Add time:07/23/2019         Source:sciencedirect.com

Moulds are able to cause spoilage in preserved foods through degradation of the preservatives using the Pad-decarboxylation system. This causes, for example, decarboxylation of the preservative sorbic acid to 1,3-pentadiene, a volatile compound with a kerosene-like odour. Neither the natural role of this system nor the range of potential substrates has yet been reported. The Pad-decarboxylation system, encoded by a gene cluster in germinating spores of the mould Aspergillus niger, involves activity by two decarboxylases, PadA1 and OhbA1, and a regulator, SdrA, acting pleiotropically on sorbic acid and cinnamic acid. The structural features of compounds important for the induction of Pad-decarboxylation at both transcriptional and functionality levels were investigated by rtPCR and GCMS. Sorbic and cinnamic acids served as transcriptional inducers but ferulic, coumaric and hexanoic acids did not. 2,3,4,5,6-Pentafluorocinnamic acid was a substrate for the enzyme but had no inducer function; it was used to distinguish induction and competence for decarboxylation in combination with the analogue chemicals. The structural requirements for the substrates of the Pad-decarboxylation system were probed using a variety of sorbic and cinnamic acid analogues. High decarboxylation activity, ~ 100% conversion of 1 mM substrates, required a mono-carboxylic acid with an alkenyl double bond in the trans (E)-configuration at position C2, further unsaturation at C4, and an overall molecular length between 6.5 Å and 9 Å. Polar groups on the phenyl ring of cinnamic acid abolished activity (no conversion). Furthermore, several compounds were shown to block Pad-decarboxylation. These compounds, primarily aldehyde analogues of active substrates, may serve to reduce food spoilage by moulds such as A. niger. The possible ecological role of Pad-decarboxylation of spore self-inhibitors is unlikely and the most probable role for Pad-decarboxylation is to remove cinnamic acid-type inhibitors from plant material and allow uninhibited germination and growth of mould spores.

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