2009_Book_FoodChemistry

98 2 Enzymes
lation are specified in addition to the name of the
enzyme preparation; for example, “ascorbate oxidase
(E.C. 1.1.10.3.3) from cucumber”. When
known, the subcellular fraction of origin (cytoplasmic,
mitochondrial or peroxisomal) is also
specified.
b) The molar catalytic activity. This can be
determined when the pure enzyme with
a known molecular weight is available. It
is expressed as “katal per mol of enzyme”
(katmol −1 ). When the enzyme has only
one active site or center per molecule, the
molar catalytic activity equals the “turnover
number”, which is defined as the number of
substrate molecules converted per unit time
by each active site of the enzyme molecule.
2.3 Enzyme Cofactors
(2.3)
A number of enzymes of interest to food chemistry
are described in Table 2.4. The number of
the section in which an enzyme is dealt with is
given in the last column.
2.2.7 Activity Units
The catalytic activity of enzymes is exhibited
only under specific conditions, such as pH,
ionic strength, buffer type, presence of cofactors
and suitable temperature. Therefore, the rate of
substrate conversion or product formation can
be measured in a test system designed to follow
the enzyme activity. The International System
of Units (SI) designation is mols −1 and its
recommended designation is the “katal” (kat * ).
Decimal units are formed in the usual way, e. g.:
Rigorous analysis has demonstrated that numerous
enzymes are not pure proteins. In addition
to protein, they contain metal ions and/or low
molecular weight nonprotein organic molecules.
These nonprotein hetero constituents are denoted
as cofactors which are indispensable for enzyme
activity.
According to the systematics (Fig. 2.3), an apoenzyme
is the inactive protein without a cofactor.
Metal ions and coenzymes participating in enzymatic
activity belong to the cofactors which
are subdivided into prosthetic groups and cosubstrates.
The prosthetic group is bound firmly to
the enzyme. It can not be removed by, e. g. dialysis,
and during enzyme catalysis it remains attached
to the enzyme molecule. Often, two substrates
are converted by such enzymes, one substrate
followed by the other, returning the prosthetic
group to its original state. On the other
µkat = 10 −6 kat = µmol · s −1 (2.4)
Concentration of enzymatic activity is given
as µkat l −1 . The following activity units are
derived from this:
a) The specific catalytic activity,i.e.theactivity
of the enzyme preparation in relation to the
protein concentration.
* The old definition in the literature may also be
used: 1 enzyme unit (U) 1µmolmin −1 (1U 16.67 ·
10 −9 kat).
Fig. 2.3. Systematics of cofactor-containing enzymes
(according to Schellenberger, 1989)