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Carotenoids
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Note: Carotene and
xanthophyll structures shown in these pages are the 'all-trans (E)'
forms. Many carotenoids also naturally occur as various Z isomers. |
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Carotenoid confusion: trivial (common) names vs. ring structures vs. semi-systematic
names
Common
names are in
red
to distinguish them from ring structures Carotenoid molecules with one or two ring structures on the ends are
derived by cyclization of lycopene. Three basic kinds of six-membered
rings are found in the carotenoids:
 The
green structure is known as a 'beta ring' and can be found on both ends of
the beta-carotene molecule as well as at one
end of the alpha-carotene molecule and the gamma-carotene molecule. The blue
structure is a 'gamma ring' and is less common than the other two. The
pink structure is called an 'epsilon ring' and is found in both
alpha- and delta-carotenes.
Uncyclized ends, for example in lycopene, are
known as a 'psi' end groups. The names of the beta and
epsilon ring structures were derived from the common names beta-carotene
and epsilon-carotene:
beta-carotene has a 'beta' ring at both ends;
epsilon-carotene has an 'epsilon' ring at both ends.
The 'gamma' ring is named after the terpenoid compound gamma-ionone. There are no 'alpha' or 'delta'
rings.
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Thus, alpha-carotene contains one 'beta'
ring and one 'epsilon' ring; beta-carotene
contains two 'beta' rings; gamma-carotene has
one 'beta' ring and one uncyclized ('psi') end; and delta-carotene
contains one 'epsilon' ring and one 'psi' end:
Why doesn't gamma-carotene contain a 'gamma'
ring, or why isn't delta-carotene called epsilon-carotene, since it has only one ring, an
'epsilon' type? Welcome to the strange world of carotene nomenclature. There
are actually three different naming systems at work, the names of the rings
themselves, the 'trivial'
names for the entire molecule, and the official IUPAC 'semi-systematic' names
for the molecules.
The trivial (common) names for these compounds are the familiar alpha-,
beta-, delta- and gamma-carotenes.* The 'semi-systematic' IUPAC names
for these carotenoids are constructed by designating the structure at both
ends of the molecule. Thus beta-carotene
(with two 'beta' rings) is 'beta,beta-carotene'; alpha-carotene
(with one 'beta' and one 'epsilon' ring) is 'beta,epsilon-carotene'; delta-carotene
(with one 'epsilon' ring and one uncyclized end) is 'epsilon,psi-carotene'
and gamma-carotene (one 'beta' ring, one uncyclized
end) is 'beta,psi-carotene.' Eyes glazed over yet?
*How did these
carotenes get their common names in the first place? They were basically
named for the order in which they were discovered.
Alpha- and beta-carotenes were the first
two compounds isolated in the 1930s from the previously known mixture of
carotenoids called simply 'carotene.' The isolation of
gamma-carotene was accomplished shortly
thereafter and delta-carotene was identified a few years
later. |
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Biosynthesis of the
carotenoids
Carotenoids are tetraterpenes originating from the
mevalonate & deoxyxylulose phosphate pathways (older sources sometimes refer
to their source as the 'isoprenoid' pathway). Two molecules of the
C20 compound
geranylgeranyldiphosphate (GGDP) condense to form the symmetrical carotenoid
skeleton. The first dedicated step in the pathway leading to carotenoids is
the formation of phytoene by the enzyme phytoene synthase.
 Phytoene
[(15Z)-Phytoene], reminiscent of a Mayan step pyramid in this
diagram, is the direct precursor of the carotenoids. The short central series of
three conjugated double bonds (triene system) is not extensive enough to
provide a colored molecule. However, as phytoene is transformed into
lycopene, the backbone of the molecule undergoes a number of sequential desaturations to form the
long conjugated series of double bonds characteristic
of the highly colored carotenoid chromophore. In the majority of molecules,
an isomerization of the
central Z bond also occurs, resulting in the all-trans configuration
of lycopene:
The next step in the biosynthesis of many carotenoids is cyclization of one
or both end groups of the molecule. As noted above, one of three
six-membered ring structures may form (beta, gamma, or epsilon), or one end
may remain uncyclized:
After cyclization, oxygen functions may be added (hydroxy or keto
groups) to form the various xanthophylls (many of these are named after the
organism from which they were first isolated):
Epoxides may be formed from the six-membered rings, as in violaxanthin. A
contraction of the epoxide structure leads to the five-membered ring as
found on the ends of capsanthin and capsorubin:
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Apocarotenoids
are carotenoid derivatives formed by the removal of fragments of the carbon
backbone from either or both ends of a C40 precursor such as
lycopene or beta,beta-carotene.
 Two
well-known apocarotenoids are bixin and crocetin. Bixin is one of several
highly colored molecules extracted from annatto seed coats (Bixa orellana) and
used as a food coloring; crocetin, occurring as
various glycosyl
esters, is the coloring principle of saffron, pollen harvested from
Crocus
sativus. Capsicum
and Citrus
spp. also contain small amounts of several apocarotenoids. |
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General
references
1. Britton, G. Overview of Carotenoid
Biosynthesis. In: Carotenoids, Vol. 3: Biosynthesis and Metabolism.
Britton, G.; Leeaen-Jensen, S.; Pfander, H. Eds.
Birkhauser Verlag:
Basel, 1998; pp 13-147.
2. Weedon, B. C. L. and G. P. Moss. Structure and Nomenclature. In:
Carotenoids, Vol. 1A: Isolation and Analysis.
Britton, G.; Leeaen-Jensen, S.; Pfander, H. Eds.
Birkhauser Verlag:
Basel, 1995; pp 27-70.
3. Dewick, Paul. Medicinal Natural Products: A Biosynthetic Approach. John
Wiley & Sons, Ltd: Chichester, UK. 2002; pp 226-231. |
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