Biomedical Engineering Reference
In-Depth Information
performs fi ve dehydrogenations producing 3,4-didehydrolycopene (as an
intermediate step in the formation of toluene) (Hausmann and Sandmann
2000).
CrtI
-type enzymes often use FAD as a cofactor (Fraser et al. 1992),
although there are exceptions, as in the case of
N. crassa
which uses NAD
(Hausmann and Sandmann 2000).
In organisms with oxygenic photosynthesis the phytoene dehydrogenase
types are
crtP
or
pds
and often use NAD (P) as cofactor (Hausmann and
Sandmann 2000). These enzymes are characterized by introducing only two
dehydrogenations in the phytoene molecule producing ζ-carotene as the
fi nal product. In organisms with
crtP
type enzymes lycopene is produced
by the action of a second type of enzymes, ζ-carotene dehydrogenases
(
zds
or
crtQ
). These enzymes act on the ζ-carotene molecule producing
lycopene as the end product. The comparison of the amino acid sequence
of phytoene dehydrogenases known to date suggests that the ζ-carotene
dehydrogenases derive phylogenetically from the
crtI-
type phytoene
dehydrogenases (Linden et al. 1993).
During the phytoene dehydrogenation process a cis to all-trans
conformation change takes place. The isomerization occurs in different steps
of the dehydrogenation process depending on the
crtI
or
crtP
enzyme types.
Thus, in the
crtI
-type isomerization takes place at the level of phytoene,
whereas in the
crtP
type it seems to take place at the ζ-carotene (Sandmann
1994b).
Lycopene Cyclation
The subsequent steps in the formation of lycopene vary signifi cantly from
one organism to another. The different processes occurring in the molecule
of lycopene (and sometimes in the neurosporene) cause a wide range of
compounds in nature. However, the most common step is the cyclation of
at least one end of the molecule. Among the six different types of rings the
β-ring is the most common. In many fungi the sequential introduction of
β-rings at each end of the lycopene molecule leads fi rst to the γ-carotene
and then to β-carotene.
The cyclation of the lycopene molecule is carried out for different
types of lycopene cyclases in different organisms. There are several types
of these enzymes distributed in nature which are similar to phytoene
dehydrogenases. Thus, in organisms without oxygenic photosynthesis
enzymes are of
crtY
type, whereas in those with oxygenic photosynthesis
the enzymes are of
licC
or
crtL
types. These enzymes have a size of about
400 amino acids, being slightly higher in plants due to the possession of
a peptide for organellar location. Although enzyme
crtY
and
crtL
types
have different primary structures, both types have conserved sequences,
such as the consensus sequences I (F X Y X
4
S X
6
E X T) and II (G X
2
A X
3
