초록
<P><B>Abstract</B></P> <P>In N<SUB>2</SUB>-fixing cyanobacteria, three enzymes are involved in the H<SUB>2</SUB> metabolism. Nitrogenase catalyzes the N<SUB>2</SUB> fixation which produces H<SUB>2</SUB> as a by-product. The produced H<SUB>2</SUB> is taken up to protons and electrons by an activity of uptake hydrogenase. Reversible enzyme catalyzes both reactions of the H<SUB>2</SUB> evolution and the H<SUB>2</SUB> 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 H<SUB>2</SUB> production and hydrogenase activity by N<SUB>2</SUB>-fixing cyanobacterium <I>Anabaena siamensis</I> TISTR 8012 isolated from rice paddle field in Thailand. The result showed that <I>A. siamensis</I> cells incubated in N-deprived BG11 medium (BG11<SUB>0</SUB>) gave clearly higher H<SUB>2</SUB> production rate and hydrogenase activity than those in normal BG11 medium. Under nitrogen deprivation, an increase of iron, nickel, and molybdenum concentrations obviously enhanced H<SUB>2</SUB> 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 H<SUB>2</SUB> production in <I>A. siamensis</I>. The optimal concentrations of iron, nickel and molybdenum ions for H<SUB>2</SUB> production rate by <I>A. siamensis</I> were 60 μM, 4 μM and 4 μM, respectively. The highest H<SUB>2</SUB> production rate of 0.057 µmolH<SUB>2</SUB> mg chl a<SUP>-1</SUP> h<SUP>-1</SUP> was observed in cells incubated in BG11<SUB>0</SUB> medium supplemented with 4 μM nickel ion.</P>