Biochemical Reactions - A First Look
61
enzyme encoded by
GDH1
uses NADPH as cofactor, the other NADH. The stoichiometry of this
reaction is:
-\N A D (P )H + CH9l50<tiN V5
= 0
(3.26)
where the parenthesis around P signifies that both co-factors can be used.
Many microorganisms such as
E. coii
(but not
S. cerevisiae)
are able to convert one of the redox
co-factors to the other by a transhydrogenase catalyzed reaction.
-N A D H -N A D P * + NAD+
+ NADPH = 0
(3.27)
For such microorganisms the two cofactors are in a sense equivalent, but usually they have to be
treated as different entities in stoichiometric equations. In organisms with specialized organelles,
such as the mitochondria in eucaryotes NADH in the cytosol can usually not be lumped with
NADH in the mitochondria, and separate balances for each cofactor must be set up in each
compartment of the cell. These facts o f biochemistry must of course be considered in any serious
quantitative study of cell metabolism as discussed in Section 5.4, but in the present context we
shall lump different redox carrying co-factors (in Section 5.2.3 we are going to consider balances
for the individual co-factors). Therefore, if a unit of redox power is defined as one H atom then
all redox-carrying co-factors are equivalent to H
2
and carry 2 redox equivalents, i.e.
Redox unit = H = 1 ;
NADH = NADPH = FADH
2
= “H2”
(3.28)
We shall now introduce a systematic way of defining the redox level of different chemical
compounds.
1.
Define a redox neutral compound for each element of interest.
2.
We choose H
2
0 , C 0 2, NHj, H
2
S 0 4, H
3
P 0
4
as the neutral compounds corresponding to the
elements O, C, N, S and P. With this set of neutral compounds and with the unit of redox
defined as H = 1 one obtains the following redox levels of the five listed elements:
0 - - 2 , C - 4 , N = -3 , S =
6
, P = 5
(3.29)
3.
Now the redox level o f any reactant in a biochemical reaction can be calculated. With our
convention to write all carbon containing compounds on a 1 C-atom basis one obtains the
following
degrees o f reduction
K,.
CH20 (glucose and other hexoses, acetic acid, lactic acid, formaldehyde), K - 4.
CH
3
Ow
(ethanol),
K
=
6
.
CH
2
0
2
(formic acid), K = 2.
CH[
8
O
0
jN
0
2
(standard biomass), K = 4.20
Table 3.3 compiles elemental composition of many different compounds and their corresponding
degree of reduction
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