Page 121 - Color Atlas of Biochemistry
P. 121
112 Metabolism
Intermediary metabolism arrows). Of the numerous metabolites in the
pool, only three particularly important repre-
Hundreds of chemical reactions are con- sentatives—pyruvate, acetyl-CoA, and gly-
stantly taking place in every cell, and taken cerol—are shown here. These molecules
together these are referred to as the metabo- represent connecting links between the
lism. The chemical compounds involved in metabolism of proteins, carbohydrates, and
this are known as metabolites.Outside of lipids. The metabolite pool also includes the
the cell, almost all of the chemical changes intermediates of the tricarboxylic acid cycle
in metabolites would only take place very (6). This cyclic pathway has both catabolic and
slowly and without any specific direction. By anabolic functions—i. e., it is amphibolic (vio-
contrast, organized sequences of chemical re- let; see p. 138).
actions with a high rate of throughput, known Waste products from the degradation of
as metabolic pathways, become possible organic substances in animal metabolism
through the existence of specific enzymes include carbon dioxide (CO 2 ), water (H 2 O),
(see p. 88). and ammonia (NH 3 ). In mammals, the toxic
substance ammonia is incorporated into urea
andexcretedin thisform (see p. 182).
A. Intermediary metabolism: overview
The most important form of storage for
A number of central metabolic pathways are chemical energy in all cells is adenosine
common to most cells and organisms. These triphosphate (ATP, see p. 122). ATP synthesis
pathways, which serve for synthesis, degra- requires energy—i. e., the reaction is ender-
dation, and interconversion of important me- gonic. Conversely, cleavage of ATP into ADP
tabolites, and also for energy conservation, and phosphate releases energy. Exergonic hy-
are referred to as the intermediary metabo- drolysis of ATP, as a result of energetic
lism. coupling (see p. 16), makes energy-depend-
In order to survive, all cells constantly re- ent (endergonic)processes possible. For ex-
quire organic and inorganic nutrients,as well ample, most anabolic pathways, as well as
as chemical energy, which is mainly derived movement and transport processes, are en-
from ATP (see below). Depending on the way ergy-dependent.
in which these needs are satisfied, organisms The most important pathway for the syn-
can be classified into autotrophic and hetero- thesis of ATP is oxidative phosphorylation
trophic groups. The autotrophs,which in- (see p. 122). In this process, catabolic path-
+
clude plants and many microorganisms, can ways first form reduced cofactors (NADH+H ,
synthesize organic molecules from inorganic QH 2 ,ETFH 2 ). Electrons are then transferred
precursors (CO 2 ). An autotrophic lifestyle is from these compounds to oxygen. This
possible through photosynthesis,for exam- strongly exergonic process is catalyzed by
ple (see p. 128). The heterotrophs—e. g., ani- the respiratory chain andusedindirectlyfor
mals and fungi—depend on organic substan- the ATP synthesis (see p. 140). In anaerobic
ces supplied in their diet. The schema shown conditions—i. e., in the absence of oxygen—
on this page provides an overview of animal most organisms can fall back on ATP that
metabolism. arises in glycolysis (3). This less ef cient
The polymeric substances contained in the type of ATP synthesis is referred to as fermen-
diet (proteins, carbohydrates, and nucleic tation (see p. 146).
acids—top) cannot be used by the organism While NADH exclusively supplies oxidative
+
directly. Digestive processes first have to de- phosphorylation, NADPH+H —a very similar
grade them to monomers (amino acids, sug- coenzyme—is the reducing agent for anabolic
+
ars, nucleotides). These are then mostly bro- pathways. NADPH + H is mainly formed in
kendownby catabolic pathways (pink ar- the pentose phosphate pathway (PPP, 1;see
rows) into smaller fragments. The metabolites p. 152).
produced in this way (generally referred to as
the “metabolite pool”) are then either used to
obtain energy through further catabolic con-
version, or are built up again into more com-
plex molecules by anabolic pathways (blue
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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