2009_Book_FoodChemistry

84 1 Amino Acids, Peptides, Proteins
about 3 kdal:
(1.162)
The reaction rate is affected by, among other
things, the nature of the amino acid residues.
Hydrophobic amino acid residues are preferably
linked together (Fig.1.51). Incorporation of
amino acid esters into protein is affected by the
alkyl chain length of the ester. Short-chain alkyl
esters have a low rate of incorporation, while
the long-chain alkyl esters have a higher rate of
incorporation. This is especially important for
the incorporation of amino acids with a short side
chain, such as alanine (cf. Table 1.40).
The plastein reaction can help to improve the biological
value of a protein. Figure 1.52 shows
the plastein enrichment of zein with tryptophan,
threonine and lysine. The amino acid composition
of such a zein-plastein product is given in
Table 1.41.
Enrichment of a protein with selected amino acids
can be achieved with the corresponding amino
Table 1.40. Plastein reaction catalyzed by papain: rate
of incorporation of amino acid esters a
Aminoacyl OEt OnBu OnHex OnOct
residue
L-Ala 0.016 0.054 0.133 0.135
D-Ala 0.0 – 0.0 –
α-Methylala 0.0 – 0.0 –
L-Val 0.005 – 0.077 –
L-Norval 0.122 – 0.155 –
L-Leu 0.119 – 0.140 –
L-Norleu 0.125 – 0.149 –
L-Ile 0.005 – 0.048 –
a µmole× mg papain −1 × min −1 .
acid esters or, equally well, by using suitable partial
hydrolysates of another protein.
Figure 1.53 presents the example of soya protein
enrichment with sulfur-containing amino
acids through “adulteration” with the partial
hydrolysate of wool keratin. The PER (protein
efficiency ratio) values of such plastein products
are significantly improved, as is seen in
Table 1.42.
Figure 1.54 shows that the production of plastein
with an amino acid profile very close to that recommended
by FAO/WHO can be achieved from
very diverse proteins.
The plastein reaction also makes it possible to
improve the solubility of a protein, for example,
by increasing the content of glutamic acid
(Fig. 1.55). A soya protein with 25% glutamic
acid yields a plastein with 42% glutamic acid.
Fig. 1.51. Plastein reaction with papain: incorporation
rates of amino acid esters as function of side chain hydrophobicity.
(according to Arai et al., 1978)
Fig. 1.52. Zein enrichment with Trp, Thr, and Lys by
a plastein reaction. (according to Aso et al., 1974)
a 1% substrate, E/S = 1/50, pH 1.6 at37 ◦ C for 72 h
b 50% substrate, hydrolyzate/AS-OEt = 10/1,
E/S = 3/100 at 37 ◦ C for 48 h
c 0.1mol/L in 50% ethanol at 25 ◦ Cfor5h