SI-FACE STEREOSPECIFICITY AT C5 OF COENZYME F-420 FOR F-420-DEPENDENTGLUCOSE-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-coenzym e F-420 is formed with a prochiral center at C5. In this study we repo rt that the F-420-dependent glucose-6-phosphate dehydrogenase from Myc obacterium smegmatis and the F-420-dependent alcohol dehydrogenase fro m Methanocelleus thermophilicus are Si-face stereospecific with respec t 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 th e number of coenzyme-F-420-dependent enzymes shown to be Si-face stere ospecific. No F-420-dependent enzyme with Re-face stereospecificity is known. This is noteworthy since coenzyme F-420 is functionally simila r to pyridine nucleotides for which both Si-face and Re-face specific enzymes have been found.