Diversity of ThDP-Dependent Enzymes Forming Chiral Tertiary Alcohols

Thiamine diphosphate (ThDP)-dependent enzymes are well known biocatalysts for CC bond-forming reactions. While this enzyme class is mainly investigated for the formation of acyloins of secondary alcohols, recent studies have expanded its scope to utilize ketones as electrophiles in asymmetric carboligation reactions for the formation of tertiary alcohols. Chiral tertiary alcohols are ubiquitous motifs in natural products and important building blocks for the synthesis of bioactive compounds. ThDP-dependent enzymes are emerging as one of the most promising classes of biocatalysts for synthesizing a wide range of products due to the variety of possible substrate combinations, accessible starting materials, high enantioselectivity, and advantageous self-regeneration of the catalytic ThDP cofactor. This review provides an overview of the ThDP-dependent enzymes (e.g., decarboxylase, DC; transketolase, TK; α-keto acid dehydrogenase 2, αKADH2) that form tertiary alcohols, focusing on the substrate scope and diversity of physiological functions. The available toolbox and the characterized reactions shall serve as a starting point for future studies. Inspired by nature, an even broader diversity of classes and substrate specificities is expected in this field.

Identification of Kibdelomycin and Related Biosynthetic Gene Clusters and Characterization of the C-Branching of Amycolose

Many bacterial natural products contain C-branched sugars, including components from the outer cell wall or antibiotically active metabolites. The enzymatic C-branching of keto sugars leading to longer side chains (≥C2) is catalyzed by thiamine diphosphate (ThDP)-dependent enzymes. Chiral tertiary α-hydroxy ketones are formed in this process. The ThDP-dependent enzymes that catalyze C-branching reactions belong to one of three enzymatic superfamilies: decarboxylases, transketolases, and α-ketoacid dehydrogenases 2, but branching of keto sugars has only been demonstrated for decarboxylases. In this study, we showed that an α-ketoacid dehydrogenase is responsible for C-branching of the deoxyketo sugar amycolose in the biosynthesis of kibdelomycin in Kibdelosporangium sp. MA7385. In addition, we characterized an amino transferase in the same biosynthetic gene cluster (BGC) that accepts a sterically demanding tertiary α-hydroxy ketone in a downstream reaction. Subsequently, we identified approximately 400 similar BGCs in silico, suggesting that there is a large diversity of possible ThDP-dependent enzymes catalyzing the C-branching of keto sugars and subsequent modifications.

Recent advances in enzymatic carbon-carbon bond formation

Enzymatic carbon-carbon (C-C) bond formation reactions have become an effective and invaluable tool for designing new biological and medicinal molecules, often with asymmetric features. This review provides a systematic overview of key C-C bond formation reactions and enzymes, with the focus of reaction mechanisms and recent advances. These reactions include the aldol reaction, Henry reaction, Knoevenagel condensation, Michael addition, Friedel-Crafts alkylation and acylation, Mannich reaction, Morita-Baylis-Hillman (MBH) reaction, Diels-Alder reaction, acyloin condensations via Thiamine Diphosphate (ThDP)-dependent enzymes, oxidative and reductive C-C bond formation, C-C bond formation through C1 resource utilization, radical enzymes for C-C bond formation, and other C-C bond formation reactions.