Enzyme-catalysed biotransformations through photochemical regeneration of nicotinamide cofactors

Photosensitized regeneration of NAD(P)H cofactors by photochemical means is reviewed. Reductive regeneration of NAD(P)H cofactors proceeds through coupling of photogenerated N,N′dimethyl-4,4′-bipyridinium radical cation, which acts as electron carrier, to the enzymes lipoamide dehydrogenase, LipDH and ferredoxin reductase, FDR, respectively. Regeneration of NAD(P)H is also accomplished by substitution of the enzymes and electron carrier by synthetic rhodium complexes acting as H-donors for the regeneration of NAD(P)H. Various photosensitizers such as Ru(II)-tris-bipyridine, Ru(bpy)²⁺₃, or Zn-meso-tetramethyl-pyridinium porphyrin, Zn-TMPyP⁴⁺, are applied in the systems. The photoinduced processes are initiated through oxidative or reductive electron-transfer quenching pathways. The photoregenerated NAD(P)H cofactors are coupled to subsequent biotransformations in such reduction of ketones and keto-acid, synthesis of amino-acids, and CO₂-fixation processes. Oxidative regeneration of NAD(P)⁺ cofactors is accomplished by photochemical means. In these systems, Sn(II)-meso-tetramethylpyridinium porphyrin, Sn-TMPyP⁴⁺, Ru(bpy)²⁺₃ or acridine dyes are used as photosensitizers. Oxidation of NAD(P)H proceeds either by reductive quenching of the excited photosensitizer by NAD(P)H or dark oxidation of NAD(P)H by the oxidized photoproduct formed in the photosensitized electron-transfer process. The systems are applied in the dehydrogenation of alcohols, hydroxy acids, and amino acids.