Food Chemistry

2.3 Enzyme Cofactors 103

2.3.2.2 Hemin.................................................

Peroxidases from food of plant origin and several
catalases contain ferri-protoporphyrin IX (hemin,
cf. Formula 2.11) as their prosthetic group and as
the chromophore responsible for the brown color
of the enzymes:

(2.11)

In catalytic reactions there is a change in the
electron excitation spectra of the peroxidases
(Fig. 2.6a) which is caused by a valence change
of the iron ion (Fig. 2.6b). Intermediary com-
pounds I (green) and II (pale red) are formed
during this change by reaction with H 2 O 2
and reducing agent AH. The reaction cycle is
completed by another single electron transfer.
Some verdoperoxidases, which are green in color
(as suggested by their name) and found in vari-
ous foods of animal origin, e. g. milk, contain an

Fig. 2.6.Peroxidase reaction with H 2 O 2 and a hydrogen donor (AH).aElectron excitation spectra of peroxidase
and intermediates I and II;bmechanism of catalysis

unidentified Fe-protoporphyrin as their prosthetic

group.

2.3.2.3 Pyridoxal Phosphate

Pyridoxal phosphate (Formula 2.12) and pyridox-

amine (Formula 2.13), derived from it, are desig-

nated as vitamin B 6 (cf. 6.3.3) and are essential

ingredients of food:

(2.12)

(2.13)

Coupled to the enzyme as a prosthetic group

through a lysyl residue, pyridoxal phosphate

is involved in conversion reactions of amino

acids. In the first step of catalysis, the amino

group of the amino acid substrate displaces

the 6-amino group of lysine from the aldimine

linkage (cf. Reaction 2.14). The positively

charged pyridine ring then exerts an electron

shift towards theα-C-atom of the amino acid

substrate; the shift being supported by the release

of one substituent of theα-C-atom. In Fig. 2.7

is shown how the ionization of the proton

attached to theα-C-atom leads totransamination