Environmental stress induces plants to increase their tannin concentration, leading to increased release of tannin into soils from leaching and leaf litter. These changes can in turn alter the soil biology at the level of the microbiome, the invertebrate populations, and the plant systems. Condensed tannin degradation by the soil microbiome, which is critical to the carbon cycle and to soil and plant ecology, is vastly unknown. Here, we are investigating the possible degradation pathway of condensed tannin by the enzyme manganese peroxidase (MnP), which is released into the soil by fungi and microbes that decompose plant matter. MnP catalyzes the oxidation of Mn2+ to Mn3+, which is then stabilized by dicarboxylic acids such as oxalate. The metal-oxalate complex is an oxidizing agent for a variety of phenolic substrates related to lignin, leading to identification of MnP as a key enzyme lignin depolymerization. We proposed to explore the possible role of MnP in decomposition of the polymeric polyphenols known as condensed tannins by examining the monomers, (+)-catechin and (-)-epicatechin. We will establish whether the Mn3+-oxalate complex acts on the flavan-3-ols to establish whether MnP could play a role in tannin decomposition. Stereochemical differences between (+)-catechin and (-)-epicatechin will give us insights into how structural features of the tannin polymer could impact its oxidation by MnP. We have monitored the products of hydrogen peroxide-dependent MnP oxidation of the two substrates using HPLC and UV-Visible spectrometry to follow kinetics and product formation. Catechin forms a more complex mixture of products than epicatechin, and the rates constants for the oxidation are different. Our findings highlight the possible importance of MnP in the oxidation of small phenolics related to the more complex condensed tannins, and serve as a first step for identifying key steps in condensed tannin turn over in soil systems.

Potential of soil enzyme manganese peroxidase to degrade catechin and related polyphenols

ACS Spring 2022

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