Coenzyme specificity of nicotinamide-adenine dinucleotide phosphate cytochrome c reductase for flavin phosphates

Effects of changes in the structures of flavin phosphates were studied to assess the coenzyme specificity of nicotinamide-adenine dinucleotide phosphate cytochrome c reductase (NADPH: cytochrome c oxidoreductase, EC 1.6.2.3) from brewer's yeast.

• 1.

  1. Among analogues of riboflavin 5′-phosphate which are symmetrically substituted in positions 6 and 7 of the isoalloxazine ring system, diethylriboflavin 5′-phosphate is as active as riboflavin 5′-phosphate, but decreasing activities are seen with the dibromo-, dichloro-, and diiodo-analogues.

• 2.

  2. With analogues which are substituted in position 6 of the ring system, satisfactory activities of the 5′-phosphates of methyl- and, somewhat less, methyl-pyridinoriboflavin are observed; however carboxy and ethoxy substituents markedly diminish activity.

• 3.

  3. Activities of 6,7-dimethylflavin phosphates bearing different side chains are progressively decreased with the shorter chains of the glycityl series of d-ribityl, d-erythrityl, and dl-glyceryl, respectively. Similarly with the alkyl chains, 2′, 3′, 4′-trideoxyribityl is better than 2′-deoxyglyceryl.

• 4.

  4. Secondary hydroxyl groups on the chain confer greater activity, but are not obligatory as seen by the activities of 2′,3′,4′-trideoxyriboflavin 5′-phosphate and 2′-deoxyglyceroflavin 3′-phosphate. A reduction in activity is also seen when the 2′-hydroxy function is missing, as in 2′-deoxyriboflavin 5′-phosphate, or in l-configuration, as in d-araboflavin 5′-phosphate.