Chlorophyll b Biosynthetic Pathway

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  • XI. The Chl b Biosynthetic Pathway: Chl b Biosynthetic Routes

    A. Chlorophyllide b (Chlide b)

    1. Biochemical Heterogeneity of Chlide b
    2. Metabolism of MV Chlide b
    a. Formation of MV Chlide b from MV Chlide a
    a. Formation of MV Chlide b from DV Chlide a and MV Chlide a via Route 4 in Greening DDV-LDV-LDDV Plants
    b. Formation of MV Chlide b from MV Chlide a via Route 14 in greening DMV-LDV-LDMV Plants
    b. Formation of MV Chlide b from MV Pchlide b in Greening Plants
    a. Possible Formation of MV Chlide b from MV Pchlide b in Green DDV-LDV-LDDV Plants via Biosynthetic Route 9
    b. Possible Formation of MV Chlide b from MV Pchlide b in Green DMV-LDV-LDMV Plants via Biosynthetic Route 11
    c. Formation of DV Chlide b via Route 6

    B. Chlorophyll b

    1. Chlorophyll b Biosynthetic Heterogeneity
    a. Biosynthetic Heterogeneity of MV Chlorophyll b
    a. Formation of MV Chl b from MV Chlide a and MV Chl a via Routes 2, and 5 and from MV Chlide b via route 4 in etiolated DDV-LDV-LDDV Plants Subjected to Illumination.
    b. Biosynthesis of MV Chl b from MV Chl a via Route 8 in Greening DDV-LDV-LDDV Plants during the Light Phases of the Photoperiod.
    g. Biosynthesis of MV Chl b from MV Pchlide b via Route 9 in Greening DDV-LDV-LDDV Plants during the Light Phases of the Photoperiod
    d. Biosynthesis of MV Chl b from MV Chl a via Route 10 in Green DMV-LDV-LDMV Plants during the Light Phases of the Photoperiod
    e. Biosynthesis of MV Chl b from MV Pchlide b via Route 11 in Greening DMV-LDV-LDMV Plants during the Light Phases of the Photoperiod
    z. Biosynthesis of MV Chl b from MV Chl a via Route 12 and from MV Chlide a via Route 14 in Etiolated DMV-LDV-LDMV Plants after Exposure to Light, and in Greening DMV-LDV-LDMV Plants during the Initial Dark Phases of the Photoperiod
    b. Biosynthesis of DV Chlorophyll b
    a. Conversion of DV Chl a to DV Chl b via Route 1
    b. Conversion of DV Chlide b to DV Chl b via Route 6

  • C. References

    XI. The Chl b Biosynthetic Pathway

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    Fig. 1. Integrated Chl a/b biosynthetic pathway depicting 15 carboxylic routes. To facilitate understanding of the text, various biosynthetic routes are designated by the numbers 1-14. Black boxes with yellow lettering joined by black arrows and numbers refer to biosynthetic routes that occur in etiolated tissues in darkness, or in greening tissues at the beginning of the light phase of the photoperiod. White boxes with green lettering joined by yllowish-brown arrows and numbers refer to biosynthetic routes that occur in green tissues, during the light phases of the photoperiod. Blue frames refer to routes that occurin DDV-LDDV greening groups. Red frames refer to routes that occurin DMV-LDMV greening groups.

    FourteenRoutesCarboxylicPW.jpg - 86601 Bytes


    The demonstration of metabolic pathways is a multistep process. It involves at least three stages: (a) the detection and characterization of metabolic intermediates, (b) the demonstration of precursor-product relationships between putative intermediates, and (c) purification and characterization of enzymes involved in the metabolic interconversions. These criteria will be applied in our evaluation of the experimental evidence that supports the operation of a multibranched Chl b biosynthetic pathway in green(ing) plants.

    A. Chlorophyllide b (Chlide b)

    MV Chlide b
    DV Chlideb

    Chlorophyllides b (Chlides b) are the immediate precursors of Chl b. Chlides b are chemically and biochemically heterogeneous. Chemical heterogeneity consists in MV and DV substitutions at position 4 of the macrocycle. Biochemical heterogeneity consists of multiple biosynthetic routes involving MV Chlide a and putative MV Pchlide b precursors.

    1. Biochemical Heterogeneity of Chlide b
    Figure. 1, depicts the operation of five biosynthetic routes that lead to the formation of four MV Chlide b, and one DV Chlide b pools. The metabolism of these pools will be discussed below.

    2. Metabolism of MV Chlide b

    MV Chlide b was simultaneously detected in greening (Duggan and Rebeiz, 1981; 1982) and green higher plant tissues (Aronoff, 1981). It was proposed as a logical immediate precursor of MV Chl b. Subsequently the conversion of exogenous MV Chlide b to MV Chl b in etiolated oat was reported (Benz and Rudiger, 1981).

