Physical Chemistry of Foods

method of excluding hydrophilic residues from the core, if they make up a

high proportion of the residues, is the formation of an elongated shape

(Figure 7.3c). On the basis of the outer shape, proteins can be classified into

three groups: globular, fibrous, and disordered.

In aglobular proteinthe peptide chain is tightly folded into a roughly

spherical shape. The secondary structures (a-helices andb-sheets) roughly

span the diameter, and they are often linked by reverse turns. The latter are

sharp bends in the peptide chain, to some extent stabilized by hydrogen

bonds. The charged amino acid residues Asp, Glu, Arg, and Lys are

predominantly at the surface, and the charge density is generally between 0

and 2.5 charges per nm^2 , depending on pH. Val, Leu, Ile, Phe, Ala, Gly, and

Cys are mostly inside. Globular proteins always have a fairly high average

hydrophobicity, over 4 kJ per mole of residues, and contain relatively few

Pro residues.

However, these statements are by no means universal rules. Globular

proteins, which make up by far the greatest proportion (99%?) of protein

species, evolved to fulfil specific functions, which are mostly related to the

capacity to bind specific molecules or groups, mostly called ‘‘ligands.’’ This

applies to enzymes, transport proteins, antibodies (immunoglobulins), etc.

Their tertiary structure thus is highly specific, and the primary structure

needed mostly does not allow complete segregation of hydrophobic and

hydrophilic residues. Consequently, the outside often contains several

hydrophobic residues, whereas some hydrophilic ones are in the core.

Nevertheless, the geometrical constraints mentioned above imply that a

protein of more than, say, 250 residues cannot assume a globular

FIGURE7.3 Illustration of the effect of size and shape on the proportion taken up
by the hydrophobic core (hatched) of a protein, taking the hydrophilic outer layer to
be of constant thickness (e.g., 0.5 nm). In (a) the proportion is 0.20, in (b) 0.50, and in
(c) 0.12. In (d) molecules with a hydrophobic patch on the surface are illustrated,
which tend to associate into a quaternary structure. Highly schematic.