Front Matter

74 Canine Sports Medicine and Rehabilitation

which eventually generate 2NADH and 4ATP
and two pyruvate molecules (Figure  4.2). ATP
is produced rapidly via glycolysis and does not
require oxygen—the rate‐limiting step in the
citric acid cycle—making glycolysis a preferred
system to generate ATP quickly and anaerobi-
cally. The excess pyruvate is converted to lactate
under anaerobic metabolism. Glycogen stores
for ATP generation, unlike fat in most cases, are
limited.
Glycolysis occurs in the cytoplasm of all
cells. During rest, glycolysis is inhibited caus-
ing a flux of glucose toward eventual glycogen
formation (Figure  4.3). In times of need,
increased glycogen phosphorylase activity
induces glycogenolysis and generation of
glucose 6‐phosphate for glycolysis and entry
into the citric acid cycle. Beta‐oxidation of fat
occurs in the mitochondria, and transport of
free fatty acid into the inner mitochondria is
required. Carnitine is bound to the carboxyl
terminus of the free fatty acid in exchange for
CoA (Figure 4.4). Once the carnitine is bound,
the fatty acid is transported across the outer

mitochondrial membrane by carnitine acyl

transferase I to the inner compartment, and

then carnitine palmitoyl transferase II will

transport the fatty acid–carnitine moiety into

the mitochondrion. The fatty acid will become

conjugated to acetyl CoA again, and then

carnitine will undergo reverse transportation

back to the cytoplasm. Interestingly during

prolonged exercise, we often observe a decline

in serum free fatty acids while acylated carnitine

derivatives rise, suggesting that transport into

mitochondria can be part of the rate‐limiting

process of beta‐oxidation of fat (Shulman &

Petersen, 2009).

Energetics of substrates

The unit of energy generated by metabolism of

protein, carbohydrate, and fat is the calorie or

joule. The kilocalorie (kcal) is the unit referred

to in lay terminology when talking about

calories, and is equivalent to 4.16 kilojoules

(kJ). One gram of protein placed in a bomb

Phospho-

glyceraldehyde

NAD/NAD

NADH/NADH

ATP

ATP

ATP

ATP

ATP

ADP

ADP

ATP

Pyruvate Pyruvate

To acetyl CoA,

citric acid cycle

Phospho-

glyceraldehyde

Fructose 1,6

diphosphate

Fructose-6

phosphate

Glucose

Figure 4.2 Basic anaerobic glycolysis showing glucose being split into two pyruvate molecules, which eventually
become acetyl CoA for entry into the citric acid cycle. Note that 2ATP are utilized, but 4ATP are made in the reaction to
provide quick anaerobic energy.