2009_Book_FoodChemistry

24 1 Amino Acids, Peptides, Proteins
1.2.4.3.4 Serine and Threonine
Acidic or alkaline hydrolysis of protein can yield
α-keto acids through β-elimination of a water
molecule:
The equilibrium constants for the reduction of
cystine at pH 7 and 25 ◦ C with mercaptoethanol
or dithiothreitol are 1 and 10 4 , respectively.
Stronger oxidation of cysteine, e. g., with performic
acid, yields the corresponding sulfonic
acid, cysteic acid:
(1.50)
In this way, α-ketobutyric acid formed from threonine
can yield another amino acid, α-aminobutyric
acid, via a transamination reaction. Reaction
1.51 is responsible for losses of hydroxy
amino acids during protein hydrolysis.
Reliable estimates of the occurrence of these
amino acids are obtained by hydrolyzing protein
for varying lengths of time and extrapolating the
results to zero time.
(1.52)
Reaction of cysteine with alkylating agents
yields thioethers. Iodoacetic acid, iodoacetamide,
dimethylaminoazobenzene iodoacetamide, ethylenimine
and 4-vinylpyridine are the most
commonly used alkylating agents:
1.2.4.3.5 Cysteine and Cystine
Cysteine is readily converted to the corresponding
disulfide, cystine, even under mild
oxidative conditions, such as treatment with I 2
or potassium hexacyanoferrate (III). Reduction
of cystine to cysteine is possible using sodium
borohydride or thiol reagents (mercaptoethanol,
dithiothreitol):
1.2.4.3.6 Methionine
(1.53)
Methionine is readily oxidized to the sulfoxide
and then to the sulfone. This reaction can result
in losses of this essential amino acid during food
processing:
(1.54)
1.2.4.3.7 Tyrosine
(1.51)
Tyrosine reacts, like histidine, with diazotized
sulfanilic acid (Pauly reagent). The coupled-