Effect of Metal Cofactors of Key Enzymes on Biohydrogen Production by Nitrogen Fixing Cyanobacterium Anabaena siamensis TISIR 8012

Abstract

In N2-fixing cyanobacteria, three enzymes are involved in the H2 metabolism. Nitrogenase catalyzes the N2 fixation which produces H2 as a by-product. The produced H2 is taken up to protons and electrons by an activity of uptake hydrogenase. Reversible enzyme catalyzes both reactions of the H2 evolution and the H2 uptake. These enzymes are all metalloenzyme. The cyanobacterial nitrogenase normally requires molybdenum and iron as cofactors; however nitrogenase of few cyanobacterial species is dependent on vanadium. The cyanobacterial uptake and reversible hydrogenase requires nickel and iron as cofactors. This research aimed to investigate the effect of these metal cofactors on H2 production and hydrogenase activity by N2-fixing cyanobacterium Anabaena siamensis TISTR 8012 isolated from rice paddle field in Thailand. The result showed that A. siamensis cells incubated in N-deprived BG11 medium (BG110) gave clearly higher H2 production rate and hydrogenase activity than those in normal BG11 medium. Under nitrogen deprivation, an increase of iron, nickel, and molybdenum concentrations obviously enhanced H2 production rate. But only higher iron concentrations increased hydrogenase activity, indicating that the iron metal assisted in the function of reversible hydrogenase activity. In addition, vanadium seemed not to be a metal cofactor of key enzymes involving in H2 production in A. siamensis. The optimal concentrations of iron, nickel and molybdenum ions for H2 production rate by A. siamensis were 60 μM, 4 μM and 4 μM, respectively. The highest H2 production rate of 0.057 µmolH2 mg chl a-1 h-1 was observed in cells incubated in BG110 medium supplemented with 4 μM nickel ion.