214
(2008) proposed that α-helical structure is associated with the amphiphilic
nature due to the presence of helix surface being made up of hydrophobic
(non polar residues) as well as hydrophyllic (polar) residues. The nonpolar
residues may also position themselves in the hydrophobic interior of the
protein molecule. The α-helixis sometimes also termed the 3.6 13 -helix, due
to the 13 backbone atoms in the hydrogen bonded loop in the structure of
this molecule. It should be noted that out of 20 amino acids, the cyclic imino
acid proline cannot form α- helices which is because of its ring structure the
N-Cα bond is unable to twist and the φ angle remains fixed at 70° due to this
reason it is also known as ‘α helix breaker’ which is because of its ring
structure. It should also be noted that of the 20 amino acids commonly
found in foods, only proline is likely to adopt the cis- isomer (Damodaran
2008; Nelson and Cox 2013).
(b). Beta-sheet proteins: Some proteins are largely formed from beta—sheets
and include some antibodies and T-cell receptors. There are two types of
β-pleated sheet structures, parallel β-sheet or antiparallel β-sheet depending
on the direction of the polypeptide strands. In parallel β-sheet the N → C
strands run parallel, whereas in the antiparallel β-sheet N → C strands run
in opposite direction. In the antiparallelβ-sheet structure hydrogen bonds
form a straight line which provides additional stability to the structure making antiparallel β-sheet more stable than its parallel β-sheet counterpart. In
the parallel β-sheet structure hydrogen bonds are formed at an angle, thus
the stability of the hydrogen bonds, and therefore the stability of the structure are reduced. Also, generally, β-sheet is more stable than the α-helix
structure; therefore, proteins with large segments of β-sheet structures are
likely to be more heat stable or have higher denaturation temperatures
(Nelson and Cox 2013; Voet et al. 2013).
(c). Alpha-helix and beta sheet proteins: Some proteins contain both alpha helix
and beta sheets. Hexokinase is an example of alpha/beta structure
3. Tertiary structure
Tertiary structure of the protein includes the total three dimensional arrangement
of the polypeptide chain comprising of hydrophobic interactions, hydrogen
bonds (non-covalent bonds in general) and sulfur-bridges. It also includes simple
dimmers to homo-oligomers and complexes with defined or variable numbers of
subunits. The tertiary structure of proteins depends largely on the sequence of
amino acids in a polypeptide and is stabilized through hydrogen bonding between
CO and NH groups e.g. heamoglobin.
4. Quaternary structure
It is the association of two or more polypeptides into a multi-subunit complex.
Heamoglobin is one example of quaternary structure another example is collagen
which is a widespread connective tissue protein and consists of three polypeptide
chains. It has been observed that proteins having molecular weight greater than
100 kDa are likely to have more than one polypeptide and more likely to have
quaternary structures. Many food proteins having quaternary structures include
N. A. Mir et al.
(2008) proposed that α-helical structure is associated with the amphiphilic
nature due to the presence of helix surface being made up of hydrophobic
(non polar residues) as well as hydrophyllic (polar) residues. The nonpolar
residues may also position themselves in the hydrophobic interior of the
protein molecule. The α-helixis sometimes also termed the 3.6 13 -helix, due
to the 13 backbone atoms in the hydrogen bonded loop in the structure of
this molecule. It should be noted that out of 20 amino acids, the cyclic imino
acid proline cannot form α- helices which is because of its ring structure the
N-Cα bond is unable to twist and the φ angle remains fixed at 70° due to this
reason it is also known as ‘α helix breaker’ which is because of its ring
structure. It should also be noted that of the 20 amino acids commonly
found in foods, only proline is likely to adopt the cis- isomer (Damodaran
2008; Nelson and Cox 2013).
(b). Beta-sheet proteins: Some proteins are largely formed from beta—sheets
and include some antibodies and T-cell receptors. There are two types of
β-pleated sheet structures, parallel β-sheet or antiparallel β-sheet depending
on the direction of the polypeptide strands. In parallel β-sheet the N → C
strands run parallel, whereas in the antiparallel β-sheet N → C strands run
in opposite direction. In the antiparallelβ-sheet structure hydrogen bonds
form a straight line which provides additional stability to the structure making antiparallel β-sheet more stable than its parallel β-sheet counterpart. In
the parallel β-sheet structure hydrogen bonds are formed at an angle, thus
the stability of the hydrogen bonds, and therefore the stability of the structure are reduced. Also, generally, β-sheet is more stable than the α-helix
structure; therefore, proteins with large segments of β-sheet structures are
likely to be more heat stable or have higher denaturation temperatures
(Nelson and Cox 2013; Voet et al. 2013).
(c). Alpha-helix and beta sheet proteins: Some proteins contain both alpha helix
and beta sheets. Hexokinase is an example of alpha/beta structure
3. Tertiary structure
Tertiary structure of the protein includes the total three dimensional arrangement
of the polypeptide chain comprising of hydrophobic interactions, hydrogen
bonds (non-covalent bonds in general) and sulfur-bridges. It also includes simple
dimmers to homo-oligomers and complexes with defined or variable numbers of
subunits. The tertiary structure of proteins depends largely on the sequence of
amino acids in a polypeptide and is stabilized through hydrogen bonding between
CO and NH groups e.g. heamoglobin.
4. Quaternary structure
It is the association of two or more polypeptides into a multi-subunit complex.
Heamoglobin is one example of quaternary structure another example is collagen
which is a widespread connective tissue protein and consists of three polypeptide
chains. It has been observed that proteins having molecular weight greater than
100 kDa are likely to have more than one polypeptide and more likely to have
quaternary structures. Many food proteins having quaternary structures include
N. A. Mir et al.