Glycyl Radical Enzyme-Associated Microcompartments: Redox-Replete Bacterial Organelles

Abstract

ABSTRACT An increasing number of microbes are being identified that organize catabolic pathways within self-assembling proteinaceous structures known as bacterial microcompartments (BMCs). Most BMCs are characterized by their singular substrate specificity and commonly employ B12-dependent radical mechanisms. In contrast, a less-well-known BMC type utilizes the B12-independent radical chemistry of glycyl radical enzymes (GREs). Unlike B12-dependent enzymes, GREs require an activating enzyme (AE) as well as an external source of electrons to generate an adenosyl radical and form their catalytic glycyl radical. Organisms encoding these glycyl radical enzyme-associated microcompartments (GRMs) confront the challenge of coordinating the activation and maintenance of their GREs with the assembly of a multienzyme core that is encapsulated in a protein shell. The GRMs appear to enlist redox proteins to either generate reductants internally or facilitate the transfer of electrons from the cytosol across the shell. Despite this relative complexity, GRMs are one of the most widespread types of BMC, with distinct subtypes to catabolize different substrates. Moreover, they are encoded by many prominent gut-associated and pathogenic bacteria. In this review, we will focus on the diversity, function, and physiological importance of GRMs, with particular attention given to their associated and enigmatic redox proteins.

Keywords

• activating enzymes
• bacterial microcompartments
• glycyl radical enzyme-associated microcompartments
• glycyl radical enzymes
• iron-sulfur proteins
• electron transfer