Biochem. J. 10.1042/bj20141237

Cellular metabolism is typically described in terms of specific pathways that work together to provide the cell with energy and biomolecular precursors. However, increasing evidence suggests that the metabolic map contains a variety of detours and dead ends, with associated clean-up crews to resolve the problems. For example, directed overflow systems divert chemicals produced in excess, whereas metabolite damage preemption or repair systems prevent or correct chemical and enzymatic damage involving reactive small molecules. Frelin et al. suspected that the undefined COG3236 domains fused to or associated with enzymes involved in riboflavin biosynthesis might serve in one of these protective functions, as this pathway has no known post-transcriptional regulatory mechanisms and the pathway intermediates are highly reactive. To test this hypothesis, the authors monitored the fate of riboflavin intermediates in the presence of one of several COG3236 homologs, observing that the first two intermediates (shown) but not the third were converted into new compounds in the presence of the COG3236 domains. Computational analysis, genomic relationships and chemical characterization confirmed that the COG3236s cleave the N-glycosidic bond (red arrows), yielding products distinct from those generated by spontaneous decomposition and confirming that the COG3236 enzymes serve an active role in directing compound degradation to innocuous byproducts. Enzyme specificity agreed with the anticipated need for the COG3236 domain: the Vibrio vulnificus COG3236 fused to an earlier enzyme in the pathway was more active on intermediate 1, whereas a plant COG3236 fused to a later enzyme preferred intermediate 2. Finally, analysis of literature data along with a two-hybrid experiment suggested that the enzymes in this pathway form a metabolon, or multienzyme complex, explaining how these COG3236 domains would have privileged access to the reactive intermediates to prevent cellular damage as well as why the overexpression of COG3236s did not influence flavin levels in Arabidopsis or Escherichia coli.