Food Chemistry

56 1 Amino Acids, Peptides, Proteins

Fig. 1.28.Tertiary structures (schematic: spiral:α-helix, arrow: pleated sheet) of theβ-chain of hemoglobin (a),
of triosephosphate isomerase (b) and carboxypeptidase (c). (according toWalton, 1981)

However, a PrPp forces the pathogenic confor-
mation 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 (Mr27-30 kDa) is released from PrPp.
This marker can be identified using the sandwich
ELISA (cf. 2.6.3).

1.4.2.3.4 QuaternaryStructures....................................

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 ir-
reversible change of native conformation (tertiary
structure) without cleavage of covalent bonds (ex-
cept for disulfide bridges). Denaturation is pos-
sible 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. 61

Papain Papaya latex 20. 71

α-Chymotrypsin Pancreas 23 1

(beef)

Trypsin Pancreas 23. 81

(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. 54

Avidin Chicken egg 68. 34

Alcohol-

dehydrogenase 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