Page 81 - Color Atlas of Biochemistry
P. 81
72 Biomolecules
Globular proteins tively cleaved again. The small plant protein
crambin (46 amino acids) contains three di-
Solubleproteins havea more complex struc- sulfide bonds and is therefore very stable. The
ture than the fibrous, completely insoluble high degree of stability of insulin (see p. 76)
structural proteins. The shape of soluble pro- has a similar reason.
teinsismore or lessspherical (globular). In
their biologically active form, globular
C. Protein dynamics
proteins have a defined spatial structure
(the native conformation). If this structure is The conformations of globular proteins are
destroyed (denaturation; see p. 74), not only not rigid, but can change dramatically on
does the biological effect disappear, but the binding of ligands or in contact with other
protein also usually precipitates in insoluble proteins. For example, the enzyme adenylate
form. This happens, for example, when eggs kinase (see p. 336) has a mobile domain (do-
are boiled; the proteins dissolved in the egg main = independently folded partial struc-
white are denatured by the heat and produce ture), which folds shut after binding of the
the solid egg white. substrate (yellow). The larger domain (bot-
To illustrate protein conformations in a tom) also markedly alters its conformation.
clear (but extremely simplified) way, Richard- There are large numbers of allosteric
son diagrams are often used. In these proteins of this type. This group includes, for
diagrams, α-helices are symbolized by red example, hemoglobin (see p. 280), calmodulin
cylinders or spirals and strands of pleated (see p. 386), and many allosteric enzymes
sheets by green arrows. Less structured areas such as aspartate carbamoyltransferase (see
of the chain, including the β-turns, are shown p.116).
as sections of gray tubing.
D. Folding patterns
A. Conformation-stabilizing interactions
The globular proteins show a high degree of
The native conformation of proteins is stabi- variability in folding of their peptide chains.
lized by a number of different interactions. Only a few examples are shown here. Purely
Among these, only the disulfide bonds (B) helically folded proteins such as myoglobin (1;
represent covalent bonds. Hydrogen bonds, see p. 74, heme yellow) are rare. In general,
which can form inside secondary structures, pleated sheet and helical elements exist
as well as between more distant residues, are alongside each other. In the hormone-binding
involved in all proteins (see p. 6). Many pro- domain of the estrogen receptor (2;see p. 378),
teins are also stabilized by complex formation a small, two-stranded pleated sheet functions
with metal ions (see pp. 76, 342, and 378, for as a “cover” for the hormone binding site
example). The hydrophobic effect is particu- (estradiol yellow). In flavodoxin, asmall flavo-
larly important for protein stability. In glob- protein with a redox function (3;FMN yel-
ular proteins, most hydrophobic amino acid low),a fan-shaped,pleated sheetmadeupof
residues are arranged in the interior of the five parallel strands forms the core of the
structure in the native conformation, while molecule. The conformation of the β subunit
the polar amino acids are mainly found on of the G-protein transducin (4;see pp. 224,
the surface (see pp. 28, 76). 358) is very unusual. Seven pleated sheets
form a large, symmetrical “β propeller.” The
N-terminal section of the protein contains one
B. Disulfide bonds
long and one short helix.
Disulfide bonds arise when the SH groups of
two cysteine residues are covalently linked as
a dithiol by oxidation. Bonds of this type are
only found (with a few exceptions) in extra-
cellular proteins, because in the interior of the
cell glutathione (see p. 284) and other reduc-
ing compounds are present in such high con-
centrations that disulfides would be reduc-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
