membrane bilayer of animal cells that are subject to physical or chemical stress.
Furthermore, among several other things, they are responsible for the AB0 blood
system of humans.
2.2.3.3 Proteins
Proteins fulfill numerous highly important functions in the cell, of which only a few
can be mentioned here. They build up the cytoskeletal framework, which forms the
cellular structure and is responsible for cell movements (motility). Proteins are also
part of the extracellular supportive framework (extracellular matrix), e.g., as collagen
in animals. As catalytic enzymes for highly specific biochemical reactions, they rule
and control the metabolism of a single cell or whole organism. Furthermore, pro-
teins regulated by transient modifications are relevant for signal transduction, e.g.,
proteins controlling cell division such as cyclin-dependent protein kinases (CDK).
A further highly important function of proteins is their ability to control the tran-
scription and translation of genes as well as the degradation of proteins (see Sec-
tion 2.4).
Proteins consist of one or more polypeptides. Each polypeptide is composed of
covalently linked amino acids; these covalent bonds are called peptide bonds. Such a
bond is formed by a condensation reaction between the amino group of one amino
acid and the carboxyl group of another (Fig. 2.6a). The primary structure of a poly-
peptide is coded by the genetic information that defines in which order amino acids
– chosen from a set of 20 different ones – appear. Figure 2.5 shows the chemical
structures of these amino acids. Common to all amino acids is a central carbon
(a-carbon), which carries an amino group (except for proline, where this is a ring-
forming imino group), a carboxyl group, and a hydrogen. Furthermore, it carries a
residual group with different physicochemical properties, due to which the amino
acids can be divided into different groups, such as amino acids that carry (1) nonpo-
lar residues that can grant lipophobic characteristics, (2) uncharged polar residues,
(3) residues that contain a carboxyl group that is negatively charged at physiological
pH and thus act as acids, and (4) residues that are usually positively charged at com-
mon pH ranges of living cells and thus show basic characteristics. Due to the combi-
nation of possibilities of these amino acids, proteins are very diverse. Usually, pro-
teins are assembled from about 50 to 1000 amino acids, but they might be much
smaller or larger. Except for glycine, the a-carbon of amino acids binds four different
residues and therefore amino acids can occur in two different isoforms that behave
like an image and its mirror image. These two forms are called the L-isoform and
the D-isoform, of which only the L-isoform is used in naturally occurring proteins.
Furthermore, amino acids of proteins are often altered posttranslational. For in-
31
2.2 Molecular Biology of the Cell
3 Fig. 2.4 (a) Fatty acids represent one part of
fats and phospholipids. They are either saturated
or unsaturated. (b) Triglycerides are formed by
condensation reactions of glycerol and three
fatty acids. (c) In phospholipids the third carbon
of glycerol is bound to a polar group via a phos-
phate group (P), which is usually ethanolamine,
choline, glycerol, serine, threonine, or inositol.
(d) Steroids constitute another major lipid class.
They are formed by four condensed carbon
rings. Cholesterol, shown here, is important,
e.g., for membrane fluidity of eukaryotic cells.