Food Chemistry

2.3 Enzyme Cofactors 99

hand, during metabolism, the cosubstrate reacts
with at least two enzymes. It transfers the hydro-
gen or the functional group to another enzyme
and, hence, is denoted as a “transport metabo-
lite” or as an “intermediary substrate”. It is dis-
tinguished from a true substrate by being regener-
ated in a subsequent reaction. Therefore the con-
centration of the intermediary substrates can be
very low. In food analysis higher amounts of co-
substrates 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 cofac-
tors are related to water-soluble vitamins (cf. 6.3).
The metal ions are dealt with separately in sec-
tion 2.3.3.

2.3.1 Cosubstrates............................................

2.3.1.1 NicotinamideAdenineDinucleotide........................

Transhydrogenases (e. g. lactate dehydrogenase,
alcohol dehydrogenase) dehydrogenate or hydro-
genate their substrates with the help of a pyridine
cosubstrate (Fig. 2.4); its nicotinamide residue
accepts or donates a hydride ion (H−) at pos-
ition 4:

(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. 2.4.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 pyri-

dine cosubstrate are readily distinguished by

absorbance readings at 340 nm (Fig. 2.5). There-

fore, 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.

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 de-

noted as A and B.