Biomedical Engineering Reference
In-Depth Information
pathways. In this sense,
Phycomyces
is feed-back regulated by the end-product,
while in
Mucor
and
Blakeslea
this regulation has not been described.
INTRODUCTION
Carotenoids Description and Structure
Carotenoids are one of the most diverse and widely distributed groups
of natural pigments including more than 600 coloured compounds.
Carotenoids are synthesized by most photosynthetic organisms like
cyanobacteria, algae and plants and some non-photosynthetic. Such
as certain fungi, yeasts and bacteria. Animals and humans cannot
synthesize them and acquire them through diet. These lipid compounds
are soluble in various organic solvents and in plants are mainly located
in membranes, where they are normally associated with proteins being
part of the antenna complexes along with chlorophyll. Carotenoids are
very sensitive to oxidation by light or oxygen in the air and to heat that
causes cis-trans isomerization.
Since the fi rst carotenoid was discovered in the early nineteenth century,
new members have been adding to this family of compounds which now
has hundreds of members (Goodwin 1980, Britton et al. 1998, Sandmann
and Misawa 2002). All of them are derived from isoprenoids, which are
characterized by being constituted by a variable number of isoprene
molecules (2-methyl-1,3-butadiene). Carotenoids are linear molecules of 40
carbon atoms, composed of 8 isoprene units assembled so that their order
is reversed in the center of the molecule. To this basic structure several
changes can be made, such as the union of six different types of rings at the
ends, changes in the level of hydrogenation or addition of functional groups
containing oxygen. Depending on whether or not they contain oxygen,
there are two types of carotenoids: the carotenes, which contain only carbon
and hydrogen and xanthophylls, which also contain oxygen. Both of them
are highly unsaturated compounds. The set of conjugated double bonds
of these molecules constitute the chromophore group, responsible for the
ability to absorb light at different wavelengths (depending on the carotenoid
chromophore group in question) and so responsible for the carotenoid´s
colour (yellow, orange, green, white or red). In general, all of them absorb
light in the visible region of the electromagnetic spectrum (370-750 nm)
and their absorption spectra look very similar, with three absorption peaks
clearly defi ned. The wavelength of each peak depends on the particular
distribution of double bonds in the molecules and whether they have linear
or cyclic ends. The distribution of double bonds in the chromophore group
also determines the existence of a carotenoid cis or trans form, trans forms
predominate over the cis forms because these latter are less stable. Thus,
