102 2 Enzymes
2.3.1.2 Adenosine Triphosphate
The nucleotide adenosine triphosphate (ATP) is
an energy-rich compound. Various groups are
cleaved and transferred to defined substrates
during metabolism in the presence of ATP. One
possibility, the transfer of orthophosphates by
kinases, is utilized in the enzymatic analysis of
food (cf. Table 2.16).
(2.6)(2.7)2.3.2 Prosthetic Groups
2.3.2.1 Flavins
Riboflavin (7,8-dimethyl-10-ribityl-isoalloxazi-
ne), known as vitamin B 2 (cf. 6.3.2), is the
building block of flavin mononucleotide (FMN)
and flavin adenine dinucleotide (FAD). Both
act as prosthetic groups for electron transfer
reactions in a number of enzymes.
Due to the much wider redox potential of the
flavin enzymes, riboflavin is involved in the
transfer of either one or two electrons. This is
different from nicotinamides which participate
in double electron transfer only. Values be-
tween+ 0 .19 V (stronger oxidizing effect than
NAD⊕)and− 0 .49 V (stronger reducing effect
than NADH) have been reported.
(2.8)An example for a flavin enzyme is glucose
oxidase, an enzyme often used in food processing
to trap residual oxygen (cf. 2.7.2.1.1). The
enzyme isolated and purified fromAspergillus
niger is a dimer (Mr= 168 , 000 ) with two
noncovalently bound FAD molecules. In contrast
to xanthine oxidase (cf. 2.3.3.2), for example,
this enzyme has no heavy metal ion. During
oxidation of a substrate, such as the oxidation
of β-D-glucose to δ-D- gluconolactone, the
flavoquinone is reduced by two single electron
transfers:(2.9)Like glucose oxidase, many flavin enzymes
transfer the electrons to molecular oxygen,
forming H 2 O 2 and flavoquinone. The fol-
lowing intermediary products appear in this
reaction:(2.10)