NUTRITION IN SPORT

One potential problem with the oxidative
regeneration of NAD+is that the reactions of
oxidative phosphorylation occur within the
mitochondria, whereas glycolysis is a cytosolic
process, and the inner mitochondrial membrane
is impermeable to NADH and to NAD+. Without
regeneration of the NAD+ in the cytoplasm,
glycolysis will stop so there must be a mecha-
nism for the effective oxidation of the NADH
formed during glycolysis. This separation is
overcome by a number of substrate shuttles
which transfer reducing equivalents into the
mitochondrion.

28 nutrition and exercise

Some of the pyruvate formed may be con-

verted to the amino acid alanine. Some may also

be converted to the four-carbon compound

oxaloacetate by the incorporation of CO 2 in a

reaction catalysed by pyruvate carboxylase. This

conversion to oxaloacetate can be the first step in

the resynthesis of glucose by the process of glu-

coneogenesis. Alternatively, this may be impor-

tant as an anapleurotic reaction: these are

reactions which maintain the intracellular con-

centration of crucial intermediates (e.g. of the

TCA cycle) which might otherwise become

depleted.

Inner mitochondrial

membrane

β-oxidation

of fatty acids

Coenzyme A

Acetyl-CoA

Pyruvate

NADH

NADH

NADH

FADH 2

CO 2

CO 2

NADH

ATP

TCA

cycle

Hydrogen is transported

by reduced coenzymes

(carrier molecules)

of the electron

NADH

FADH 2

NADH

NAD+

H+

H+H+

e O 2

• e–e–
  H 2 O
  ADP + Pi H+
  ATP
  ADP

ATP

ADP

ATP synthase

O 2

Outer mitochondrial

membrane

Pyruvate Fatty acids

CO 2

Electron transport chain:

reduced coenzyme

complexes are oxidized

Fig. 2.5Schematic diagram showing the relationship of the tricarboxylic (TCA) cycle to the electron transport
chain. The main function of the TCA cycle is to reduce the coenzymes NAD+and flavin adenine dinucleotide (FAD)
to NADH and FADH 2 which act as carriers of H+ions and electrons which are donated to the electron transport
chain. Molecular oxygen acts as the terminal electron acceptor and the hydrogen ion gradient generated across the
inner mitochondrial membrane is used to drive the synthesis of ATP from ADP and Pi.