Kinetic and modelling studies of NAD+ and poly(ethylene glycol)-bound NAD+ in horse liver alcohol dehydrogenase

Poly(ethylene glycol)-bound nicotinamide adenine dinucleotide (PEG-NAD+) has been successfully employed in the continuous production of L-amino acids from the corresponding alpha-keto acids by stereospecific reductive amination. Like many other dehydrogenases also horse liver alcohol dehydrogenase (HLADH) appears to be active with PEG-NAD+ as coenzyme, although the turnover number is three to four times lower. The possibilities were considered that the PEG-tail of a PEG-NAD+ bound to one active site of the HLADH dimer prevents the binding of another PEG-NAD+ to the second site, or that the PEG-tail causes destabilization of the active dimer. Both could be ruled out by kinetic studies. Neither can the observed lower intrinsic reactivity of PEG-NAD+ account for the diminished activity of the enzyme. Molecular dynamics studies, on the other hand, show that the pulling action of the polymer chain shifts the NAD position in the active site in the outside direction, causing small but significant changes in the enzyme/coenzyme interactions of a sufficient extent to explain the experimental results.