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

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