Food Chemistry

98 2 Enzymes

lation are specified in addition to the name of the
enzyme preparation; for example, “ascorbate ox-
idase (E.C. 1.1.10.3.3) from cucumber”. When
known, the subcellular fraction of origin (cyto-
plasmic, mitochondrial or peroxisomal) is also
specified.

(2.3)

A number of enzymes of interest to food chem-
istry 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 ActivityUnits

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.:

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) Thespecific 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).

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 EnzymeCofactors.......................................

Rigorous analysis has demonstrated that numer-

ous 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 apoen-

zyme is the inactive protein without a cofactor.

Metal ions and coenzymes participating in en-

zymatic activity belong to the cofactors which

are subdivided into prosthetic groups and cosub-

strates. The prosthetic group is bound firmly to

the enzyme. It can not be removed by, e. g. dial-

ysis, and during enzyme catalysis it remains at-

tached to the enzyme molecule. Often, two sub-

strates are converted by such enzymes, one sub-

strate followed by the other, returning the pros-

thetic group to its original state. On the other

Fig. 2.3.Systematics of cofactor-containing enzymes

(according toSchellenberger, 1989)