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Chapter 14 · Building Blocks of Life - Amino Acids
14
5 Spherical molecules are also called globular proteins (spheroidal proteins). They
are soluble in aqueous systems and can
diffuse easily. They therefore usually have
a dynamic function in the organism. Many
enzymes are globular proteins, such as the
proteins serum albumin and haemoglobin
contained in the blood.
5 Quaternary structure: Several peptide chains
(which are not covalently linked) form a
larger association. Haemoglobin, for example, consists of four protein subunits packed
together. This arrangement can often be
determined by X-ray structure analysis.
External influences can change the superstructure of the proteins. The tertiary structure can be
reversibly or irreversibly eliminated by increased
temperatures, pH-value changes or the addition of organic solvents, salts or detergents. This
process is called denaturation. One example is
liquid egg white, which coagulates into a solid
during cooking.
In addition to the chemical composition, the
steric structure is of great importance for the biological activity of proteins. A distinction is made
between:
5 Primary structure: It is determined by the
amino acid sequence.
5 Secondary structure: It takes into account
that the peptide chain is not stretched, but
that preferred conformations are formed.
α-Keratin, for example, forms an α-helix.
5 Tertiary structure: In addition to the secondary structure, the molecule also develops
larger folds, which are often induced by
disulfide bridges. Either folded fibrillar or
spherical molecules are formed:
5 The folded molecular fibrillary proteins are
also known as sclerotic or fibrous proteins.
Due to their fiber structure, they have a
high mechanical strength and are insoluble in water. They often perform protective
functions in the organism. Typical examples are α-keratin, the main component of
hair, skin and nails, and collagen, the main
component of tendons.
BOX: The Optimally Grilled Steak
The amino acids of proteins can
undergo chemical reactions.
In 1912, the Frenchman Louis
Camille Maillard found that
reducing sugars react with the
amino group of proteins to give
the proteins a brown color, a
“non-enzymatic browning”. In
the first step of the reaction
sequence, an N-glycosylamine
is formed, which is then
rearranged, dehydrated, cyclized
and degraded in numerous
further steps which have not
yet been fully elucidated.
Anyone grilling a steak can
easily follow this Maillard
reaction. The brown pigments,
known as melanoidins, are also
responsible for the delicious
roast aroma. However, there
are a few rules to follow when
grilling:
5 The higher the temperature,
the faster the chemical
reactions take place. At
temperatures significantly
above 180 °C and during
long grilling times, however,
unhealthy, sometimes
carcinogenic coke products
can be formed. Thus, it is
better to grill at medium
temperatures with limited
grilling time.
5 Adding a little sugar in the
meat marinade accelerates
Maillard reaction.
5 Dab the steak well
beforehand: Excess water
prolongs the preparation
time. The meat is rather
cooked than grilled. Less
aroma substances are
formed.
Chapter 14 · Building Blocks of Life - Amino Acids
14
5 Spherical molecules are also called globular proteins (spheroidal proteins). They
are soluble in aqueous systems and can
diffuse easily. They therefore usually have
a dynamic function in the organism. Many
enzymes are globular proteins, such as the
proteins serum albumin and haemoglobin
contained in the blood.
5 Quaternary structure: Several peptide chains
(which are not covalently linked) form a
larger association. Haemoglobin, for example, consists of four protein subunits packed
together. This arrangement can often be
determined by X-ray structure analysis.
External influences can change the superstructure of the proteins. The tertiary structure can be
reversibly or irreversibly eliminated by increased
temperatures, pH-value changes or the addition of organic solvents, salts or detergents. This
process is called denaturation. One example is
liquid egg white, which coagulates into a solid
during cooking.
In addition to the chemical composition, the
steric structure is of great importance for the biological activity of proteins. A distinction is made
between:
5 Primary structure: It is determined by the
amino acid sequence.
5 Secondary structure: It takes into account
that the peptide chain is not stretched, but
that preferred conformations are formed.
α-Keratin, for example, forms an α-helix.
5 Tertiary structure: In addition to the secondary structure, the molecule also develops
larger folds, which are often induced by
disulfide bridges. Either folded fibrillar or
spherical molecules are formed:
5 The folded molecular fibrillary proteins are
also known as sclerotic or fibrous proteins.
Due to their fiber structure, they have a
high mechanical strength and are insoluble in water. They often perform protective
functions in the organism. Typical examples are α-keratin, the main component of
hair, skin and nails, and collagen, the main
component of tendons.
BOX: The Optimally Grilled Steak
The amino acids of proteins can
undergo chemical reactions.
In 1912, the Frenchman Louis
Camille Maillard found that
reducing sugars react with the
amino group of proteins to give
the proteins a brown color, a
“non-enzymatic browning”. In
the first step of the reaction
sequence, an N-glycosylamine
is formed, which is then
rearranged, dehydrated, cyclized
and degraded in numerous
further steps which have not
yet been fully elucidated.
Anyone grilling a steak can
easily follow this Maillard
reaction. The brown pigments,
known as melanoidins, are also
responsible for the delicious
roast aroma. However, there
are a few rules to follow when
grilling:
5 The higher the temperature,
the faster the chemical
reactions take place. At
temperatures significantly
above 180 °C and during
long grilling times, however,
unhealthy, sometimes
carcinogenic coke products
can be formed. Thus, it is
better to grill at medium
temperatures with limited
grilling time.
5 Adding a little sugar in the
meat marinade accelerates
Maillard reaction.
5 Dab the steak well
beforehand: Excess water
prolongs the preparation
time. The meat is rather
cooked than grilled. Less
aroma substances are
formed.