272
Chapter 9
Skeletal Muscle Energy Metabolism
As we have seen, ATP performs three functions directly related
to muscle fi ber contraction and relaxation (see Table 9–1). In
no other cell type does the rate of ATP breakdown increase
so much from one moment to the next as in a skeletal muscle
fi ber when it goes from rest to a state of contractile activity.
The ATP breakdown may change 20- to several hundred-fold
depending on the type of muscle fi ber. The small supply of
preformed ATP that exists at the start of contractile activity
would only support a few twitches. If a fi ber is to sustain con-
tractile activity, metabolism must produce molecules of ATP
as rapidly as they break down during the contractile process.
There are three ways a muscle fi
ber can form ATP
(
Figure 9–22
): (1) phosphorylation of ADP by
creatine
phosphate,
(2) oxidative phosphorylation of ADP in the
mitochondria, and (3) phosphorylation of ADP by the glyco-
lytic pathway in the cytosol.
Phosphorylation of ADP by creatine phosphate (CP) pro-
vides a very rapid means of forming ATP at the onset of con-
tractile activity. When the chemical bond between creatine
(C) and phosphate is broken, the amount of energy released is
about the same as that released when the terminal phosphate
bond in ATP is broken. This energy, along with the phosphate
group, can be transferred to ADP to form ATP in a reversible
reaction catalyzed by creatine kinase:
creatine
kinase
CP + ADP
34
C + ATP
Although creatine phosphate is a high-energy molecule, its
energy cannot be released by myosin to drive cross-bridge
activity. During periods of rest, muscle fi bers build up a con-
centration of creatine phosphate approximately fi ve times that
of ATP. At the beginning of contraction, when the ATP con-
centration begins to fall and that of ADP to rise, owing to
the increased rate of ATP breakdown by myosin, mass action
favors the formation of ATP from creatine phosphate. This
energy transfer is so rapid that the concentration of ATP in
a muscle fi ber changes very little at the start of contraction,
whereas the concentration of creatine phosphate falls rapidly.
Although the formation of ATP from creatine phosphate
is very rapid, requiring only a single enzymatic reaction, the
amount of ATP that this process can form is limited by the
initial concentration of creatine phosphate in the cell. (Many
athletes in sports that require rapid power output consume
creatine supplements in hopes of increasing the pool of
immediately available ATP in their muscles.) If contractile
activity is to continue for more than a few seconds, how-
ever, the muscle must be able to form ATP from the other
two sources listed previously. The use of creatine phosphate
at the start of contractile activity provides the few seconds
necessary for the slower, multienzyme pathways of oxidative
phosphorylation and glycolysis to increase their rates of ATP
formation to levels that match the rates of ATP breakdown.
At moderate levels of muscular activity, most of the
ATP used for muscle contraction is formed by oxidative phos-
phorylation, and during the fi rst 5 to 10 min of such exercise,
breakdown of muscle glycogen to glucose provides the major
fuel contributing to oxidative phosphorylation. For the next
30 min or so, blood-borne fuels become dominant, blood
glucose and fatty acids contributing approximately equally;
beyond this period, fatty acids become progressively more
important, and the muscle’s glucose utilization decreases.
3
2
1
ATP
Oxidative
phosphorylation
Glycolysis
Lactic acid
Glycogen
Creatine phosphate
Creatine
ADP + P
i
Fatty acids
Amino acids
Proteins
Ca
2+
-ATPase
Myosin ATPase
contraction
relaxation
Muscle fiber
Glucose
Oxygen
Fatty acids
Blood
Figure 9–22
The three sources of ATP production during muscle contraction: (1) creatine phosphate, (2) oxidative phosphorylation, and (3) glycolysis.
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