350
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
In the strict aerobe Hydrogenomonas, this enzyme is coupled with a
respiratory chain where oxygen is the terminal electron acceptor:
2H+ + 2e + y 2 0 2 -* H 2 0
A hydrogenase is also present in the strict anaerobe Desulfovibrio desulfuricans, but in this case the terminal acceptor is sulfate which is
reduced to sulfide:
2H+ + 2e + MS0 4
2
~ -> MS
2
" + H 2 0
Moreover, Thiobacillus denitrificans, a bacterium which derives its energy from the litho-oxidation of thiosulfate to sulfate, can do so either
aerobically in air or anaerobically if nitrate is present. These facts and
many similar observations demonstrate that no sharp separation can be
drawn between aerobic and anaerobic reactions.
B. ENERGETICS OF BIOLOGICAL LITHO-OXIDATIONS
1. Heat of Combustion and Free Energy Change
The energetics of the biological litho-oxidations have been estimated
by many authors from the heat of combustion of the substrate, i.e., from
the total energy change of the reaction at constant temperature and
pressure. According to the classical notation of Lewis and Randall (7)
and Wurmser (8), the heat of reaction is expressed by the symbol
-AH.
A more precise and meaningful estimate is given by the free energy
change (— AF°), i.e., the portion of the total energy exchanged which
is convertible into work. At a given absolute temperature T, —AF° and
—AH are defined reciprocally by the equation
-AF° T = -AH T - TAS
where TAS is the energy not convertible into work and AS is the increase of entropy in the system.
Table III shows the numerical values of — AH and —AF° for several
litho-oxidations at the absolute temperature of 298°K (+25°C), under
the standard conditions where the solids and liquids are dissolved in
the aqueous phase at a concentration of 1.0 M and the gases are at a
partial pressure of 1 atm. As pointed out by Marjorie Stephenson (i)
in her book, the concentrations of the reactants have no effect on —AH
but have a significant influence on the values of —AF. When the reactants are highly diluted or at a very low partial pressure, as in most
biological systems, — AF tends to increase and to approach the value
of -ΔΗ.
It may be seen from the data of Table III that many litho-oxidations
are strongly exergonic reactions. However, none of them provides as
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
In the strict aerobe Hydrogenomonas, this enzyme is coupled with a
respiratory chain where oxygen is the terminal electron acceptor:
2H+ + 2e + y 2 0 2 -* H 2 0
A hydrogenase is also present in the strict anaerobe Desulfovibrio desulfuricans, but in this case the terminal acceptor is sulfate which is
reduced to sulfide:
2H+ + 2e + MS0 4
2
~ -> MS
2
" + H 2 0
Moreover, Thiobacillus denitrificans, a bacterium which derives its energy from the litho-oxidation of thiosulfate to sulfate, can do so either
aerobically in air or anaerobically if nitrate is present. These facts and
many similar observations demonstrate that no sharp separation can be
drawn between aerobic and anaerobic reactions.
B. ENERGETICS OF BIOLOGICAL LITHO-OXIDATIONS
1. Heat of Combustion and Free Energy Change
The energetics of the biological litho-oxidations have been estimated
by many authors from the heat of combustion of the substrate, i.e., from
the total energy change of the reaction at constant temperature and
pressure. According to the classical notation of Lewis and Randall (7)
and Wurmser (8), the heat of reaction is expressed by the symbol
-AH.
A more precise and meaningful estimate is given by the free energy
change (— AF°), i.e., the portion of the total energy exchanged which
is convertible into work. At a given absolute temperature T, —AF° and
—AH are defined reciprocally by the equation
-AF° T = -AH T - TAS
where TAS is the energy not convertible into work and AS is the increase of entropy in the system.
Table III shows the numerical values of — AH and —AF° for several
litho-oxidations at the absolute temperature of 298°K (+25°C), under
the standard conditions where the solids and liquids are dissolved in
the aqueous phase at a concentration of 1.0 M and the gases are at a
partial pressure of 1 atm. As pointed out by Marjorie Stephenson (i)
in her book, the concentrations of the reactants have no effect on —AH
but have a significant influence on the values of —AF. When the reactants are highly diluted or at a very low partial pressure, as in most
biological systems, — AF tends to increase and to approach the value
of -ΔΗ.
It may be seen from the data of Table III that many litho-oxidations
are strongly exergonic reactions. However, none of them provides as