nature and extent of which is de-
scribed as the secondary structure.
The three-dimensional shape of the
coiled or pleated polypeptides is
called the tertiary structure. Quater-
nary structure speciÜes the structural
relationship of the component
polypeptides.
Proteins may be broadly classiÜed
into globular proteins andÜbrous
proteins. Globular proteins have
compact rounded molecules and are
usually water-soluble. Of prime im-
portance are the *enzymes, proteins
that catalyse biochemical reactions.
Other globular proteins include the
antibodies, which combine with for-
eign substances in the body; the car-
rier proteins, such as *haemoglobin;
the storage proteins (e.g. casein in
milk and albumin in egg white), and
certain hormones (e.g. insulin). Fi-
brous proteins are generally insolu-
ble in water and consist of long
coiled strands orÛat sheets, which
confer strength and elasticity. In this
category are keratin and collagen.
Actin and myosin are the principal
Übrous proteins of muscle, the inter-
action of which brings about muscle
contraction. Blood clotting involves
theÜbrous protein calledÜbrin.
When heated over 50°C or sub-
jected to strong acids or alkalis, pro-
teins lose their speciÜc tertiary
structure and may form insoluble co-
agulates (e.g. egg white). This usually
inactivates their biological proper-
ties.protein engineering The tech-
niques used to alter the structure of
proteins (especially enzymes) in
order to improve their use to hu-
mans. This involves artiÜcially modi-
fying the DNA sequences that encode
them so that, for example, new
amino acids are inserted into existing
proteins. Synthesized lengths of
novel DNA can be used to producenew proteins by cells or other sys-
tems containing the necessary fac-
tors for transcription and translation.
Alternatively, new proteins can be
synthesized by solid state synthesis,
in which polypeptide chains are as-
sembled under the control of chemi-
cals. One end of the chain is
anchored to a solid support and the
chemicals selectively determine
which amino acids are added to the
free end. The appropriate chemicals
can be renewed during the process;
when synthesized, the polypeptide is
removed and puriÜed. Protein engi-
neering is used to synthesize en-
zymes (so-called ‘designer enzymes’)
used in biotechnology. The three-
dimensional tertiary structure of pro-
teins is crucially important for their
function, and this can be investigated
using computer-aided modelling.protein synthesis The process by
which living cells manufacture pro-
teins from their constituent amino
acids, in accordance with the genetic
information carried in the DNA of
the chromosomes. This information
is encoded in messenger *RNA,
which is transcribed from DNA in
the nucleus of the cell: the sequence
of amino acids in a particular protein
is determined by the sequence of nu-
cleotides in messenger RNA. At the
ribosomes the information carried by
messenger RNA is translated into the
sequence of amino acids of the pro-
tein in the process of translation.proteolysis The enzymic splitting
of proteins. See protease.proteolytic enzyme See protease.proton An elementary particle that
is stable, bears a positive charge
equal in magnitude to that of the
*electron, and has a mass of
1.672 614 × 10 –27kg, which is
1836.12 times that of the electron.
The proton is a hydrogen ion and oc-protein engineering 442p