Food Biochemistry and Food Processing (2 edition)

BLBS102-c26 BLBS102-Simpson March 21, 2012 13:51 Trim: 276mm X 219mm Printer Name: Yet to Come

504 Part 4: Milk

Table 26.5.Properties of Equine and Bovineβ-Lactoglobulin (β-Lg) and Equine Bovine and Humanα-Lactalbumin

(α-La) and Lactoferrin.

Protein Species Variant

Primary

Accession

Numbera

Amino

Acid

Residues

Molecular

Mass (Da)

Isoelectric

point

GRAVY

Scoreb

Disulphide

Bridges

β-Lg Equine I P02758 162 18500.2 4.85 −0.386 2

II P07380 163 18261.6 4.71 −0.300 2

Bovine A P02754 162 18367.3 4.76 −0.167 2

B P02754 162 18281.2 4.83 −0.162 2

α-La Equine A P08334 123 14223.2 4.95 −0.416 4 c

B P08896 123 14251.2 4.95 −0.503 4 c

C P08896 123 14249.3 5.11 −0.438 4 c

Bovine P00711 123 14186.0 4.80 −0.453 4

Human P00709 123 14078.1 4.70 −0.255 4

Lactoferrin Equine O77811 689 75420.4 8.32 −0.376 17

Bovine P24627 689 76143.9 8.67 −0.350 16 d

Human P02788 691 76165.2 8.47 −0.415 16

Source: Modified from Uniacke et al. 2010.

Values were calculated from the amino acid sequences of the mature proteins provided on http://au.expasy.org.

aPrimary accession number for the protein in SWISS-PROT database.

bGrand average hydropathy (GRAVY) score using the scale of Kyte and Doolittle (1982).

cEstimated from structural similarity with bovine and humanα-La.

dEstimated from structural similarity with human lactoferrin.

−0.386 and−0.300, respectively (Table 26.5). Bovineβ-Lg A

and B have a GRAVY score of−0.167 and−0.162, respectively

(Table 26.5), and are, therefore, considered to be less hydrophilic

than equineβ-Lg I and II. Both equine and bovineβ-Lg contain

two intramolecular disulphide bridges, linking Cys 66 to Cys 160

and Cys 106 to Cys 119 in equineβ-Lg I, Cys 66 to Cys 161 and Cys 106

to Cys 120 in equineβ-Lg II and Cys 66 to Cys 160 and Cys 106 to

Cys 119 or Cys 121 in bovineβ-Lg A and B. Bovineβ-Lg contains

one sulphydryl group at Cys 119 or Cys 121. Equineβ-Lg contains

only four cysteine residues and lacks a sulphydryl group that

has major implications for denaturation and aggregation of the

protein (see later).

At physiological conditions (neutral pH andβ-Lg concentra-

tion> 50 μM), bovineβ-Lg occurs predominantly in dimeric

form and at its isoelectric point (pH 3.7–5.2) the dimers as-

sociate into octamers but below pH 3.4 and above pH 8.0 the

protein dissociates into its monomeric form (Gottschalk et al.

2003). Equine and asinineβ-Lg I exist in the monomeric form

only (Godovac-Zimmermann et al. 1990).

α-Lactalbumin

α-La, a unique milk protein, is homologous with the well-

characterised C-type Lyz. It is a calcium metalloprotein, in which

the Ca^2 +plays a crucial role in folding and structure and has

a regulatory function in the synthesis of lactose (Larson 1979,

Brew 2003, Neville 2009).

Similar to theα-La of asinine, bovine, caprine, ovine, camelid

and human milk, equineα-La contains 123 amino acids (Brew

2003). Equineα-La occurs as three genetic variants, A, B

and C, which differ by only a few single amino acid replace-

ments (Godovac-Zimmermann et al. 1987). Bovineα-La occurs

as two, or possibly three, genetic variants (Bell et al. 1981b)

and humanα-La has two genetic variants, one of which has

been identified only recently (Chowanadisai et al. 2005). The

primary structure of equine, bovine and humanα-La differ only

by a few single amino acid replacements, and the proteins have

similar properties (Table 26.5). Equineα-La A, B and C have an

isoelectric point at pH 4.95, 4.95 and 5.11, respectively, whereas

bovine and humanα-La have isoelectric point at pH 4.80 and

4.70, respectively (Table 26.5). The GRAVY scores of equine

and bovineα-La are comparable, whereas that of humanα-La is

distinctly higher (Table 26.5), indicating a lower hydrophobicity.

The eight-cysteine residues of bovine and humanα-La form four

intramolecular disulphide bonds, linking Cys 6 to Cys 120 ,Cys 28

to Cys 111 ,Cys 61 to Cys 77 and Cys 73 to Cys 93. On the basis of the

very high similarity between equine, bovine and humanα-Las,

as well as theα-La of other species, it is very likely that equine

α-La also contains four intramolecular disulphide bridges, in the

aforementioned positions. Equine, bovine or humanα-La does

not contain a sulphydryl group.

Three genetic variants of equineα-La have been reported

but asinine milk has only one (123 amino acid residues, Mw

approximately 14.2 kDa and four disulphide bonds), although

some heterogeneity has been shown. Two isoforms, A and B, of

asinineα-La (whose isoelectric points differ by 0.23 units) have

been reported but subsequent analysis showed that the protein

has only one form and misidentification in earlier work was

probably due to differences in calcium binding by asinineα-La

(Giuffrida et al. 1992). The primary structure of asinineα-La