From: http://www.fao.org/docrep/v7180e/v7180e05.htm#4.3
Proteins
.3 Proteins
The proteins in fish muscle tissue can be divided into the following three groups:
1. Structural proteins (actin, myosin, tropormyosin and actomyosin), which
constitute 70-80 % of the total protein content (compared with 40 % in
mammals). These proteins are soluble in neutral salt solutions of fairly high ionic
strength (³0.5 M).
2. Sarcoplasmic proteins (myoalbumin, globulin and enzymes) which are soluble in
neutral salt solutions of low ionic strength (<0.15 M). This fraction constitutes 2530 % of the protein.
3. Connective tissue proteins (collagen), which constitute approximately 3 % of the protein in teleostei and about 10 % in elasmobranchii (compared with 17 % in
mammals).
The structural proteins make up the contractile apparatus responsible for the muscle
movement as explained in section 3.2. The amino-acid composition is approximately the
same as for the corresponding proteins in mammaliam muscle, although the physical
properties may be slightly different. The isoelectric point (pI) is around pH 4.5-5.5. At the
corresponding pH values the proteins have their lowest solublity, as illustrated in Figure
4.4.
The conformational structure of fish proteins is easily changed by changing the physical
environment. Figure 4.4 shows how the solubility characteristics of the myofibrillar
proteins are changed after freeze-drying. Treatment with high salt concentrations or heat
may lead to denaturation, after which the native protein structure has been irreversibly
changed.
When the proteins are denatured under controlled conditions their properties may be
utilized for technological purposes. A good example is the production of surimi-based
products, in which the gel forming ability of the myofibrillar proteins is used. After salt
and stabilizers are added to a washed, minced preparation of muscle proteins, and after
a controlled heating and cooling procedure the proteins form a very strong gel (Suzuki,
1981).
Figure 4.4 Solubility of myofibrillar protein before and after freeze drying at pH values
ranging from 2 to 12 (Spinelli et al.,1972)
The majority of the sarcoplasmic proteins are enzymes participating in the cell
metabolism, such as the anaerobic energy conversion from glycogen to ATP. If the
organelles within the muscle cells are broken, this protein fraction may also contain the
metabolic enzymes localized inside the endoplasmatic reticulum, mitochondria and
lysosomes.
The fact that the composition of the sarcoplasmic protein fraction changes when the
organelles are broken was suggested as a method for differentiating fresh from frozen
fish, under the assumption that the organelles were intact until freezing (Rehbein et al.,
1978, Rehbein, 1979, Salfi et al., 1985). However, it was later stated that these methods
should be used with great caution, as some of the enzymes are liberated from the
organelles also during iced storage of fish (Rehbein, 1992).
The proteins in the sarcoplasmic fraction are excellently suited to distinguishing between
different fish species, as all the different species have their characteristic band pattern
when separated by the isoelectric focusing method. The method was succesfully
introduced by Lundstrom (1980) and has been used by many laboratories and for many
fish species. A review of the literature is given by Rehbein (1990).
The chemical and physical properties of collagen proteins are different in tissues such as
skin, swim bladder and the myocommata in muscle (Mohr, 1971). In general, collagen
fibrils form a delicate network structure with varying complexity in the different
connective tissues in a pattern similar to that found in mammals. However, the collagen
in fish is much more thermolabile and contains fewer but more labile cross-links than
collagen from warm-blooded vertebrates. The hydroxyprolin content is in general lower
in fish than in mammals, although a total variation between 4.7 and 10 % of the collagen
has been observed (Sato et at, 1989).
Different fish species contain varying amounts of collagen in the body tissues. This has
led to a theory that the distribution of collagen may reflect the swimming behaviour of the
species (Yoshinaka et at, 1988). Further, the varying amounts and varying types of
collagen in different fishes may also have an influence on the textural properties of fish
muscle (Montero and Borderias, 1989). Borresen (1976) developed a method for
isolation of the collagenous network surrounding each individual muscle cell. The
structure and composition of these structures has been further characterized in cod by
Almaas (1982).
The role of collagen in fish was reviewed by Sikorsky et al. (1984). An excellent, more
recent review is given by Bremner (1992), in which the most recent literature of the
different types of collagen found in fish is presented.
Fish proteins contain all the essential amino-acids and, like milk, eggs and mammalian
meat proteins, have a very high biological value (Table 4.3).
Table 4.3 Essential amino-acids (percentage) in various proteins
Amino-acid
Fish Milk Beef Eggs
Lysine
8.8
8.1
9.3
6.8
Tryptophan
1.0
1.6
1.1
1.9
Histidine
2.0
2.6
3.8
2.2
Phenylalanine
3.9
5.3
4.5
5.4
Leucine
8.4 10.2 8.2
8.4
Isoleucine
6.0
7.2
5.2
7.1
Threonine
4.6
4.4
4.2
5.5
Methionine-cystine 4.0
4.3
2.9
3.3
Valine
7.6
5.0
8.1
6.0
SOURCES: Braekkan, 1976; Moustgard, 1957
Cereal grains are ususally low in lysine and/or the sulphur-containing amino-acids
(methionine and cysteine), whereas fish protein is an excellent source of these
aminoacids. In diets based mainly on cereals, a supplement of fish can, therefore, raise
the biological value significantly.
In addition to the fish proteins already mentioned there is a renewed interest in specific
protein fractions that may be recovered from by-products, particularly in the viscera. One
such example is the basic protein or protamines found in the milt of the male fish. The
molecular weight is usually below 10 000 kD and the pI is higher than 10. This is a result
of the extreme amino-acid composition that may show as much as 65 % arginine.
The presence of the basic proteins has long been known, and it is also known that they
are not present in all fish species (Kossel, 1928). The best sources are salmonids and
herring, whereas ground fish like cod are not found to contain protamines.
The extreme basic character of protamines makes them interesting for several reasons.
They will adhere to most other proteins less basic. Thus they have the effect of
enhancing functional properties of other food proteins (Poole et al., 1987; Phillips et al.,
1989). However, there is a problem in removing all lipids present in the milt from the
protein preparation, as this results in an off-flavour in the concentrations to be used in
foods.
Another interesting feature of the basic proteins is their ability to prevent growth of
microorganisms (Braekkan and Boge, 1964; Kamal et al., 1986). This appears to be the
most promising use of these basic proteins in the future.