nature and extent of which is described as the secondary structure.
The three-dimensional shape of the
coiled or pleated polypeptides is
called the tertiary structure. Quaternary structure speciÜes the structural
relationship of the component
polypeptides.
Proteins may be broadly classiÜed
into globular proteins and Übrous
proteins. Globular proteins have
compact rounded molecules and are
usually water-soluble. Of prime importance are the *enzymes, proteins
that catalyse biochemical reactions.
Other globular proteins include the
antibodies, which combine with foreign substances in the body; the carrier proteins, such as *haemoglobin;
the storage proteins (e.g. casein in
milk and albumin in egg white), and
certain hormones (e.g. insulin). Fibrous proteins are generally insoluble in water and consist of long
coiled strands or Ûat sheets, which
confer strength and elasticity. In this
category are keratin and collagen.
Actin and myosin are the principal
Übrous proteins of muscle, the interaction of which brings about muscle
contraction. Blood clotting involves
the Übrous protein called Übrin.
When heated over 50°C or subjected to strong acids or alkalis, proteins lose their speciÜc tertiary
structure and may form insoluble coagulates (e.g. egg white). This usually
inactivates their biological properties.
protein engineering The techniques used to alter the structure of
proteins (especially enzymes) in
order to improve their use to humans. This involves artiÜcially modifying 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 produce
new proteins by cells or other systems containing the necessary factors for transcription and translation.
Alternatively, new proteins can be
synthesized by solid state synthesis,
in which polypeptide chains are assembled under the control of chemicals. 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 engineering is used to synthesize enzymes (so-called ‘designer enzymes’)
used in biotechnology. The threedimensional tertiary structure of proteins is crucially important for their
function, and this can be investigated
using computer-aided modelling.
protein synthesis The process by
which living cells manufacture proteins 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 nucleotides in messenger RNA. At the
ribosomes the information carried by
messenger RNA is translated into the
sequence of amino acids of the protein 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
–27 kg, which is
1836.12 times that of the electron.
The proton is a hydrogen ion and ocprotein engineering
442
p
www.AzShimi.ir www.AzShimi.com
The three-dimensional shape of the
coiled or pleated polypeptides is
called the tertiary structure. Quaternary structure speciÜes the structural
relationship of the component
polypeptides.
Proteins may be broadly classiÜed
into globular proteins and Übrous
proteins. Globular proteins have
compact rounded molecules and are
usually water-soluble. Of prime importance are the *enzymes, proteins
that catalyse biochemical reactions.
Other globular proteins include the
antibodies, which combine with foreign substances in the body; the carrier proteins, such as *haemoglobin;
the storage proteins (e.g. casein in
milk and albumin in egg white), and
certain hormones (e.g. insulin). Fibrous proteins are generally insoluble in water and consist of long
coiled strands or Ûat sheets, which
confer strength and elasticity. In this
category are keratin and collagen.
Actin and myosin are the principal
Übrous proteins of muscle, the interaction of which brings about muscle
contraction. Blood clotting involves
the Übrous protein called Übrin.
When heated over 50°C or subjected to strong acids or alkalis, proteins lose their speciÜc tertiary
structure and may form insoluble coagulates (e.g. egg white). This usually
inactivates their biological properties.
protein engineering The techniques used to alter the structure of
proteins (especially enzymes) in
order to improve their use to humans. This involves artiÜcially modifying 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 produce
new proteins by cells or other systems containing the necessary factors for transcription and translation.
Alternatively, new proteins can be
synthesized by solid state synthesis,
in which polypeptide chains are assembled under the control of chemicals. 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 engineering is used to synthesize enzymes (so-called ‘designer enzymes’)
used in biotechnology. The threedimensional tertiary structure of proteins is crucially important for their
function, and this can be investigated
using computer-aided modelling.
protein synthesis The process by
which living cells manufacture proteins 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 nucleotides in messenger RNA. At the
ribosomes the information carried by
messenger RNA is translated into the
sequence of amino acids of the protein 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
–27 kg, which is
1836.12 times that of the electron.
The proton is a hydrogen ion and ocprotein engineering
442
p
www.AzShimi.ir www.AzShimi.com