Food Chemistry

24 1 Amino Acids, Peptides, Proteins

1.2.4.3.4 SerineandThreonine

Acidic or alkaline hydrolysis of protein can yield
α-keto acids throughβ-elimination of a water
molecule:

(1.50)

In this way,α-ketobutyric acid formed from thre-
onine can yield another amino acid,α-amino-
butyric acid, via a transamination reaction. Re-
action 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.

1.2.4.3.5 CysteineandCystine

Cysteine is readily converted to the corres-
ponding 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):

(1.51)

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 per-

formic acid, yields the corresponding sulfonic

acid, cysteic acid:

(1.52)

Reaction of cysteine with alkylating agents

yields thioethers. Iodoacetic acid, iodoacetamide,

dimethylaminoazobenzene iodoacetamide, ethyl-

enimine and 4-vinylpyridine are the most

commonly used alkylating agents:

(1.53)

1.2.4.3.6 Methionine.............................................

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...............................................

Tyrosine reacts, like histidine, with diazotized

sulfanilic acid (Pauly reagent). The coupled-