9.4.1 Fermentation Hydrogen Generation Pathways
and Metabolites
Organisms oxidize sugars such as glucose to produce adenosine triphosphate
(ATP) as an energy source. Three chief sugar metabolism pathways have been
identified in microorganism: the Embden-Meyerhof (EM) pathway, the
Entrner-Doudoroff (ED) pathway, and the pentose phosphate (PP) pathway. Each of
them has been acquired by microorganisms for the production of ATP (Wrighton
et al. 2012).
As an example, bacteria from the genus Enterobacter chiefly use the EM pathway
to oxidize glucose to produce 2 mol each of the reduction product nicotinamide
adenine dinucleotide (NADH, reduced), pyruvic acid, and ATP through the
following reaction:
C 6 H 12 O 6 þ 2NAD
þ
þ 2ADP þ 2Pi ! 2CH 3 COCOOH þ 2NADH þ 2H
þ
þ 2ATP
ð9:1Þ
This alone does not generate sufficient ATP. To decompose new glucose,
metabolites are produced through a reaction between reduced NADH and pyruvic
acid, and NADH is oxidized to NAD
+ for reuse. It is at this point that hydrogen is
generated, although this generation does not occur in the production process for all
metabolites. As shown in Fig. 9.6, it chiefly occurs with the selection of a pathway
where metabolites are generation through production of acetyl-CoA.
For example, not only does a lactic acid production reaction not use the
acetyl-CoA production pathway, but, as shown in Formula (9.2), pyruvic acid and
NADH react at a 1:1 ratio for lactic acid production and do not contribute to
hydrogen generation. In the production of acetic acid, butyric acid, acetone, or
butanol, however, the acetyl-CoA production pathway is used and hydrogen is
generated (Tanisho 2011).
Lactic acid production reaction
CH 3 COCOOH þ NADH þ H
þ
! CH 3 CHOHCOOH þ NAD
þ
ð9:2Þ
Acetic acid production reaction
CH 3 COCOOH þ H 2 O þ FerredoxineðFdÞ ! CH 3 COOH þ CO 2 þ FdH 2 ð9:3Þ
FdH 2 ! Fd þ H 2
ð9:4Þ
NADH þ H
þ
! NAD
þ
þ H 2
ð9:5Þ
316
9 Marine Bioenergy Production
and Metabolites
Organisms oxidize sugars such as glucose to produce adenosine triphosphate
(ATP) as an energy source. Three chief sugar metabolism pathways have been
identified in microorganism: the Embden-Meyerhof (EM) pathway, the
Entrner-Doudoroff (ED) pathway, and the pentose phosphate (PP) pathway. Each of
them has been acquired by microorganisms for the production of ATP (Wrighton
et al. 2012).
As an example, bacteria from the genus Enterobacter chiefly use the EM pathway
to oxidize glucose to produce 2 mol each of the reduction product nicotinamide
adenine dinucleotide (NADH, reduced), pyruvic acid, and ATP through the
following reaction:
C 6 H 12 O 6 þ 2NAD
þ
þ 2ADP þ 2Pi ! 2CH 3 COCOOH þ 2NADH þ 2H
þ
þ 2ATP
ð9:1Þ
This alone does not generate sufficient ATP. To decompose new glucose,
metabolites are produced through a reaction between reduced NADH and pyruvic
acid, and NADH is oxidized to NAD
+ for reuse. It is at this point that hydrogen is
generated, although this generation does not occur in the production process for all
metabolites. As shown in Fig. 9.6, it chiefly occurs with the selection of a pathway
where metabolites are generation through production of acetyl-CoA.
For example, not only does a lactic acid production reaction not use the
acetyl-CoA production pathway, but, as shown in Formula (9.2), pyruvic acid and
NADH react at a 1:1 ratio for lactic acid production and do not contribute to
hydrogen generation. In the production of acetic acid, butyric acid, acetone, or
butanol, however, the acetyl-CoA production pathway is used and hydrogen is
generated (Tanisho 2011).
Lactic acid production reaction
CH 3 COCOOH þ NADH þ H
þ
! CH 3 CHOHCOOH þ NAD
þ
ð9:2Þ
Acetic acid production reaction
CH 3 COCOOH þ H 2 O þ FerredoxineðFdÞ ! CH 3 COOH þ CO 2 þ FdH 2 ð9:3Þ
FdH 2 ! Fd þ H 2
ð9:4Þ
NADH þ H
þ
! NAD
þ
þ H 2
ð9:5Þ
316
9 Marine Bioenergy Production