Biology Reference
In-Depth Information
retinol
retinal
VITAMIN A
retinoic acid
VITAMIN D
VITAMIN E
FIGURE 5.26
Structures of the lipid-soluble vitamins: The family of vitamin As, retinol, retinal and retinoic
acid; vitamin D; and vitamin E.
derived from a Greek word meaning 'to carry a pregnancy' and was discovered in wheat
germ oil in 1922 by Herbert Evans and Katherine Bishop. The vitamin's primary function
is to prevent lipid peroxidation in biological membranes. It is the major anti-oxidant in the
human body where it has been linked to prevention of a variety of human afflictions
including cellular aging and cardiovascular disease.
Superficially
-tocopherol resembles cholesterol, having a polar OH anchoring the mole-
cule to the aqueous interface, a rigid chromanol ring and a floppy hydrophobic tail extending
into the membrane interior. A major problem in understanding how
a
-tocopherol functions
in a membrane is deducing how such low levels of the vitamin (estimated to be between ~0.1
to 1 mol% of phospholipids) can protect the large excess of polyunsaturated fatty acids from
being peroxidized. This is the equivalent of one molecule of
a
a
-tocopherol for every 100
e
1,000
phospholipids. The anti-oxidant role of
-tocopherol exemplifies many biological processes.
While there is a basic understanding of the process itself, it is not at all clear why the process
is so efficient.
The lipid-soluble vitamins A, D, and E were all discovered in less than a ten year period in
the early 20
th
century.
a
Ubiquinone (Coenzyme Q)
Coenzyme Q (also known as CoQ or ubiquinone,
Figure 5.27
) is a major electron and
proton carrying component of the mitochondrial electron transport system
[42]
. CoQ was
discovered in 1957 by Frederick Crane from an extract of beef heart. The molecule has
a water-soluble quinone anchored to the hydrophobic membrane interior by a very long
methyl branched (isoprenoid) chain. One of the most common ubiquinones has a 50-carbon
