Biomedical Engineering Reference
In-Depth Information
Phytoene Synthesis
The reaction of phytoene synthesis is mediated by a highly conserved
enzyme, phytoene synthase. Although this enzyme is common to the
majority of carotenoid-producing organisms, in algae and higher plants
there are some variations in the amino terminal region due to the presence
of a signal peptide responsible for the localization of these enzymes in
chloroplasts and chromoplasts. The peptide is processed at the plastids
enttrance resulting in shorter proteins (Bartley et al. 1992). They are
usually membrane-associated proteins, as in
Neurospora
(Harding and
Turner 1981, Mitzka-Schnabel et al. 1981), but can also be located in the
cytoplasm as in
Mycobacterium
(Gregonis and Rilling 1974). They have a
size between 35 and 39 kDa (Chamovitz et al. 1992, Misawa et al. 1994),
are monomeric and associated with divalent cations (Dogbo et al. 1988).
The comparison of amino acid sequences of several phytoene synthase
enzymes (encoded by
crtB
bacteria genes and
psy
genes in plants, algae
and cyanobacteria) shows an area with several conserved regions that
could be the GGPP binding motif.
Dehydrogenation of Phytoene
Phytoene dehydrogenation takes place through a series of successive
desaturations as a result of which four double bonds are progressively
introduced in the phytoene molecule. The introduction of these double
bonds takes place in a symmetric way with respect to the central axis of
the molecule, leading to the formation of lycopene as the fi nal product
through a series of intermediates such as phytofl uene, ΞΆ-carotene and
neurosporene (through the introduction of one, two and three double
bonds, respectively). This sequential dehydrogenation was proposed by
Porter and Lincoln in 1950, before the process intermediates were known.
There are two different types of phytoene dehydrogenases, one
characteristic of organisms with oxygenic photosynthesis and the other
of organisms that lack it. These two types of enzymes have quite different
characteristics (Sandmann 1994a, b) which have been considered as an
example of convergent evolution, without a common origin (Pecker et al.
1992).
In organisms without oxygenic photosynthesis (fungi, archaea and
some eubacteria) the type of phytoene dehydrogenases are
crtI
. This type
of enzyme is characterized by introducing four dehydrogenations in the
phytoene molecule resulting in lycopene. Within this group there are some
exceptions, such as in the case of
Rhodobacter capsulatus
where this enzyme
introduces only three dehydrogenations giving rise to neurosporene as the
fi nal compound (Armstrong 1997), or
Neurospora crassa
where the enzyme
