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FUNGAL METABOLISM AND FUNGAL PRODUCTS 125

The net result of this whole sequence is that one
molecule of glucose is completely oxidized to six
molecules of CO 2 (and six molecules of water), accord-
ing to the empirical equation for aerobic respiration:

C 6 H 12 O 6 +6O 2 →6CO 2 +6H 2 O

remembering that every step after the cleavage of
fructose-1,6-biphosphate into two 3-carbon com-
pounds occurs twice, as shown in Fig. 7.2.
The oxidation of any substance must always be
coupled with a corresponding reduction of another
substance, and this role is served by three nucleotides:
NAD+(nicotinamide adenine dinucleotide), NADP+,
and FAD (flavin adenine dinucleotide). As shown in
Fig. 7.2, these compounds accept electrons and are
correspondingly reduced to NADH, NADPH and
FADH 2. These nucleotides then need to be reoxidized
for the whole process to continue, and this is achieved
by passing their electrons along an electron transport
chain, where oxygen is the terminal electron acceptor.
The electron transport chain is shown in simplified,
diagrammatic form in Fig. 7.3. It consists of a series
of electron carriers which are located in the mito-
chondrial membrane, and in several cases span this
membrane. They are aligned in a specific sequence.
Initially, NADH [or NAD(P)H] is reoxidized to NAD+
[or NAD(P)+] by transferring its electrons to the
carrier molecule, flavin mononucleotide (FMN), which
becomes reduced to FMNH. (Note that, for our purposes,
we can regard FMN/FMNH as equivalent to FAD/
FADH 2 ; the essential point is that a flavin molecule acts
as a carrier in the electron-transport chain.) The flavin
nucleotide, in turn, transfers the electrons to coenzyme
Q , and so on down the chain, until the final step when
oxygen accepts the electrons and is, itself, reduced to
water. The carriers in the electron transport chain gen-
erate a proton motive force, because protons (H+
ions) are extruded to the outside of the mitochondrial
membrane while OH−ions accumulate inside. This
polarization of the membrane is used to drive the syn-
thesis of ATP (from ADP + inorganic phosphate) when
protons re-enter through a membrane-located ATPase
(also termed ATP synthase).
As shown in Fig. 7.3, the electron transport chain is,
essentially, an electrochemical gradient, and at three

stages along this gradient sufficient energy is released

to synthesize a molecule of ATP from the oxidation of

NADH/NAD(P)H. But only two molecules of ATP are

produced from the oxidation of flavin nucleotides

(e.g. FMN).

The energy yield from aerobic respiration

We can calculate the theoretical energy yield from

glucose during aerobic respirationby calculating the

number of ATP molecules that could be synthesized

(Fig. 7.2):

2 ATP from the EM pathway down to pyruvic acid

(4 ATP produced but 2 ATP used initially to

phosphorylate glucose)

2 ATP from the TCA cycle (one in each turn of

the cycle, but 2 molecules of pyruvate must be

processed through this cycle)

30 ATP from the reoxidation of 10 pyridine

nucleotides (NADH / NADPH)

4 ATP from the reoxidation of 2 flavin nucleotides

Total 38 ATP

However, the actual ATP yieldwill be much less than

this, for at least two reasons:

1 Intermediates are continuously drawn from the

pathways for biosynthetic reactions, so this represents

a loss of potential ATP.

2 The major role of NADP/NADPH (as opposed to

NAD+/ NADH) is to provide reducing power for

biosynthetic reactions, rather than to generate ATP.

At this stage we should mention the special role of the

pentose-phosphate pathway(Fig. 7.2). It can be used

as an alternative to the EM pathway for generating

energy from sugars (giving 1 ATP instead of the 2 ATP

from the EM route). But its major role is in biosynthesis

• it generates some important intermediates, such as
  ribose-5-phosphate for the synthesis of nucleic acids and
  erythrose-4-phosphate for the synthesis of aromatic
  amino acids.

Fig. 7.3Outline of the respiratory electron transport chain. CoQ =coenzyme Q; Cyt =cytochrome.