3 Protein–Protein Complex Formation
Protein molecules are special polyelectrolytes in that their charge sign and charge
density are a function of the pH (see Fig. 2). Moreover, the charge density of these
molecules is rather low because there are only a few amino acids that are charged
(arginine, histidine and lysine are positively charged; aspartic acid and glutamic
acid are negatively charged). Most protein molecules form globular structures in an
aqueous environment due to a delicate interplay between hydrophobic and hydrophilic amino acids. The specific folding of these globular structures enables protein
molecules to perform special tasks as, e.g., enzymes or as building blocks.
Studying complex formation between protein molecules can give insight into
biological processes. The cytosol of living cells is a complex mixture containing
many different protein molecules that differ in size, charge (sign) and function.
Electrostatic interactions between these proteins might play an important role in
the stability of the cytoplasm. Also, in food, protein molecules are important for the
structure and texture, and for the way the food is experienced in the mouth. In the
context of this chapter we will discuss protein–protein complex formation as a
special kind of polyelectrolyte complex formation.
Oppositely charged protein molecules could be an interesting model system for
studying the interactions between similar, but oppositely charged nanoparticles.
Biesheuvel et al. studied complex formation between lysozyme and succinylated
lysozyme. The latter protein is a chemically modified form of lysozyme and has the
opposite charge to lysozyme (i.e., negative instead of positive) at pH 7.5 [81]. At
low salt concentration, mixtures of these two protein molecules form precipitates.
These precipitates dissolve when the ionic strength or temperature is increased [82].
The redissolution of these +/À protein precipitates at elevated temperature
indicates that attractive forces other than electrostatic attraction (e.g., hydrophobic
interactions and hydrogen bonding) are important in this system because electrostatic attraction is not, or only minimally, dependent on temperature. This is not
entirely surprising because many amino acids are hydrophobic or are able to form
hydrogen bonds.
Figure 17 shows a plot of F
+ versus salt concentration for compositions with a
fixed turbidity (measured as transmission of 95%). Like the plot of DLS intensity as
a function of composition (see Sect. 2.1 on DLS), a maximum intensity is found at
F
+
¼ 0.5. The experimental results of this system could theoretically be described
using a heterogeneous Poison–Boltzmann cell model. This model assumes stronger
correlations than, e.g., Voorn and Overbeek, taking the spatial structure within the
complex into account [19]. It was developed to estimate the electrostatic free
energy of strong complexes of oppositely charged flexible polyelectrolytes.
In cell models, each charged colloid, or in this case protein molecule, is surrounded
by solvent and small (salt) ions, which form one cell. The Poison–Boltzmann
equation is solved for the space between the protein molecule and the cell edge.
In one-component cell models, the boundary conditions at each cell edge are fixed
and each cell itself is electroneutral [83–86]. Here, we are dealing with a system
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S. Lindhoud and M.A. Cohen Stuart
Protein molecules are special polyelectrolytes in that their charge sign and charge
density are a function of the pH (see Fig. 2). Moreover, the charge density of these
molecules is rather low because there are only a few amino acids that are charged
(arginine, histidine and lysine are positively charged; aspartic acid and glutamic
acid are negatively charged). Most protein molecules form globular structures in an
aqueous environment due to a delicate interplay between hydrophobic and hydrophilic amino acids. The specific folding of these globular structures enables protein
molecules to perform special tasks as, e.g., enzymes or as building blocks.
Studying complex formation between protein molecules can give insight into
biological processes. The cytosol of living cells is a complex mixture containing
many different protein molecules that differ in size, charge (sign) and function.
Electrostatic interactions between these proteins might play an important role in
the stability of the cytoplasm. Also, in food, protein molecules are important for the
structure and texture, and for the way the food is experienced in the mouth. In the
context of this chapter we will discuss protein–protein complex formation as a
special kind of polyelectrolyte complex formation.
Oppositely charged protein molecules could be an interesting model system for
studying the interactions between similar, but oppositely charged nanoparticles.
Biesheuvel et al. studied complex formation between lysozyme and succinylated
lysozyme. The latter protein is a chemically modified form of lysozyme and has the
opposite charge to lysozyme (i.e., negative instead of positive) at pH 7.5 [81]. At
low salt concentration, mixtures of these two protein molecules form precipitates.
These precipitates dissolve when the ionic strength or temperature is increased [82].
The redissolution of these +/À protein precipitates at elevated temperature
indicates that attractive forces other than electrostatic attraction (e.g., hydrophobic
interactions and hydrogen bonding) are important in this system because electrostatic attraction is not, or only minimally, dependent on temperature. This is not
entirely surprising because many amino acids are hydrophobic or are able to form
hydrogen bonds.
Figure 17 shows a plot of F
+ versus salt concentration for compositions with a
fixed turbidity (measured as transmission of 95%). Like the plot of DLS intensity as
a function of composition (see Sect. 2.1 on DLS), a maximum intensity is found at
F
+
¼ 0.5. The experimental results of this system could theoretically be described
using a heterogeneous Poison–Boltzmann cell model. This model assumes stronger
correlations than, e.g., Voorn and Overbeek, taking the spatial structure within the
complex into account [19]. It was developed to estimate the electrostatic free
energy of strong complexes of oppositely charged flexible polyelectrolytes.
In cell models, each charged colloid, or in this case protein molecule, is surrounded
by solvent and small (salt) ions, which form one cell. The Poison–Boltzmann
equation is solved for the space between the protein molecule and the cell edge.
In one-component cell models, the boundary conditions at each cell edge are fixed
and each cell itself is electroneutral [83–86]. Here, we are dealing with a system
166
S. Lindhoud and M.A. Cohen Stuart