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1.4 Proteins 71

for example, lactose or glucose, which yield

protein-bound ε-N-deoxylactulosyl-1-lysine or

ε-N-deoxyfructosyl-1-lysine, respectively. Lysine

is not biologically available in these forms.

Acidic hydrolysis of such primary reaction

products yields lysine as well as the degradation

products furosine and pyridosine in a constant

ratio (cf. 4.2.4.4):

In the case of cystine, the eliminated thiolcysteine

can form a second dehydroalanine residue:

(1.149)

Alternatively, cleavage of the cystine disulfide

bond can occur by nucleophilic attack on sulfur,

yielding a dehydroalanine residue via thiol and

sulfinate intermediates:

(1.150)

(1.147)

A nonreducing sugar (e. g. sucrose) can also

cause a loss of lysine when conditions for sugar

hydrolysis are favorable.

Losses of available lysine, cystine, serine, threonine,

arginine and some other amino acids occur

at higher pH values. Hydrolysates of alkalitreated

proteins often contain some unusual

compounds, such as ornithine, β-aminoalanine,

lysinoalanine, ornithinoalanine, lanthionine,

methyllanthionine and D-alloiso-leucine, as well

as other D-amino acids.

The formation of these compounds is based on the

following reactions: 1,2-elimination in the case of

hydroxy amino acids and thio amino acids results

in 2-amino-acrylic acid (dehydroalanine) or 2-

aminocrotonic acid (dehydro-aminobutyric acid):

(1.151)

(1.152)

Intra- and interchain cross-linking of proteins

can occur in dehydroalanine reactions involving

additions of amines and thiols. Ammonia may

also react via an addition reaction:

(1.153)

(1.148)

Acidic hydrolysis of such a cross-linked protein

yields the unusual amino acids listed in

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