Diverse Activities of Carotenoid Cleavage Oxygenases - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 401

marinus MIT9313 was inactive under conditions tested as was the second CCO paralog present in
Synechocystis sp. PCC6803 (SYC1).
Four different species of cyanobacteria ( Nostoc sp., Nostoc punctiforme , Synechocystis PCC6803,
Synechococcus elongatus ) have been shown to cleave (apo)carotenoids into retinal which is the
chromophore for retinylidene proteins (see Table 19.4 for an overview of cleavage activities). A
sensory rhodopsin protein (ASR) was recently identii ed in Nostoc sp. PCC 7120 and shown to have
a unique photocycle and exhibit light-induced reversible interconversion between 13- cis - and all-
trans -retinal (Jung et al. 2003, Sineshchekov and Spudich 2004, Vogeley et al. 2004, Sineshchevkov
et al. 2005). ASR may act as a sensor for light-regulated processes such as chromatic adaptation in
Nostoc sp. PCC 7120. Intriguingly the other three strains that also generate retinal by specii c caro-
tenoid cleavage enzymes do not have ASR homologs in their genomes. The role of retinal in these
organisms is a major unanswered question given retinal's prevalence in nature as a signaling mol-
ecule (see Section 19.5). Another interesting observation that may have biological signii cance is the
presence of multiple cleavage enzymes with different functions in single cyanobacterial genomes.
Filamentous cyanobacteria have several CCO paralogs in their genomes compared to unicellular
cyanobacteria that have only one or two CCO paralogs, although carotenoid biosynthetic pathways
are not signii cantly duplicated or more complicated in i lamentous Nostoc species (Liang C 2006).
Multiple CCO enzymes in these strains may act sequentially on cleavage products to synthesize
molecules similar to those produced by the CCO activities of CCD7 and CCD8 from Arabidopsis .
This is an area of research that merits further investigation.

19.3.5 B ACTERIAL CCO S
The cleavage of the interphenyl a,b-double bond of lignostilbenes by molecular oxygen to the
corresponding aldehydes is a reaction analogous to the carotenoid cleavage reaction catalyzed by
NCED in the biosynthesis of the plant hormone ABA (Figure 19.4) (Kamoda and Saburi 1993a,b,
Han et al. 2002, Schwartz et al. 2003). Enzymes (four isoenzymes corresponding to two genes)
from Sphingomonas paucimobilis TMY1009 have been shown to cleave stilbene-type intermedi-
ates which can arise from the degradation of dimeric lignin compounds (Kamoda and Saburi 1995).
Protein sequences of NCEDs and LSD isoenzymes are similar and consequently many members of
the bacterial CCO family are annotated in databases as lignostilbene-a,b-dioxygenases (LSD, EC
1.13.11.43). The enzymatic reaction was shown to require molecular oxygen and ferrous iron as do
other CCO catalyzed reactions. LSDs are proposed to be involved in degradation of the cell wall
constituent lignin, although this has never been shown with natural substrates. Lignin degradation
products include metabolites belonging to the l avonoid/stilbene class of compounds.
Descriptions of the cloning, enzyme purii cation, and inhibitor studies on four LSD isoforms
are described in a series of papers (Kamoda and Saburi 1993a,b, 1995, Kamoda et al. 1997, 2003,
2005). The four isoforms have different substrate specii cities for substituted stilbenoids (Kamoda
and Saburi 1993, Kamoda et al. 2003). In vitro assays with stilbene derivatives containing various
functional groups showed that a 4 -hydroxyl group was essential for cleavage (Kamoda et al. 2003).

Inhibitor studies suggest that the binding of biphenyl substrates in LSDs is different from carotenoid
binding in NCEDs as LSD inhibitors did not inhibit NCED enzymes (Han et al. 2002).
Recently, two enzymes NOV1 (YP_496081) and NOV2 (YP_498079) from Novosphingobium
aromaticivorans DSM12444 have been identii ed that also cleave stilbene compounds (Figure 19.4)
(Marasco and Schmidt-Dannert 2008). Both enzymes cleave stilbenes with a range of functional
group substitutions and required a 4

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-oxygen functional group. Unlike the Sphingomonas enzymes,
the NOV enzymes had similar substrate specii cities (Marasco and Schmidt-Dannert 2008). Both
mono- and dioxygenase mechanisms have been attributed to CCO enzymes, but labeling studies
with the NOV enzymes favors a monooxygenase mechanism (Marasco and Schmidt-Dannert 2008)
(see Section 19.4).

′