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Abstract
Tissue cultures of the vanilla orchid, Vanilla planifolia, produce the flavor compound
vanillin (4-hydroxy-3-methoxybenzaldehyde) and vanillin precursors such as 4-hydroxybenzaldehyde.
A constitutively expressed enzyme activity catalyzing chain shortening of a hydroxycinnamic
acid, believed to be the first reaction specific for formation of vanilla flavor compounds,
was identified in these cultures. The enzyme converts 4-coumaric acid non-oxidatively
to 4-hydroxybenzaldehyde in the presence of a thiol reagent but with no co-factor
requirement. Several forms of this 4-hydroxybenzaldehyde synthase (4HBS) were resolved
and partially purified by a combination of hydrophobic interaction, ion exchange and
gel filtration chromatography. These forms appear to be interconvertible. The unusual
properties of the 4HBS, and its appearance in different protein fractions, raise questions
as to its physiological role in vanillin biosynthesis in vivo.