Biomedical Engineering Reference
In-Depth Information
Table 2.3
Amino acid composition (mol-%) of the total proteins of flours from various cereals [
56
]
Amino acid
Wheat
Rye
Barley
Oats
Rice
Millet
Corn
Asx
a
4.2
6.9
4.9
8.1
8.8
7.7
5.9
Thr
3.2
4.0
3.8
3.9
4.1
4.5
3.7
Ser
6.6
6.4
6.0
6.6
6.8
6.6
6.4
Glx
a
31.1
23.6
24.8
19.5
15.4
17.1
17.7
Pro
12.6
12.2
14.3
6.2
5.2
7.5
10.8
Gly
6.1
7.0
6.0
8.2
7.8
5.7
4.9
Ala
4.3
6.0
5.1
6.7
8.1
11.2
11.2
Cys
1.8
1.6
1.5
2.6
1.6
1.2
1.6
Val
4.9
5.5
6.1
6.2
6.7
6.7
5.0
Met
1.4
1.3
1.6
1.7
2.6
2.9
1.8
Ile
3.8
3.6
3.7
4.0
4.2
3.9
3.6
Leu
6.8
6.6
6.8
7.6
8.1
9.6
14.1
Tyr
2.3
2.2
2.7
2.8
3.8
2.7
3.1
Phe
3.8
3.9
4.3
4.4
4.1
4.0
4.0
His
1.8
1.9
1.8
2.0
2.2
2.1
2.2
Lys
1.8
3.1
2.6
3.3
3.3
2.5
1.4
Arg
2.8
3.7
3.3
5.4
6.4
3.1
2.4
Trp
0.7
0.5
0.7
0.8
0.8
1.0
0.2
Amide group
31.0
24.4
26.1
19.2
15.7
22.8
19.8
a
Asx
Asp+Asn,
Glx
Glu+Gln
while globulins are insoluble in pure water but soluble in dilute salt solutions.
Prolamins are classically defined as cereal proteins soluble in aqueous alcohols, for
example 60-70% ethanol. Originally, glutelins were described as proteins that were
insoluble in water, salt solution, aqueous alcohols and soluble in dilute acids or
bases. Later, it was ascertained that notable portions of glutelins are insoluble in
dilute acids such as acetic acid, and that extraction with strong bases destroys the
primary structure of proteins. Nowadays, complete solubility of glutelins is achieved
by solvents containing a mixture of aqueous alcohols (e.g., 50% propanol), reduc-
ing agents (e.g., dithiothreitol), and disaggregating compounds (e.g., urea).
Regarding their functions, most of the albumins and globulins are metabolic
proteins, for example enzymes or enzyme inhibitors (see Sect.
2.3.4
). Oats are an
exception containing considerable amounts of legume-like globulins such as 12S
globulin [
58
]. Albumins and globulins are concentrated in the aleurone layer, bran,
and germ, whereas their concentration in the starchy endosperm is relatively low.
Predominantly, prolamins and glutelins are the storage proteins of cereal grains (see
Sects.
2.3.2
and
2.3.3
). Their only biological function is to supply the seedling with
nitrogen and amino acids during germination. They are located only in the starchy
endosperm; in white flours, their proportions based on total proteins amount to
70-90%. In general, none of the Osborne fractions consists of a single protein, but
of a complex mixture of different proteins. A small portion of proteins does not fall
into any of the four solubility fractions. Together with starch, they remain in the
insoluble residue after Osborne fractionation and mainly belong to the class of lipo
(membrane) proteins.
