2009_Book_FoodChemistry

56 1 Amino Acids, Peptides, Proteins
Fig. 1.28. Tertiary structures (schematic: spiral: α-helix, arrow: pleated sheet) of the β-chain of hemoglobin (a),
of triosephosphate isomerase (b) and carboxypeptidase (c). (according to Walton, 1981)
However, a PrPp forces the pathogenic conformation
on a PrPn. The stability towards serine
proteinase K from the fungus Tritirachium
album is used to differentiate between PrPp
and PrPn. Serine proteinase K, which attacks
the carboxyl side of hydrophobic amino acids,
largely hydrolyzes PrPn while a characteristic
peptide (M r 27-30 kDa) is released from PrPp.
This marker can be identified using the sandwich
ELISA (cf. 2.6.3).
1.4.2.3.4 Quaternary Structures
In addition to the free energy gain by folding
of a single peptide chain, association of more
than one peptide chain (subunit) can provide
further gains in free energy. For example,
hemoglobin (4 associated peptide chains)
G 0 = −46 kJ mole −1 and the trypsin-trypsin
inhibitor complex (association of 2 peptide
chains) G 0 = − 75.2kJmole −1 . In principle
such associations correspond to the folding of
a larger peptide chain with several structural
domains without covalently binding the subunits.
Table 1.26 lists some proteins which partially
exhibit quaternary structures.
1.4.2.4 Denaturation
The term denaturation denotes a reversible or irreversible
change of native conformation (tertiary
structure) without cleavage of covalent bonds (except
for disulfide bridges). Denaturation is possible
with any treatment that cleaves hydrogen
Table 1.26. Examples of globular proteins
Name Origin Molecular Number
weight of
(Kdal) subunits
Lysozyme Chicken egg 14.6 1
Papain Papaya latex 20.7 1
α-Chymotrypsin Pancreas 23 1
(beef)
Trypsin Pancreas 23.8 1
(beef)
Pectinesterase Tomato 27.5
Chymosin Stomach 31
(calf)
β-Lactoglobulin Milk 35 2
Pepsin A Stomach 35 1
(swine)
Peroxidase Horseradish 40 1
Hemoglobin Blood 64.5 4
Avidin Chicken egg 68.3 4
Alcoholdehydrogenase
Liver (horse) 80 2
Yeast 150 4
Hexokinase Yeast 104 2
Lactate
dehydrogenase Heart (swine) 135 4
Glucose oxidase P. notatum 152
Pyruvate kinase Yeast 161 8
A. niger 186
β-Amylase Sweet potato 215 4
Catalase Liver (beef) 232 4
M. lysodeikticus 232
Adenosine
triphosphatase Heart (beef) 284 6
Urease Jack beans 483 6
Glutamine
synthetase E. coli 592 12
Arginine
decarboxylase E. coli 820 10