Page 61 - Color Atlas of Biochemistry
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52 Biomolecules
Isoprenoids tabolism, but cannot be produced by them
independently, are vitamins; this group
includes vitamins A, D, E, and K. Due to its
A. Activated acetic acid as a component of
structure and function, vitamin D is now usu-
lipids
ally classified as a steroid hormone (see
Although the lipids found in plant and animal pp. 56, 330).
organisms occur in many different forms, Isoprene metabolism in plants is very com-
they are all closely related biogenetically; plex. Plants can synthesize many types of ar-
they are all derived from acetyl-CoA,the “ac- omatic substances and volatile oils from iso-
tivated acetic acid” (see pp.12, 110). prenoids. Examples include menthol (I= 2 ),
1. One major pathway leads from acetyl- camphor (I = 2), and citronellal (I = 2). These
CoA to the activated fatty acids (acyl-CoA;for C 10 compounds are also called monoterpenes.
details, see p.168). Fats, phospholipids, and Similarly, compounds consisting of three iso-
glycolipids are synthesized from these, and prene units (I = 3) are termed sesquiterpenes,
fatty acid derivatives in particular are formed. and the steroids (I = 6) are called triterpenes.
Quantitatively, this is the most important Isoprenoids that have hormonal and sig-
pathway in animals and most plants. naling functions form an important group.
2. The second pathway leads from acetyl- These include steroid hormones (I = 6) and
CoA to isopentenyl diphosphate (“active iso- retinoate (the anion of retinoic acid; I = 3) in
prene”), the basic component for the isopren- vertebrates, and juvenile hormone (I = 3) in
oids. Its biosynthesis is discussed in connec- arthropods. Some plant hormones also belong
tion with biosynthesis of the isoprenoid, cho- to the isoprenoids—e. g., the cytokinins, absci-
lesterol (see p.172). sic acid, and brassinosteroids.
Isoprene chains are sometimes used as
lipid anchors to fix molecules to membranes
B. Isoprenoids
(see p. 214). Chlorophyll has a phytyl residue (I
Formally, isoprenoids are derived from a sin- =4) as a lipid anchor. Coenzymeswith iso-
gle common building block, isoprene (2- prenoid anchors of various lengths include
methyl-1,3-butadiene), a methyl-branched ubiquinone (coenzyme Q; I = 6–10), plastoqui-
compound with five C atoms. Activated none (I = 9), and menaquinone (vitamin K; I =
isoprene, isopentenyl diphosphate, is used by 4–6). Proteins can also be anchored to mem-
plants and animals to biosynthesize linear branes by isoprenylation.
and cyclic oligomers and polymers. For the In some cases, an isoprene residue is used
isoprenoids listed here—which only represent as an element to modify molecules chemi-
a small selection—the number of isoprene cally. One example of this is N'-isopentenyl-
units (I) is shown. AMP, which occurs as a modified component
From activated isoprene, the metabolic in tRNA.
pathway leads via dimerization to activated
geraniol (I = 2) and then to activated farnesol (I
= 3). At this point, the pathway divides into
two. Further extension of farnesol leads to
chains with increasing numbers of isoprene
units—e. g., phytol (I = 4), dolichol (I = 14–24),
and rubber (I = 700–5000). The other pathway
involves a “head-to-head” linkage between
two farnesol residues, giving rise to squalene
(I = 6), which, in turn, is converted to choles-
terol (I = 6) and the other steroids.
The ability to synthesize particular iso-
prenoids is limited to a few species of plants
and animals. For example, rubber is only
formed by a few plant species, including the
rubber tree (Hevea brasiliensis). Several iso-
prenoids that are required by animals for me-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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