Kinetics and inhibition effects of furfural and hydroxymethyl furfural on enzymes in yeast

Fermentatively produced ethanol from lignocellulose by yeast is a renewable alternative to fossil fuels. However, inhibitors originating from the breakdown process of the raw material decrease the fermentation rate. Previous studies have made known inhibition effects by the furan compounds furfural and hydroxymethyl furfural (HMF). In the current work, the kinetics and inhibition effect of furfural, and in some cases also HMF, on the enzymes alcohol dehydrogenase (ADH), aldehyde dehydrogenase (AlDH), and pyruvate dehydrogenase (PDH) were studied. This was done using in vitro spectro-photometric assays with commercially available pure enzymes. The study showed that furfural strongly inhibited both AlDH and PDH, but the inhibition effects of HMF on AlDH and PDH were not as big. Furthermore, ADH was much less affected than AlDH and PDH by furfural, and was similarly inhibited by both furans. The kinetic analysis suggested that furfural and HMF most likely inhibited ADH and AlDH in a competitive manner, whereas PDH was inhibited in a mixed mode in the presence of furfural. The present results offer an explanation to the strong effects exerted by furans on the yeast metabolism.