Identification of Hydroxypyrazine O-Methyltransferase Genes in Coffea arabica: A Potential Source of Methoxypyrazines That Cause Potato Taste Defect.

The goal of this study is to identify Coffea arabica O-methyltransferase (OMT) genes involved in the biosynthesis of methoxypyrazines. High levels of 2-isopropyl-3-methoxypyrazine (IPMP) and 2-isobutyl-3-methoxypyrazine (IBMP) in coffee beans are associated with the potato taste defect (PTD). Among the 34 putative O-methyltransferase genes identified in the published genome of C. canephora, three genes are highly homologous to known hydroxypyrazine OMT genes. Genes of interest were amplified and sequenced from genomic DNA of single C. arabica beans grown in eight different locations, including regions with endemic PTD. Although C. arabica OMT target sequences were almost identical regardless of source location, individual beans shared numerous polymorphisms in each of the target genes. Two of the predicted C. arabica OMT enzymes were successfully expressed in Escherichia coli and purified, and one enzyme shows slow yet measurable turnover of both 3-isobutyl-2-hydroxypyrazine (IBHP) and 3-isopropyl-2- hydroxypyrazine (IPHP), supporting a possible role of the coffee plant in PTD.