responsible for the accumulation of endogenous or exogenous metabolites by grapes in glucoside, disaccharide or even trisaccharide forms, or the variation observed in glycoconjugate profiles between leaves and fruit. Given glycoconjugates impart varied sensory outcomes (e.g., more highly glycosylated volatile phenols have little sensory impact because they are not readily hydrolysed in-mouth), there may be opportunities to favourably influence wine sensory qualities by better understanding the role and regulation of glycosyltransferase enzymes. For example, by enhancing the accumulation of secondary metabolites that positively impact wine aroma as readily hydrolysable glucosides and/or facilitating the accumulation of metabolites that negatively impact wine aroma as glycoconjugates that are less susceptible to hydrolysis during either fermentation or consumption.

The structure-function relationships of glycosyltransferase enzymes responsible for disaccharide or trisaccharide formation are not well established. Understanding their amino acid composition and associated activity are important to the development of practices that can modify their function, i.e., to enable moderation of grape secondary metabolites.

Research approach

In this study, glucosyltransferase enzymes will be isolated from different grapevine tissues, at different phenological stages, to investigate the factors that affect gene expression activity. Recombined glucosyltransferases will be cloned into expression vectors, and heterologous proteins expressed to allow identification by LC-MS/MS. The AlphaFold algorithm will be used to predict 3D protein structure from amino acid sequence, and understand enzyme function. Enzyme mutation technology will then be applied to change substrate selectivity.

Sector benefits

The research aims to provide the sector with an improved understanding of grapevine secondary metabolism, and developing strategies/practices that can be implemented in the vineyard to improve grape, and therefore wine composition.