2009_Book_FoodChemistry

2.3 Enzyme Cofactors 99
hand, during metabolism, the cosubstrate reacts
with at least two enzymes. It transfers the hydrogen
or the functional group to another enzyme
and, hence, is denoted as a “transport metabolite”
or as an “intermediary substrate”. It is distinguished
from a true substrate by being regenerated
in a subsequent reaction. Therefore the concentration
of the intermediary substrates can be
very low. In food analysis higher amounts of cosubstrates
are often used without regeneration.
Only those cofactors with enzymatic activities of
importance in enzymatic analysis of food and/or
in food processing will be presented. Some cofactors
are related to water-soluble vitamins (cf. 6.3).
The metal ions are dealt with separately in section
2.3.3.
2.3.1 Cosubstrates
2.3.1.1 Nicotinamide Adenine Dinucleotide
Transhydrogenases (e. g. lactate dehydrogenase,
alcohol dehydrogenase) dehydrogenate or hydrogenate
their substrates with the help of a pyridine
cosubstrate (Fig. 2.4); its nicotinamide residue
accepts or donates a hydride ion (H − ) at position
4:
Fig. 2.4. Nicotinamide adenine dinucleotide (NAD) and
nicotinamide adenine dinucleotide phosphate (NADP);
R = H: NAD; R = PO 3 H 2 : NADP
the pro-R-hydrogen from the A * side, whereas
glutamate or glucose dehydrogenases transfer the
pro-S-hydrogen from the B * side * .
The oxidized and reduced forms of the pyridine
cosubstrate are readily distinguished by
absorbance readings at 340 nm (Fig. 2.5). Therefore,
whenever possible, enzymatic reactions
which are difficult to measure directly are
coupled with an NAD(P)-dependent indicator
reaction (cf. 2.6.1.1) for food analysis.
(2.5)
The reaction proceeds stereospecifically (cf.
2.4.1.2.1); ribose phosphate and the −CONH 2
group force that pyridine ring of the cosubstrate
to become planar on the enzyme surface. The
role of Zn 2+ ions in this catalysis is outlined
in section 2.3.3.1. The transhydrogenases differ
according to the site on the pyridine ring involved
in or accessible to H-transfer. For example,
alcohol and lactate dehydrogenases transfer
Fig. 2.5. Electron excitation spectra of NAD (1) and
NADH (2)
* Until the absolute configuration of the chiral center is
determined, the two sides of the pyridine ring are denoted
as A and B.