Chemistry Reference
In-Depth Information
A chain
S
S
Thr
Ser
Ser
Leu
Asn
H
2
N
Gly
Ile
Val
Glu
Gln
Cys
Cys
Ile
Cys
Tyr
Gin
Leu
Glu
Asn
Tyr
Cys
COOH
1
5
10
15
S
S
S
S
B chain
His
Leu
Glu
Ala
Glu
Phe
Val
Asn
Gin
Leu
Cys
Gly
Ser
His
Val
Leu
Tyr
Leu
Val
Cys
Gly
H
2
N
Arg
15
20
1
5
10
Gly
Phe
Phe
Lys
Pro
Thr
Trp
HOOC
Thr
25
30
Figure 15.23
The structure of a protein - human insulin (the different coloured circles represent different amino acids in this protein).
Proteins fall broadly into two groups: they can be
fibrous or globular.
•
Fibrous proteins - these have linear molecules,
are insoluble in water and resistant to alkalis and
acids. Collagen (in tendons and muscles), keratin
(in nails, hair, horn and feathers) and elastin (in
arteries) are all fibrous proteins.
•
Globular proteins - these have complicated
three-dimensional structures and are soluble in
water. They are easily affected by acids, alkalis
and temperature increase, when they are said to
be denatured. Casein (in milk), albumen (in egg
white) and enzymes are examples of globular
proteins.
Analysis of the products of the hydrolysis of
proteins
How can you determine which amino acids are
present in a particular protein? This involves
hydrolysis of the peptide (amide) bonds in the
protein so that the individual amino acids are
released. This can only be done by heating
the protein with dilute hydrochloric acid. The
mixture of amino acids is then separated by thin
layer chromatography (TLC) (Figure 15.24) or
electrophoresis. In both cases, a locating agent
(Chapter 2, p. 23), such as ninhydrin, is used. This
ensures that the spots of amino acid are visible.
Figure 15.24
Amino acids can be separated and identified by TLC.
If you are trying to show only the presence of a
protein, a quick test to carry out is known as the
Biuret test
. A mixture of dilute sodium hydroxide
and 1% copper(
ii
) sulfate solution is shaken with
a sample of the material under test. If a protein is
present, a purple colour appears after about three
minutes (Figure 15.25).
