NUTRITION IN SPORT

cules of ADP to generate one molecule of ATP

(and one molecule of adenosine monophos-

phate, AMP). This reaction is catalysed by the

enzyme called myokinase:

ADP+ADPfiATP+AMP–31kJ·mol–1ADP

This reaction only becomes important during

exercise of high intensity. Even then, the amount

22 nutrition and exercise

of energy it makes available in the form of ATP is

extremely limited and the real importance of the

reaction may be in the formation of AMP which

is a potent allosteric activator of a number of

enzymes involved in energy metabolism.

It is known that the total adenylate pool can

decline rapidly if the AMP concentration of the

cell begins to rise during muscle force genera-

Fats Carbohydrates Proteins

Triacylglycerols

Fatty acids + glycerol Glucose/glycogen Amino acids

Transamination

deamination

Ammonia

Urea

Glutamate

Glycine

Lactate

Acetoacetate

Pyruvate

Glycolysis

Alanine

Acetyl-CoA

O 2

O (^2) H 2 O
CO 2
β-oxidation
Oxaloacetate Citrate
TCA
cycle
NADH + H+
NAD+
Electron transport
chain
Fig. 2.2Summary of the main
pathways of energy metabolism
using carbohydrate, fats and protein
as energy sources. Carbohydrate
may participate in both anaerobic
and aerobic pathways. In glycolysis,
glucose or glycogen are broken
down to lactate under anaerobic
conditions and pyruvate under
aerobic conditions. The pyruvate is
converted to acetyl-coenzyme A
(CoA) and is completely oxidized in
the tricarboxylic (TCA) cycle. Fats in
the form of triacylglycerols are
hydrolysed to fatty acids and
glycerol, the latter entering the
glycolytic pathway (in liver but not
in muscle) and the fatty acids being
converted via the b-oxidation
pathway to acetyl-CoA and
subsequently oxidized in the TCA
cycle. Protein catabolism can
provide amino acids that can be
converted by removal of the amino
group into either TCA cycle
intermediates or into pyruvate or
acetoacetate and subsequent
transformation to acetyl-CoA.
Table 2.2aCapacity and power of anaerobic systems for the production of adenosine triphosphate (ATP).
Capacity Power
(mmol ATP · kg-^1 dm) (mmol ATP · kg-^1 dm · s-^1 )
Phosphagen system 55–95 9.0
Glycolytic system 190–300 4.5
Combined 250–370 11.0
Values are expressed per kilogram dry matter (dm) of muscle and are based on estimates of ATP provision during
high-intensity exercise of human vastus lateralis muscle.