Si-face stereospecificity at C5 of coenzyme F-420 for F-420-dependent glucose-6-phosphate dehydrogenase from Mycobacterium smegmatis and F-420-dependent alcohol dehydrogenase from Methanoculleus thermophilicus

Coenzyme F-420 is a 5-deazaflavin. Upon reduction, 1,5-dihydro-coenzyme F-420 is formed with a prochiral center at C5. In this study we report that the F-420-dependent glucose-6-phosphate dehydrogenase from Mycobacterium smegmatis and the F-420-dependent alcohol dehydrogenase from Methanocelleus 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 H-3 into F-420 from tritium-labeled substrates. Our findings bring to eight the number of coenzyme-F-420-dependent enzymes shown to be Si-face stereospecific. No F-420-dependent enzyme with Re-face stereospecificity is known. This is noteworthy since coenzyme F-420 is functionally similar to pyridine nucleotides for which both Si-face and Re-face specific enzymes have been found.