Si‐Face Stereospecificity at C5 of Coenzyme F420 for F420‐Dependent Glucose‐6‐Phosphate Dehydrogenase from Mycobacterium smegmatis and F420‐Dependent Alcohol Dehydrogenase from Methanoculleus thermophilicus

Coenzyme F420 is a 5‐deazaflavin. Upon reduction, 1,5‐dihydro‐coenzyme F420 is formed with a prochiral center at C5. In this study we report that the F420‐dependent glucose‐6‐phosphate dehydrogenase from Mycobacterium smegmatis and the F420‐dependent alcohol dehydrogenase from Methanoculleus thermophilicus are Si ‐face stereospecific with respect to C5 of the 5‐deazaflavin. These results were obtained by following the stereochemical course of the reversible incorporation of 3H into F420 from tritium‐labeled substrates. Our findings bring to eight the number of coenzyme‐F420‐dependent enzymes shown to be Si ‐face stereospecific. No F420‐dependent enzyme with Re ‐face stereospecificity is known. This is noteworthy since coenzyme F420 is functionally similar to pyridine nucleotides for which both Si ‐face and Re ‐face specific enzymes have been found.