16 Part I: Principles
some of the enzymatic reactions proposed in the
degradation of chlorophyll a(Table 1.12).
MEVALONATE ANDISOPENTYLDIPHOSPHATE
BIOSYNTHESIS PRIOR TOFORMATION OF
CAROTENOIDS
Table 1.13 lists the sequence of reactions in the for-
mation of (R)-mevalonate from acetyl-CoA, and
from (R)-mevalonate to isopentyl diphosphate. Iso-
pentyl diphosphate is a key building block for caro-
tenoids (Croteau et al. 2000). Carotenoids are the
group of fat-soluble pigments that provides the yel-
low to red colors of many common fruits such as
yellow peaches, papayas, and mangoes. During post-
harvest maturation, these fruits show intense yellow
to yellowish orange colors due to synthesis of caro-
tenoids from its precursor isopentyl diphosphate,
which is derived from (R)-mevalonate. Biosyntheses
of carotenoids and terpenoids have a common pre-
cursor, (R)-mevalonate derived from acetyl-CoA
(Table 1.13). (R)-mevalonate is also a building block
for terpenoid biosynthesis (Croteau et al. 2000;
IUBMB website).NARINGENINCHALCONEBIOSYNTHESISFlavonoids are a group of interesting compounds
that not only give fruits and vegetables various
red, blue, or violet colors, but also are related to
the group of bioactive compounds called stilbenes.
They have a common precursor of trans-cinnamate
branching out into two routes, one leading to the
flavonoids, and the other leading to stilbenes (Table
1.14; IUBMB website). Considerable interest has
been given to the stilbene trans 3,5,4’-trihydroxystil-
bene (commonly called reveratrol or resveratrol) in
red grapes and red wine that may have potent antitu-
mor properties and to another stilbene, combretas-
tatin, with potential antineoplastic activity (CroteauTable 1.12.Degradation of Chlorophyll
Enzyme Reaction
Chlorophyllase (EC 3.1.1.4) Chlorophyll →chlorophyllide phytol
Magnesium dechelatase (EC not available) Chlorophyllide a →phyeophorbide aMg^2
Phyeophorbide aoxygenase (EC not available) Phyeophorbide aO 2 →red chlorophyll
catabolite (RCC)
RCC reductase (EC not available) RCC →fluorescent chlorophyll catabolite (FCC)
Various enzymes FCC →nonfluorescent chlorophyll catabolites
(NCC)
Sources:Eskin 1990, Dangl et al. 2000, IUBMB-NC website (www.iubmb.org).
Table 1.13.Mevalonate and Isopentyl Diphosphate Biosyntheses
Enzyme Reaction
Acetyl-CoA C-acetyltransferase (EC 2.3.1.9) 2 acetyl-CoA →acetoacetyl-Co-A CoA
Hydroxymethylglutaryl-CoA-synthase Acetoacetyl-CoA acetyl-CoA H 2 O →(S)-
(EC 2.3.3.10) 3-hydroxy-3-methylglutaryl CoA CoA
Hydroxymethylglutaryl-CoA reductase (NADPH 2 )(S)-3-hydroxy-3-methylglutaryl-CoA 2 NADPH 2
(EC 1.1.1.34) →(R)-mevalonate CoA 2 NADP
Mevaldate reductase (EC 1.1.1.32) (R)-mevalonate NAD →mevaldate NADH 2
Mevalonate kinase (EC 2.7.1.36) (R)-mevalonate ATP →(R)-5-
phosphomevalonate ADP
Phosphomevalonate kinase (EC 2.7.4.2) (R)-5-phosphomevalonate ATP →(R)-5-
diphosphomevalonate ADP
Diphosphomevalonate decarboxylase (EC 4.1.1.33) (R)-5-diphosphomevalonate ATP →isopentyl
diphosphate ADP phosphate CO 2
Sources:Croteau et al. 2000, IUBMB-NC Enzyme website (www.iubmb.org).