2009_Book_FoodChemistry

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:
unidentified Fe-protoporphyrin as their prosthetic
group.
2.3.2.3 Pyridoxal Phosphate
Pyridoxal phosphate (Formula 2.12) and pyridoxamine
(Formula 2.13), derived from it, are designated
as vitamin B 6 (cf. 6.3.3) and are essential
ingredients of food:
(2.12)
(2.13)
(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 compounds
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 various
foods of animal origin, e. g. milk, contain an
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 the α-C-atom of the amino acid
substrate; the shift being supported by the release
of one substituent of the α-C-atom. In Fig. 2.7
is shown how the ionization of the proton
attached to the α-C-atom leads to transamination
Fig. 2.6. Peroxidase reaction with H 2 O 2 and a hydrogen donor (AH). a Electron excitation spectra of peroxidase
and intermediates I and II; b mechanism of catalysis