Identification and characterization of two families of F₄₂₀H₂‐dependent reductases from Mycobacteria that catalyse aflatoxin degradation

Abstract

Aflatoxins are polyaromatic mycotoxins that contaminate a range of food crops as a result of fungal growth and contribute to serious health problems in the developing world because of their toxicity and mutagenicity. Although relatively resistant to biotic degradation, aflatoxins can be metabolized by certain species of Actinomycetales. However, the enzymatic basis for their breakdown has not been reported until now. We have identified nine Mycobacterium smegmatis enzymes that utilize the deazaflavin cofactor F420H2 to catalyse the reduction of the α,β-unsaturated ester moiety of aflatoxins, activating the molecules for spontaneous hydrolysis and detoxification. These enzymes belong to two previously uncharacterized F420H2 dependent reductase (FDR-A and -B) families that are distantly related to the flavin mononucleotide (FMN) dependent pyridoxamine 5′-phosphate oxidases (PNPOxs). We have solved crystal structures of an enzyme from each FDR family and show that they, like the PNPOxs, adopt a split barrel protein fold, although the FDRs also possess an extended and highly charged F420H2 binding groove. A general role for these enzymes in xenobiotic metabolism is discussed, including the observation that the nitro-reductase Rv3547 from Mycobacterium tuberculosis that is responsible for the activation of bicyclic nitroimidazole prodrugs belongs to the FDR-A family.

Biosynthetic pathway toward carbohydrate-like moieties of alnumycins contains unusual steps for C-C bond formation and cleavage

“…The in vitro confirmation that Aln4 alone was able to catalyze the last biosynthetic step refuted the initial anticipation that also Aln3 (155 amino acids, 16.5 kDa), which belongs to a very recently described family of F420-dependent reductases involved in aflatoxin degradation (26), would be required. The genes aln3 and aln4 putatively reside in the same operon in the alnumycin gene cluster ( Fig.…”

The methanogenic redox cofactor F420 is widely synthesized by aerobic soil bacteria

“…F 420 H 2 -dependent reductases mediate the biodegradation of nitroaromatic explosives (Ebert et al, 2001), triphenyl dyes (Guerra-Lopez et al, 2007) and furanocoumarins (Taylor et al, 2010), as well as the biosynthesis of tetracycline and pyrrolobenzodiazepine antibiotics Wang et al, 2013). A role for F 420 in equivalent processes seems particularly likely for the Chloroflexi and Tectomicrobia; both phyla contain an abundance of F 420 H 2 -dependent reductases (Supplementary Tables S3 and S6) and are reputed for their biosynthetic versatility and biodegradative capacities (Björnsson et al, 2002;Wilson et al, 2014).…”

“…Among them are the one-carbon reactions of methanogenesis (Thauer, 1998;Shima et al, 2000;Hagemeier et al, 2003), the biosynthesis pathways of tetracycline antibiotics (Wang et al, 2013) and the biodegradation of picrate and aflatoxins (Ebert et al, 2001;Taylor et al, 2010;Lapalikar et al, 2012). The cofactor appears to have been selected for these roles because of its unique electrochemical properties compared with the ubiquitous flavin and nicotinamide cofactors FMN (flavin mononucleotide), FAD (flavin adenine dinucleotide) and NAD(P) (nicotinamide adenine dinucleotide (phosphate)) (Walsh, 1986;Greening et al, 2016a).…”

A new role for coenzyme F420 in aflatoxin reduction by soil mycobacteria

“…The blue-green fluorescent aflatoxins can be readily detected, yet their environmental fate and mode of degradation are unknown. A new report (Taylor et al, 2010) identified two oxidoreductase protein families from the soil bacterium Mycobacterium smegmatis that initiate aflatoxin breakdown. The key to these enzymes is another fluorescent molecule, coenzyme F 420.…”