7.1.5 Other Aspects
Some proteins do not fit in the scheme of globular versus fibrous, and they
may be calleddisordered. Such proteins have a range of conformations
rather than one. The group includes some proteins of which the natural
function is nutritional, such as the caseins and some storage proteins, e.g.,
the glutelins in wheat grains. Caseins have a primary structure that prevents
tight folding of the peptide chain, partly due to the many proline residues;
see Figure 7.1. Also several denatured globular proteins and gelatin (derived
from collagen) can be considered disordered. Although the greatest number
of protein species occurring in nature is globular and the greatest mass
probably fibrous, disordered proteins are rather important in foods. Casein
and gelatin are often applied because of their functional properties, and
globular proteins often are denatured during processing.
Disordered does not mean that no secondary or other regular
structure exists, but that the conformation is (much) closer to a random
coil than in the other proteins. Only gelatin above 30 8 C andb-casein at
about 5 8 C have been shown to behave like random coil polyelectrolytes.
Glutelins have a very high molar mass and are highly branched,
undoubtedly due to 22 S 22 S 22 bridges linking the originally formed smaller
peptides.
The linking of protein molecules to form larger units is termed
quaternary structure. The term is especially used to denote association by
noncovalent bonds. Globular proteins (‘‘monomers’’) then form dimers or
trimers or larger aggregates, but in a specific orientation. The monomers
may be identical, which is the more common case, or not. The bonds mostly
are hydrophobic, although salt bridges may also be involved. In general,
globular proteins that have several apolar side groups at their surface tend
to form a quaternary structure. The driving force is, of course, minimizing
contact between water and apolar groups; see Figure 7.3d. The structures
can often be dissociated by altering the pH (generally if farther away from
the isoelectric point), or the temperature (either to high or to low values), or
the ionic strength (generally if lower).
Homogeneity and Purity. Unlike polysaccharides, proteins are
synthesized in a precisely prescribed way, i.e., as encoded in the DNA. This
would mean that all molecules of a certain protein species are exactly equal.
There are, however, various disturbing factors.
(a) For all proteins studied,genetic variantsexist that differ in one or
more amino acids. Unless the protein is obtained from one
organism that produces only one variant, it will be a mixture.