2009_Book_FoodChemistry

72 1 Amino Acids, Peptides, Proteins
Table 1.29. Ornithine is formed during cleavage
of arginine (Reaction 1.54).
(1.154)
Formation of D-amino acids occurs through
abstraction of a proton via a C2-carbanion.
The reaction with L-isoleucine is particularly
interesting. L-Isoleucine is isomerized to
D-alloisoleucine which, unlike other D-amino
acids, is a diastereoisomer and so has a retention
time different from L-isoleucine, making its
determination possible directly from an amino
acid chromatogram:
Table 1.29. Formation of unusual amino acids by alkali
treatment of protein
Name
Formula
3-N 6 -Lysinoalanine COOH COOH
(R = H) | |
3-N 6 -Lysino-3-methyl- CHNH 2 CHNH 2
alanine(R = CH 3 ) | |
CHR—NH—(CH 2 ) 4
3-N 5 -Ornithinoalanine COOH COOH
(R = H) | |
3-N 5 -Ornithino-3- CHNH 2 CHNH 2
methylalanine (R = CH 3 ) | |
CHR—NH—(CH 2 ) 3
Lanthionine (R = H) COOH COOH
3-Methyllanthionine | |
(R = CH 3 ) CHNH 2 CHNH 2
| |
CHR——S——CH 2
3-Aminoalanine (R =H) COOH
2,3-Diamino |
butyric acid (R = CH 3 ) CHNH 2
|
CHRNH 2
(1.155)
Heating proteins in a dry state at neutral pH
results in the formation of isopeptide bonds
between the ε-amino groups of lysine residues
and the β-orγ-carboxamide groups of asparagine
and glutamine residues:
These isopeptide bonds are cleaved during acidic
hydrolysis of protein and, therefore, do not
contribute to the occurrence of unusual amino
acids. A more intensive heat treatment of proteins
in the presence of water leads to a more extensive
degradation.
Oxidative changes in proteins primarily involve
methionine, which relatively readily forms methionine
sulfoxide:
(1.157)
(1.156)
The formation of methionine sulfoxide was observed
in connection with lipid peroxidation, phenol
oxidation and light exposure in the presence
of oxygen and sensitizers such as riboflavin.
After in vivo reduction to methionine, proteinbound
methionine sulfoxide is apparently biologically
available.
Figure 1.37 shows the effect of alkaline treatment
of a protein isolate of sunflower seeds. Serine,
threonine, arginine and isoleucine concentrations