    The pool of MV Chlide b exhibited the spectrofluorometric properties of MV Chl b in diethyl ether at 298 and 77 K, but had the chromatographic mobility and solubility of a monocarboxylic phorbin. The presence of a free carboxylic group and a formyl group was demonstrated by methylation withdiazomethane and conversion to a Chlide b oxime upon reaction with hydroxylamine (Duggan and Rebeiz, 1982). The concentration of Chlide b in green tissues was in the same range as that of MV Pchlide a and MV Chlide a. It was estimated that less than 15% of the Chlide b pool could have arisen by hydrolysis of phytol at position 7 of ther microcycle via chlorophyllase activity in vitro as confirmed by the extent of hydrolysis of 14C-labeled MV Chl b added to green tissues just before pigment extraction (Duggan and Rebeiz, 1982).

    The source of oxygen of the formyl group at position 3 of the macrocycle has been investigated by Porra et al (1993; 1994). Mass spectra of [7-hydroxymethyl]-Chl b extracted from leaves greened in the presence of either 18O2 or H218O2 revealed that 18O was incorporated only from molecular oxygen into the 3-formyl group of Chl b. The high enrichment using 18O2, and the absence of labeling by H218O2, suggested that molecular oxygen is the sole precursor of the 3-formyl oxygen of Chl(ide) b in greening maize leaves. This in turn suggested that a mono-oxygenase is involved in the oxidation of the methyl group to a formyl.

    The biosynthesis of MV Chlide b is highly heterogeneous (vide infra). In the elucidation of this biosynthetic heterogeneity, extensive use was made of kinetic analysis of precursor-product relationships in vivo. For that purpose, equations were derived to investigate possible precursor-product relationships in branched, and interconnected pathways (Rebeiz, et al, 1999; Tripathy and Rebeiz, 1988 ; Rebeiz, et al, 1988). It was shown that for any two compounds A and B, formed from a common precursor P such as ALA, and having a possible direct precursor-product relationship between them, for any number of time intervals t1 to t2, an equation can be derived that describes the relationship between (a) the specific radioactivity of compound A, and the possible radiolabel incorporation from compound A into compound B, and (b) the possible net synthesis of compound B from compound A (Rebeiz et al, 1988).

    a. Formation of MV Chlide b from MV Chlide a

    As depicted in Fig. 1, two of the four MV Chlide b pools are formed from MV Chlide a via routes 4, and 14. These two biosynthetic routes are discussed below.

    a. Formation of MV Chlide b from DV Chlide a and MV Chlide a via Route 4 in Greening DDV-LDV-LDDV Plants

    Conversion of DV Pchlide a to MV Chlide b via route 4 i. e. via DV Chlide a, and MV Chlide a, in greening DDV-LDV-LDDV plants is supported by the conversion of exogenous ALA to MV Chlide b and Chl b in etiochloroplasts prepared from etiolated cucumber cotyledons subjected to 4 h of illumination prior to etiochloroplast isolation (Kolossov et al, 1999). In such systems ALA is converted mainly to DV Pchlide a (Tripathy and Rebeiz, 1986) which is readily convertible to MV Chlide a via DV Chlide a.

    b. Formation of MV Chlide b from MV Chlide a via Route 14 in greening DMV-LDV-LDMV Plants

    Conversion of MV Chlide a to MV Chlide b via route 14 in DMV-LDV-LDMV plants is supported by precursor-product relationships analysis in vivo between MV Chlide a and MV Chl b in greening corn seedlings, (Rebeiz et al, 1999). After 5 hours of greening of etiolated corn seedlings, about 27-36% of the MV Chl b was formed from MV Chlide a. Under these conditions MV Chlide a is formed in turn from MV Pchlide a (Fig. 1). Under the same conditions, conversion of MV Chl a to MV Chl b by route 12 was low, and amounted to a maximum of 5% after 7 hours of greening (Rebeiz et al, 1999). These results indicated that under these conditions, the fate of the nascent MV Chlide a was conversion to MV Chl b by way of MV Chlide b rather than conversion to MV Chl b via MV Chl a. After 7 hours of greening, the rate of MV Chl b formation from MV Chlide a and b increased to 56-68% (Rebeiz et al, 1999).

    b. Formation of MV Chlide b from MV Pchlide b in Greening Plants

    Two MV Chlide b pools are putatively formed form MV Pchlide b via routes 9, and 11, (Fig. 1). These two putative biosynthetic routes are discussed below.

