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126 CHAPTER 7

What happens when oxygen is limiting?

In the absence of a terminal electron acceptor (usually
oxygen), the electron transport chain cannot operate,
so the pool of reduced nucleotides (NADH, etc.) accu-
mulates and metabolism would rapidly cease. But
fungi, and many other organisms, can still obtain
some energy in the absence of oxygen by one of the
following reactions:

In both cases the end-product (ethanol or lactic acid)
is more reduced than pyruvic acid, so the reactions
are coupled with the reoxidation of NADH to NAD+.
This allows the Embden–Myerhof pathway to continue,
and the cells release either lactic acid or ethanol into
the surrounding medium. Most yeasts and mycelial
fungi produce ethanol – this is the basis of the alco-
holic drinks industry. But several Chytridiomycota
produce lactic acid (e.g. Allomyces, Blastocladiella), as do
humans when our tissue oxygen level is depleted.
Energy-yielding reactions of this type – where an
internal inorganic compoundis the terminal electron
acceptor – are defined by the biochemical term fermen-
tation. In the first equation above the terminal electron
acceptor is acetaldehyde, because this accepts elec-
trons from NADH and is, itself, reduced to ethanol.
Similarly, in the second equation, pyruvic acid accepts
electrons and is itself reduced to lactic acid.
In terms of energy yield, the conversion of pyruvic
acid to either ethanol or lactic acid is very inefficient

• only 2 moles of ATP are produced from every mole
  of sugar metabolized (compared with the potential
  38 ATP from aerobic respiration). Fungi therefore need
  an abundant supply of sugars for growth in anaerobic
  conditions. We noted in Chapter 6 that fungi also need
  to be supplied with a wide range of other nutrients
  in anaerobic conditions, because the TCA cycle and
  several other reactions do not operate to provide the
  precursors for biosynthesis.

Alternative terminal electron acceptors

At least some fungi (e.g. Neurospora crassa, Emericella

nidulans) have an alternative means of coping with

anaerobic conditions: they use nitrate in place of

oxygen as the terminal electron acceptor in the electron

transport chain, so that the full TCA cycle operates.

This provides a theoretical yield of 26 ATP from each

molecule of glucose metabolized. The yield is lower

than when oxygen is the terminal electron acceptor,

because the energy differential along the electron-

transport chain from NADH/NADPH to nitrate is

sufficient to generate only 2 ATP (and only one ATP

from flavin nucleotides to nitrate).

All energy-yielding pathways which (i) involve an

electron-transport chain and (ii) use external inorganic

substances as the terminal electron acceptor are described

by the term respiration. However, a distinction is

made between aerobic respiration(when oxygen is the

terminal electron acceptor) and anaerobic respiration

(when nitrate is the terminal electron acceptor).

Summary: the central role of the

energy-yielding pathways

From Fig. 7.2 it can be seen that many types of substrate

can be used as potential energy sources, provided

that they can be fed into one of the energy-yielding

pathways. For example, pentose sugars such as xylose

(a major component of hemicelluloses) can be fed

into the pentose-phosphate pathway and metabolized

to give energy. An amino acid like glutamic acid can

be deaminated to an organic acid (α-ketoglutaric acid

in this case) which will feed into the TCA cycle and

yield a theoretical 9 ATP during its conversion to

oxaloacetate. Similarly, if acetate is supplied as a sub-

strate it can be linked to coenzyme A, and acetyl-CoA

can then combine with oxaloacetate (giving citrate)

and be processed through the TCA cycle. Fatty acids

can be degraded to acetyl-CoA by a process termed

β-oxidation (Fig. 7.4) and can then be metabolized in

the same way. The only limitation in all these cases is

that oxygen (or perhaps nitrate) is required as a ter-

minal electron acceptor. The only substrates that can sup-

ply energy through fermentationare sugars and sugar

derivatives, through the Embden–Meyerhof pathway or

the pentose-phosphate pathway.

Coordination of metabolism: balancing the

pathways

We noted earlier that several intermediates of the

basic energy-yielding pathways serve as precursor

metabolites for biosynthesis. For example:

1

2

CH 3 .CO.COOH

(pyruvic acid)

then:

acetaldehyde + CO 2

pyruvic dehydrogenase

CH 3 .CHO

(acetaldehyde)

CH 3 CH 2 OH

(ethanol)

+ NADH

+ NAD+

alcohol dehydrogenase

CH 3 .CO.COOH

(pyruvic acid)

CH 3 .CHOH.COOH

(lactic acid)

+ NADH

+ NAD+

lactic dehydrogenase