380 Part III: Muscle Foods
it is mainly the “classical” seafood groups—fish,
shellfish, and molluscs—that are dealt with. Muscle
enzymes are mostly considered, since muscle is the
organ of which most seafood products primarily
consist. Various other types of seafood enzymes,
such as those used in digestion, are only dealt with
to a lesser extent. Further information on seafood
enzymes can be found in the book Seafood En-
zymes,edited by Haard and Simpson (2000), and the
references therein.
COLDADAPTATION OFENZYMES
Most seafood species are poikilotherme 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 spe-
cies 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, cata-
lytic activities similar to those that homologous me-
sophilic enzymes of warm-blooded animals exhibit
at higher in vivo temperatures. 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 regula-
tory 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 homolo-
gous bovine enzymes; they have also shown marine
enzymes to be much lower in thermal stability.
Although the attempt has been made to explain the
temperature adaptation of the enzymes of marine
species in terms of molecular flexibility in a general
sense, studies of the molecular 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 minor structural adjustments of varying char-
acter that provide the necessary flexibility of the pro-
tein structure for achieving cold adaption (Smalås et
al. 2000, Fields 2001, de Backer et al. 2002).
Since the habitat temperature of most marine ani-
mals used as seafood tends to be near to the typical
cold storage temperature of approximately 0–5°C,
the enzymatic reaction rates are not thermally de-
pressed at storage temperatures as they usually would
be in the meat of warm-blooded animals. With a habi-
tat temperature of seafood of about 5°C andQ 10 val-
ues in the normal range of 1.5–2.0, the enzyme reac-
tions 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 OFPRESSURE ONENZYME
REACTIONSAlthough the oceans, with their extreme depth, rep-
resent the largest habitat on earth, it is often forgot-
ten that hydrostatic pressure is an important biologi-
cal variable, affecting the equilibrium constants of
all reactions involving changes in volume. Regard-
ing enzyme reactions, pressure affects the molecular
stability of enzymes and the intermolecular interac-
tions between enzymes and substrates, cofactors,
and macromolecules. Pressure alterations may there-
fore 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
apparent KM 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 pres-
sure sensitivity of LDH and habitat pressure have
been found for the LDH of other marine species
(Siebenaller and Somero 1979, Dahlhoff and Som-
ero 1991). In this case, only the LDH substrate inter-
action was found to be affected by pressure, where-
as the turnover number appeared to be unaffected by
pressure.
Very high pressures, of 10^8 Pa (ca. 1000 bar), cor-
responding to the pressure in the deepest trenches on
earth can lead to denaturization of proteins. Some
deep-sea fish have been found to accumulate particu-
larly high levels of trimethylamine-N-oxide (TMAO).
TMAO stabilizes most protein structures and may
counteract the effect of pressure on proteins. Adap-
tation to high pressure thus seems to be achieved both
by evolution of pressure-resistant proteins and by
adjustments of the intracellular environment.
The metabolic rates of deep-sea fish tend to be
lower than those of related species living in shallow