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248 Part 2: Biotechnology and Enzymologydigestion, are only dealt with to a lesser extent. Further infor-
mation on seafood enzymes can be found in the bookSeafood
Enzymes, edited by Haard and Simpson (2000), and the refer-
ences therein.Cold Adaptation of EnzymesMost seafood species are poikilotherm and are thus forced to
adapt to the temperature of their habitat, which may range from
the freezing point of oceanic seawater, of approximately−1.9◦C,
to temperatures similar to those of the warm-blooded species.
Since most of the aquatic environment has a temperature of
below 5◦C, the enzyme systems of seafood species are often
adapted to low temperatures. Although all enzyme reaction rates
are positively correlated with temperature, the enzyme systems
of most cold-adapted species exhibit, at low temperatures, cat-
alytic activities similar to those that homologous mesophilic
enzymes of warm-blooded animals exhibit at higher in vivo tem-
peratures. This is primarily due to the higher turnover number
(kcat) of the cold-adapted enzymes; substrate affinity, estimated
by the Michaelis-Menten constant (KM), and other regulatory
mechanisms are generally conserved (Somero 2003).
Several studies reviewed by Simpson and Haard (1987) have
found the level of activity of cold-adapted marine enzymes to be
higher than that of homologous bovine enzymes; they have also
shown marine enzymes to be much lower in thermal stability.
Although the attempt has been made to explain the tempera-
ture adaptation of the enzymes of marine species in terms of
molecular flexibility in a general sense, studies of the molecu-
lar structure of such enzymes have not revealed any common
structural features that can provide a universal explanation of
temperature adaptation. Rather, there appear to be various mi-
nor structural adjustments of varying character that provide the
necessary flexibility of the protein structure for achieving cold
adaption (Smalas et al. 2000, Fields 2001, de Backer et al. 2002). ̊
Since the habitat temperature of most marine animals used
as seafood tends to be near to the typical cold storage temper-
ature of approximately 0–5◦C, the enzymatic reaction rates are
not thermally depressed at storage temperatures as they usually
would be in the meat of warm-blooded animals. With a habitat
temperature of seafood of about 5◦CandQ 10 values in the nor-
mal range of 1.5–2.0, the enzyme reactions during ice storage
can thus be expected to proceed at rates four to nine times as
high as in warm-blooded animals under similar conditions.Effects of Pressure on Enzyme ReactionsAlthough the oceans, with their extreme depth, represent the
largest habitat on earth, it is often forgotten that hydrostatic
pressure is an important biological variable, affecting the equi-
librium constants of all reactions involving changes in volume.
Regarding enzyme reactions, pressure affects the molecular sta-
bility of enzymes and the intermolecular interactions between
enzymes and substrates, cofactors, and macromolecules. There-
fore, pressure alterations may change both the concentration of
catalytically active enzymes and their kinetic parameters, such
as the Michaelis-Menten constant,KM.Siebenaller and Somero (1978) showed that the apparentKM
values of a lactate dehydrogenase (LDH) enzyme from a deep-
sea fish species are less sensitive to pressure changes than its
homologous enzyme from a closely related fish species living in
shallow water. Similar relations between the pressure sensitivity
of LDH and habitat pressure have been found for the LDH of
other marine species (Siebenaller and Somero 1979, Dahlhoff
and Somero 1991). In this case, only the LDH substrate interac-
tion was found to be affected by pressure, whereas the turnover
number appeared to be unaffected by pressure.
Very high pressures, of 10^8 Pa (ca. 1000 bar), correspond-
ing to the pressure in the deepest trenches on earth can lead
to denaturization of proteins. Some deep-sea fish have been
found to accumulate particularly high levels of trimethylamine-
N-oxide (TMAO). TMAO stabilizes most protein structures and
may counteract the effect of pressure on proteins. Adaptation to
high pressure thus seems to be achieved both by evolution of
pressure-resistant proteins and by adjustments of the intracellu-
lar environment.
The metabolic rates of deep-sea fish tend to be lower than
those of related species living in shallow waters. Several lines
of evidence suggest that the decrease in metabolic rate with
increasing depth is largely due to the food consumption levels
being lower and to a reduced need for locomotor capability in
the darkness (Childress 1995, Gibbs 1997). The latter makes
intuitive sense if one considers the often far from streamlined,
and sometimes bizarre, anatomy of deep-sea fish species.Osmotic Adaptation of EnzymesMarine teleosts (bonefish) are hypoosmotic, having intracellu-
lar solute concentrations that are similar to those of terrestrial
vertebrates. By contrast, elasmobranches (sharks and rays) are
isoosmotic, employing the organic compounds urea and TMAO
as osmolytes. Although the concentration of osmolytes can be
high, the stability and functionality of enzymes remain mostly
unaffected due to the compatibility and balanced counteracting
effects of the osmolytes (Yancey et al. 1982).ENZYMATIC REACTIONS IN THE
ENERGY METABOLISM OF SEAFOODMost key metabolic pathways such as the glycolysis are found in
all animal species, and the majority of animal enzymes are thus
homologous. Nevertheless, the energy metabolism of seafood
species shows a number of differences from that of terrestrial
animals, and these differences contribute to the characteristics
of seafood products.
The red and white fish muscle cells are anatomically separated
into minor portions of red muscle and major portions of white
muscle tissues (see Fig. 13.2 in the Section Postmortem Proteol-
ysis in Fresh Fish). The energy metabolism and the histology of
the two muscle tissues differ, the anaerobic white muscle having
only few mitochondria and blood vessels. When blood circula-
tion stops after death, the primary effect on the white muscle
metabolism is therefore not the seizure of oxygen supply but the
accumulation of waste products that are no longer removed.