    a. Possible Formation of MV Chlide b from MV Pchlide b in Green DDV-LDV-LDDV Plants via Biosynthetic Route 9

    The operation of this hypothetical route is suggested by (a) the formation of MV Pchlide b in green cucumber seedlings, and (b) the structural relationship between MV Pchlide a and MV Pchlide b. The first traces of MV Pchlide b in greening etiolated cucumber cotyledons are detected after 14 h of greening (Ioannides, 1993). In photoperiodically grown green seedlings, higher amounts of MV Pchlide b are detected (Ioannides et al, 1997; Kolossov and Rebeiz, 2003). To ascertain the operation biosynthetic route 9, in DDV-LDV-LDMV plants, precursor product relationships between MV Pchlide a and MV Pchlide b and between MV Pchlide b and MV Chlide b need to be established.

    b. Possible Formation of MV Chlide b from MV Pchlide b in Green DMV-LDV-LDMV Plants via Biosynthetic Route 11

    The operation of this hypothetical route is suggested by (a) the formation of MV Pchlide b in green photoperiodically-grown barley (Kolossov and Rebeiz, 2003), and wheat seedlings (Rebeiz, unpublished), (b) the structural relationship between MV Pchlide a and MV Pchlide b, and (c) the reported photoconversion of MV Pchlide b to MV Chlide b (Klement et al, 1999). To ascertain the operation biosynthetic route 11, in DDV-LDV-LDMV plants, precursor product relationships between MV Pchlide a and MV Pchlide b need to be established.

    c. Formation of DV Chlide b via Route 6

    The DV Chlide b pool is putatively formed from DV Chlide a via biosynthetic route 6. DV Chlide b has so far been detected only in the Nec 2 maize mutant (that used to be known as the ON 8147 mutant) (C. A. Rebeiz, unpublished). This mutation is a lethal mutation, the leaves are pale yellow, and accumulate only DV Chl a and b (Bazzaz, 1981). Nec2 maize leaves accumulate DV Chlide b to the extent of about 1.00 nmoles per gram of fresh leaves (C. A. Rebeiz, unpublished). DV Chlide b may also be present in the prochlorophyte picoplankton of the subtropical waters of the North Atlantic as well as in the picoplankton of the euphotic zone of the world tropical and temperate oceans, and the Mediterranean sea, where DV Chl a and b are the predominant Chl species (Chisholm et al, 1988; 1992; Goerike and Repeta, 1992; Veldhuis and Kraay, 1990). DV Chlide b exhibits the same electronic spectroscopic properties as DV Chl b (see below) but differs from the latter by its solubility in organic solvents and its chromatographic mobility.

    Conversion of DV Pchlide a to DV Chlide b via DV Chlide a, is suggested by the absence of DV Pchlide b occurrence in the Nec 2 corn mutant which forms and accumulate only DV Chl a and DV Chl b. The establishment of precursor- product relationships in-vivo and in-vitro is required however, to validate the operation of route 6 in the Nec 2 corn mutant and the picoplankton of the euphotic zone of the world tropical and temperate oceans, and the Mediterranean sea.

    B. Chlorophyll b

    MV Chl b
    DV Chl b

    1. Chlorophyll b Biosynthetic Heterogeneity

    The biosynthetic heterogeneity of Chl b is more complex than that of Chl a since it is based on the biosynthetic heterogeneity of Chl a, of MV Pchlide b and MV Chlide b. As a consequence 12 different Chl b pools destined to different Chl-protein complexes appear to be formed during greening.

    a. Biosynthetic Heterogeneity of MV Chlorophyll b

    Eight different pools of MV Chl b formed from MV Chlide b, MV Chl a and MV Pchlide b, are depicted in Fig. 1. The biosynthesis of these pools is discussed below.

    a. Formation of MV Chl b from MV Chlide a and MV Chl a via Routes 2, and 5 and from MV Chlide b via route 4 in etiolated DDV-LDV-LDDV Plants Subjected to Illumination.

    We have repeatedly observed that in DDV-LDV-LDDV plant tissues such as etiolated cucumber cotyledons subjected to a brief light illumination then returned to darkness, MV Chl b accumulation as monitored by sensitive fluorescence techniques (Rebeiz, 2002) is observed after 15 to 30 min of dark-incubation. By using less sensitive spectrophotometric techniques, it is observed that etiolated cucumber cotyledons subjected to continuous illumination, start accumulating measurable amounts of MV Chl b after 2 h of illumination (Rebeiz, 1967). We propose that the biosynthesis of this MV Chl b can originate in routes 2, 4 and 5, as described below.

