Berberine bridge enzyme-like proteins: From characterization to application

Flavoproteins are a diverse protein class employing an isoalloxazine ring for catalysis in form of the flavin mononucleotide (FMN) or the flavin adenine dinucleotide (FAD). Among them, is the berberine bridge enzyme-like (BBE-like) protein family (pfam 08031) that was named after the berberine bridge enzyme (EcBBE) from California poppy (Eschscholzia californica). BBE-like proteins form a multigene family in plants and the number of members varies from one in the moss Physcomitrella patens to 28 in Arabidopsis thaliana and 57 in the Western Balsam Poplar (Populus trichocarpa). Despite of the frequent occurrence of these proteins their function is largely unknown. Therefore, we chose to investigate the BBE-like proteins occurring in A. thalina to broaden our understanding of this protein family. Presented will be the structural and biochemical characterization of AtBBE-like protein 15 and 28. Our analysis of the AtBBE-like protein family reveals that there are four frequently occurring active site types. AtBBE-like protein 15 is a representative of the most abundant type, therefore the enzyme can serve as a paradigm for the majority of BBE-like proteins1. The enzyme was identified as monolignol oxidoreductase, an activity that was not recognized for this protein family before. The active site type found in AtBBE-like protein 28 is restricted to the plant family Brassicaceae2. Additionally, the potential of AtBBE-like protein 15 as biocatalyst was elucidated. This enzyme was identified as a monolignol dehydrogenase, i.e. the enzyme inhibits the reaction between the reduced flavin and oxygen. The enzyme was rationally engineered towards higher oxygen reactivity and the potential of the enzyme as biocatalyst for oxidative reactions was tested. The pH- and temperature optimum (pH 7, 50°C) was determined. The enzyme shows enhanced activity in the presence of various organic solvents, it accepts allylic and benzylic alcohols as substrates. Secondary allylic alcohols were converted with good to excellent enantioselectivity (E>34 to E>200).