Page 345 - Color Atlas of Biochemistry
P. 345
336 Tissues and organs
Muscle metabolism I maintain glucose degradation and thus ATP
formation. If there is a lack of O 2 ,this is
Muscle contraction is associated with a high achieved by the formation of lactate,which
level of ATP consumption (see p. 332). With- is released into the blood and is resynthesized
out constant resynthesis, the amount of ATP into glucose in the liver (Cori cycle; see
available in the resting state would be used up p. 338).
in less than 1 s of contraction. Muscle-specific auxiliary reactions for ATP
synthesis exist in order to provide additional
ATP in case of emergency. Creatine phosphate
A. Energy metabolism in the white and red
(see B) acts as a buffer for the ATP level.
muscle fibers
Another ATP-supplying reaction is catalyzed
Muscles contain two types of fibers, the pro- by adenylate kinase [1] (see also p. 72). This
portions of which vary from one type of disproportionates two molecules of ADP into
muscle to another. Red fibers (type I fibers) ATP and AMP. The AMP is deaminated into
are suitable for prolonged effort. Their metab- IMP in a subsequent reaction [2] in order to
olism is mainly aerobic and therefore depends shift the balance of the reversible reaction [1]
on an adequate supply of O 2 . White fibers in the direction of ATP formation.
(type II fibers) are better suited for fast, strong
contractions. These fibers are able to form
B. Creatine metabolism
suf cient ATP even when there is little O 2
available. With appropriate training, athletes Creatine (N-methylguanidoacetic acid) and its
and sports participants are able to change the phosphorylated form creatine phosphate
proportions of the two fiber types in the mus- (a guanidophosphate) serve as an ATP buffer
culature and thereby prepare themselves for in muscle metabolism. In creatine phosphate,
the physiological demands of their disciplines the phosphate residue is at a similarly high
in a targeted fashion. The expression of func- chemical potential as in ATP and is therefore
tional muscle proteins can also change during easily transferred to ADP. Conversely, when
thecourseof training. there is an excess of ATP, creatine phosphate
Red fibers provide for their ATP require- can arise from ATP and creatine. Both proce-
ments mainly (but not exclusively) from fatty sses are catalyzed by creatine kinase [5].
acids,which arebrokendown via β-oxidation, In resting muscle, creatine phosphate
the tricarboxylic acid cycle, and the respira- forms due to the high level of ATP. If there is
tory chain (right part of the illustration). The a risk of a severe drop in the ATP level during
red color in these fibers is due to the mono- contraction, the level can be maintained for a
meric heme protein myoglobin,which they short time by synthesis of ATP from creatine
use as an O 2 reserve. Myoglobin has a much phosphate and ADP. In a nonenzymatic reac-
higher af nity for O 2 than hemoglobin and tion [6], small amounts of creatine and crea-
therefore only releases its O 2 when there is tine phosphate cyclize constantly to form cre-
a severe drop in O 2 partial pressure atinine, which can no longer be phosphory-
(cf. p. 282). lated and is therefore excreted with the urine
At a high level of muscular effort—e. g., (see p. 324).
during weightlifting or in very fast contrac- Creatine does not derive from the muscles
tionssuch asthose carried out by the eye themselves, but is synthesized in two steps in
muscles—the O 2 supply from the blood the kidneys and liver (left part of the illustra-
quickly becomes inadequate to maintain the tion). Initially, the guanidino group of argi-
aerobic metabolism. White fibers (left part of nine is transferred to glycine in the kidneys,
the illustration) therefore mainly obtain ATP yielding guanidino acetate [3]. In the liver,
from anaerobic glycolysis. They have supplies N-methylation of guanidino acetate leads to
of glycogen from which they can quickly re- the formation of creatine from this [4]. The
lease glucose-1-phosphate when needed (see coenzyme in this reaction is S-adenosyl methi-
p. 156). By isomerization, this gives rise to onine (SAM; see p.110).
glucose-6-phosphate, the substrate for glycol-
+
ysis. The NADH+H formed during glycolysis
+
has to be reoxidized into NAD in order to
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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