Page 73 - Color Atlas of Biochemistry

P. 73

64 Biomolecules

Peptides and proteins: overview hormone somatotropin and its receptor is
shown here as an example (middle). Here,
the extracellular domains of two receptor
A. Proteins
molecules here bind one molecule of the hor-
When amino acids are linked together by mone. This binding activates the cytoplasmic
acid–amide bonds, linear macromolecules domains of the complex, leading to further
(peptides) are produced. Those containing conduction of the signal to the interior of
more than ca. 100 amino acid residues are thecell(seep. 384).The smallpeptide
described as proteins (polypeptides). Every hormone insulin is discussed in detail else-
organism contains thousands of different pro- where (see pp. 76, 160). DNA-binding proteins
teins, which have a variety of functions. At a (transcription factors; see p.118) are decisively
magnification of ca. 1.5 million, the semi- involved in regulating the metabolism and in
schematic illustration shows the structures differentiation processes. The structure and
of a few intra and extracellular proteins, giv- function of the catabolite activator protein
ing an impression of their variety. The func- (top left) and similar bacterial transcription
tions of proteins can be classified as follows. factors have been particularly well investi-
Establishment and maintenance of struc- gated.
ture. Structural proteins are responsible for Catalysis. Enzymes, with more than 2000
the shape and stability of cells and tissues. A known representatives, are the largest group
small part of a collagen molecule is shown as of proteins in terms of numbers (see p. 88).
an example (right; see p. 70). The complete The smallest enzymes have molecular masses
molecule is 1.5 300 nm in size, and at the of 10–15 kDa. Intermediatesized enzymes,
magnification used here it would be as long as such as alcohol dehydrogenase (top left) are
three pages of the book. Histones are also around 100–200 kDa, and the largest—
structural proteins. They organize the ar- including glutamine synthetase with its 12
rangement of DNA in chromatin. The basic monomers (top right)—can reach more than
components of chromatin, the nucleosomes 500 kDa.
(top right; see p. 218) consist of an octameric Movement. The interaction between actin
complex of histones, around which the DNA is and myosin is responsible for muscle contrac-
coiled. tion and cell movement (see p. 332). Myosin
Transport. A wellknown transport protein (right), with a length of over 150 nm, is
is hemoglobin in the erythrocytes (bottom among the largest proteins there are. Actin
left). It is responsible for the transport of oxy- filaments (F-actin)arise dueto the polymer-
gen and carbon dioxide between the lungs ization of relatively small protein subunits (G-
and tissues (see p. 282). The blood plasma actin). Along with other proteins, tropomyo-
also contains many other proteins with trans- sin, which is associated with F-actin, controls
port functions. Prealbumin (transthyretin; contraction.
middle), for example, transports the thyroid Storage. Plants contain special storage pro-
hormones thyroxin and triiodothyronine. Ion teins, which are also important for human
channels and other integral membrane pro- nutrition (not shown). In animals, muscle
teins (see p. 220) facilitate the transport of proteins constitute a nutrient reserve that
ions and metabolites across biological mem- can be mobilized in emergencies.
branes.
Protection and defense. The immune sys-
tem protects the body from pathogens and
foreign substances. An important component
of this system is immunoglobulin G (bottom
left; see p. 300). The molecule shown here is
bound to an erythrocyte by complex forma-
tion with surface glycolipids (see p. 292).
Control and regulation. In biochemical sig-
nal chains, proteins function as signaling sub-
stances (hormones) and as hormone recep-
tors. The complex between the growth

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