9D Influence of Phenolics on Wine Organoleptic Properties 555
9D.3.4 Protein Structures
The interaction between tannins and proteins can be affected by the size of the pro-
tein (Hagerman and Butler 1981; Maury et al. 2003), its charge (Hagerman and
Butler 1981), the presence of side chains (Maury et al. 2003), and its conformation
(Wroblewski et al. 2001).
As already mentioned, the presence of proline is apparently a common char-
acteristic of proteins with high binding affinities towards tannins (Hagerman and
Butler 1981). Apart from being a binding site, proline residues are also useful for
maintaining the peptide in an extended conformation, thereby providing a bigger
surface of protein to binding (Baxter et al. 1997).
Saliva is produced by two groups of glands: parotid and submandibular/
sublingual. Submandibular/sublingual saliva is produced in non-stimulated con-
ditions (between meals) and the major organic components are mucins, proteins
that have lubricatory properties. Parotid saliva is secreted in stimulated conditions
and is mainly composed of 30% of -amilase, and 70% of Proline-Rich Proteins
(PRPs) with high levels of proline (35–40%) (Dodds et al. 2005). PRPs can be
divided into three major groups: acidic, basic and glycosylated proteins (Kauffman
and Keller 1979). All salivary PRPs show affinity towards dietary tannins (Bacon
and Rhodes 1998) and are easily precipitated (Bacon and Rhodes 1998; Yan and
Bennick 1995; Kallithraka et al. 1998). Basic PRPs seem to be more effective
in precipitating tannins when compared to acidic and glycosilated PRPs (Lu and
Bennick 1998). A group of eleven proteins (IB1-IB7, IB8a, b and c and IB9) can
be found among the family of basic PRPs having very similar sequences and dis-
playing repetitive patterns, in which some sequences of amino acids can be found
several times in the same protein and in different proteins (Kauffman et al. 1991).
It has been proposed that the presence of multiple repeated regions rich in proline
could provide sites favourable for tannin binding, as well as flexible hinges on the
protein. This latter feature allows the protein to fold and “wrap around” the tan-
nin, thereby increasing the association by cooperative intermolecular interactions
(Charlton et al. 1996).
Previous works have shown that a basic PRP (IB8c) binds to condensed tannins
much more effectively than -amylase (de Freitas and Mateus 2002). This can be
explained by the 3D structure of proteins: -amylase is a globular protein, and IB8c
is likely to adopt an extended random coil conformation, which would allow the
protein to offer more contact sites to interact with tannins. However, -amylase
seems to be more specific and selective than PRPs in the aggregation with samples
containing different amounts of procyanidins (Mateus et al. 2004c).
The astringency sensation is felt differently by different tasters (Gawel et al.
- probably due to differences in individuals’ saliva, namely its protein composi-
tion (Lesschaeve and Noble 2005; Horne et al. 2002). Recently, a study concerning
the characterization of basic PRPs in thirteen normal adults has demonstrated that
the protein IB9 was only detected in nine adults saliva, while IB7 was not detected
at all (Messana et al. 2004). The salivary flow rate was also described as a fac-
tor that varies between subjects and can affect astringency perception (Lesschaeve