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PROTEINS
Proteins and their properties are of significance in food
composition, processing, preparation and storage. The chemical,
physical and physico-chemical characteristics of the protein is
influenced by the chemical nature and physical conformation of
protein molecules.
Chemically proteins are high polymers of the basic residue-an
amino acide. Amino acids usually contain an amino group and a
carboxyl group. In he table below the general formula for an
amino acid is shown. The "R" group designates that portion of
the amino acid which varies to give the different types of amino
acids of varying characteristics. There are approximately 20
common amino acids.
Amino acids are amphoteric organic compounds, otherwise, they
have both basic and acidic characteristics. on the basis of the
characteristics of the general formula this is indicated by the
following:
H2NCHROOO-
(pH about 10:basic)
+H3NCHROOO-
(pH around 4 to 9)
+H3NCHFROOOH
(pH around 2 to 3: acid)
The ionized amino acid at pH 4 to 9, depending upon the type of
amino acid, is sometimes called a ZWITTERION as both the carboxyl
and the amine group are ionized. When this situation is present
we sometimes indicate that this is the isoelectric point of the
amino acid complex - a protein. The table below indicates the
validity of the statement that the side chains of the amino acid
have basic or acidic characteristics and thus would alter the
extent of ionization at any given pH
The amino acids form the protein due to the reaction
amine group of one amino acid and the carboxyl group
The conformation of a protein molecule in the native
determined by: the primary structure, the secondary
tertiary structure.
between the
of another.
state is
structure, a
The PRIMARY STRUCTURE is the combination of amino acids in a
proper sequence by means of the peptide bonds. no other forces
or bonds are implied by this structural level designation.
SECONDARY STRUCTURE is that which forms a pleated or helix
structure. The alpha-helix is stabilized by hydrogen bonding
between carbonyl and the amide groups of the peptide bonds which
generally appear in a regular sequence along the chain of amino
acides. A TERTIARY structure is the folding of the coiled chain
or chains and is thought to influence the degree of hydration
associated with the "R" groups. The stabilization of this
structure has been ascribed to the side chain characteristics
and their interaction.
Covalent, hydrogen and hydrophobic bonding, ionic bridges,
disulfide linkages and van der Waals forces may be involved in
the structural organization of protein molecules.
AMINO ACID
General Formula
H
|
HOOC -C--R
|
NH2
hydrophilic-hydrophobic
glycine
-- H
hydrophobic
alanine
-- CH3
hydrophobic
leucine
-- CH2-CH(CH3)2
phenylalanine
-- CH2 - benezene ring
cysteine
-- CH2SH
cystine
-- CH2 - S
|
-- CH2 - S
hydroxyproline
hydrophilic
hydrophobic
hydrophobic
hydrophilic
hydrophilic
HOOC-CH -- CH2
|
|
|
\
CH-OH
/
HN- -- CH2
aspartic acid
-- CH2COOH
lysine
hydrophilic
-- CH2(CH2)3HN2
hydrophilic