Chemistry Reference
In-Depth Information
Chapter 11
Electrostatic Interactions between
Lactoferrin and
b
-Lactoglobulin in
Oil-in-Water Emulsions
Aiqian Ye and Harjinder Singh
RIDDET CENTRE, MASSEY UNIVERSITY, PRIVATE BAG 11 222,
PALMERSTON NORTH, NEW ZEALAND
11.1 Introduction
Lactoferrin is a glycoprotein with around 700 amino acid residues and a molec-
ular weight of
8
10
4
Da.
1
The polypeptide is folded into two globular lobes,
representing its N- and C-terminal halves, which are commonly referred to as the
N lobe and the C lobe. These two lobes are linked together by a short a-helix
peptide and there is some 40% amino acid sequence identity between the N and C
lobes.
1-3
The most important feature of lactoferrin is its very high anity for iron;
consequently, its biological functions with respect to its strong bacteriostatic
properties depend on the iron-binding properties. The surface of the lactoferrin
molecule has several regions with high concentrations of positive charge, giving it
a high isoelectric point (pI
B
9). This positive charge is one of the features that
distinguishes lactoferrin from other milk proteins, such as b-lactoglobulin, which
have isoelectric points in the range 4.5-5.5 and are negatively charged at neutral
pH. Consequently, lactoferrin has been shown to interact with other milk
proteins. This interaction appears to have a certain degree of specificity, as it
occurs with b-lactoglobulin but not with a-lactalbumin.
4
The adsorption of lactoferrin has been studied onto solid surfaces
5,6
and at
air-water interfaces.
7
Wahlgren et al.
5
showed that electrostatic interactions
between lactoferrin and b-lactoglobulin caused an increase in the measured
amount of protein adsorbed on a hydrophilic silica surface, as compared with
single proteins. Neutron reflectivity measurements have revealed
7
a strong
structural unfolding of the lactoferrin molecule when adsorbed at the air-water
interface from a neutral pH buffer solution over a wide concentration range.
Two distinct regions, a top dense layer of 15-20 A
˚
on the air side and a bottom
diffuse layer of some 50 A
˚
E
into the aqueous phase, characterizes the unfolded
interfacial layer.
167
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