초록
<P><B>Abstract</B></P> <P>It is known that <I>Escherichia coli</I> produces hydrogen gas (H<SUB>2</SUB>) during mixed acid fermentation via hydrogenase (Hyd) enzymes utilizing sugars, alcohols or organic acids. However, there are many sugars including lactose which are not studied well. Therefore, H<SUB>2</SUB> production and responsible Hyd enzymes activity and role have been investigated. During lactose fermentation (1–5 g L<SUP>−1</SUP>) by <I>E. coli</I> mainly Hyd-4 and partially Hyd-3 have been shown to affect cell growth and H<SUB>2</SUB> production at different pHs. At 2 g L<SUP>−1</SUP> lactose fermentation in <I>hyfG</I> (lacking large subunit of Hyd-4) mutant at pH 5.5 and 6.5 resulted in the decrease of specific growth rate ∼2.5-fold and ∼1.8-fold, respectively, compared to wild type. Hyd-3 contribution to cell growth was observed at pH 6.5 with 1 g L<SUP>−1</SUP> and at pH 7.5 with 5 g L<SUP>−1</SUP> lactose fermentation. In addition, external pH during 24 or 48 h was less acidified in <I>hyfG</I> mutant cells grown at pH 6.5 than in wild type. Moreover, the H<SUB>2</SUB> yield in wild type cells was 6.00 ± 0.017 mmol L<SUP>−1</SUP>. Hyd-3 and Hyd-4 were responsible for H<SUB>2</SUB> production at pH 7.5, 6.5 and 5.5, as shown by using <I>hycE</I> (lacking large subunit of Hyd-3) and <I>hyfG</I> single and <I>hycE hyfB-R</I> (lacking Hyd-3 large subunit and several subunits of Hyd-4) double mutants. These findings provide novel insights into the H<SUB>2</SUB> producing activity of <I>E. coli</I> Hyd-3 and Hyd-4 during utilization of lactose and might be useful for improving H<SUB>2</SUB> production from lactose or lactose containing various wastes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> H<SUB>2</SUB> production by <I>E. coli</I> was detected till 48 h at pH 7.5 and 5 g L<SUP>−1</SUP> lactose. </LI> <LI> Optimal conditions for cell growth was pH 6.5 and 5 g L<SUP>−1</SUP> lactose. </LI> <LI> In <I>hyfG</I> mutant external pH was less acidified compared to wild type. </LI> <LI> Hyd-4 and partially Hyd-3 affect cell growth and H<SUB>2</SUB> production at different pHs. </LI> </UL> </P>