    In route 2, MV Chl b would be formed from MV Chl a which is formed in turn from MV Chlide a and MV Pchlide (Fig. 1). In route 5, MV Chl b would be formed from MV Chl a which is formed in turn from DV Chlide a, and MV Chlide a (Fig. 1). In both cases the conversion of MV Chl a to MV Chl b is substantiated by favorable precursor-product relationships between these two tetrapyrroles in DDV-LDV-LDDV plants (Rebeiz et al, 1999). In route 4, MV Chl b would be formed by esterification of nascent MV Chlide b which is formed in turn from DV Chlide a and MV Chlide a (Fig. 1). Esterification of MV Chlide b with GG followed by stepwise hydrogenation of MV Chl b-GG is strongly suggested by the detection of MV Chlide b-GG, Chlide -DHGG, MV Chlide -THGG, and MV Chl b-phytol in greening etiolated cucumber cotyledons (Shio and Sasa, 1983).

    Since during the initial phases of greening of etiolated tissues, most of the Chl consists of MV Chl, located in PSI and PSII (Alberte et al, 1972, Akoyunoglou, 1978, Akoyunoglou et al, 1981), we propose that the MV Chl b formed via routes 2, 4, and 5 is destined to PSI and/or PSII inner antenna Chl-protein complexes. It is not certain at this stage whether the MV Chl b formed via routes 2, and 5 during the very early phases of greening is convertible to MV Chl a or not as has been reported for later stages of greening (Ohtsuka et al, 1997).

    b. Biosynthesis of MV Chl b from MV Chl a via Route 8 in Greening DDV-LDV-LDDV Plants during the Light Phases of the Photoperiod.

    Most of the MV Chl b accumulation in DDV-LDV-LDDV plants takes place during the light phases of the photoperiod. In route 8, MV Chl b is formed from MV Chl a which is formed in turn from DV Chlide a and MV Chlide a (Fig. 1). In this case too, conversion of MV Chl a to MV Chl b is supported by favorable precursor product relationship between MV Chl a and MV Chl b after 7 and 8 h of illumination of DDV-LDV-LDDV plant tissues (Rebeiz et al, 1999).

    Since under continuous illumination, most of the synthesized Chl consists of MV Chl a, and b located in antenna Chl-protein complexes, (Alberte et al, 1972, Akoyunoglou, 1978, Akoyunoglou et al, 1981), we propose that the MV Chl a formed via route 8 is destined to LHCII and other outer antenna Chl-protein complexes.

    It is presently acknowledged that under certain conditions, MV Chl b is converted to MV Chl a in green cucumber cotyledons (Ohtsuka et al, 1997). These authors proposed that during the light phase of the photoperiod, the photosynthetic apparatus is reorganized during acclimation to various light environments. This reorganization involves release of MV Chl b from the light-harvesting Chl a/b protein complex of PSII. The released MV Chl b is then converted to MV Chl a by a Chl b formyl reductase. The nascent MV Chl a is then used for the formation of core complexes of PSI and PSII. On the basis of these results we propose that MV Chl b formed via route 8, is convertible to MV Chl a in green DDV-LDV-LDDV plant tissues.

    g. Biosynthesis of MV Chl b from MV Pchlide b via Route 9 in Greening DDV-LDV-LDDV Plants during the Light Phases of the Photoperiod

    In route 9, MV Chl b is formed from MV Chlide b which is formed in turn from MV Pchlide a and MV Pchlide b in green DDV-LDV-LDDV plants (Fig. 1). The conversion of MV Pchlide b to MV Chlide b is suggested by the detection of MV Pchlide b ( Ioannides et al, 1997; Kolossov and Rebeiz, 2003) and MV Chlide b (Duggan and Rebeiz, 1982) in green(ing) cucumber cotyledons. A specific precursor-product relationship remains to be established however between MV Pchlide b and MV Chlide b in DDV-LDV-LDDV plant tissues. As in the case with other Chl biosynthetic routes that operate during the light phases of the photoperiod, It is our guess that the putative MV Chl b formed via route 9 is destined to LHCII and other outer antenna Chl-protein complexes.

    d. Biosynthesis of MV Chl b from MV Chl a via Route 10 in Greening DMV-LDV-LDMV Plants during the Light Phases of the Photoperiod

    As was observed in DDV-LDV-LDDV plants, in DMV-LDV-LDMV plants, most of the MV Chl b accumulation takes place during the light phases of the photoperiod. In route 10, MV Chl b is formed from MV Chl a which is formed in turn from MV Chlide a and MV Pchlide a (Fig. 1). In this case too conversion of MV Chl a to MV Chl b is supported by the favorable precursor product relationship between MV Chlide a and MV Chl b in DMV-LDV-LDMV plant tissues after 7 and 8 h of illumination (Rebeiz et al, 1999). At this stage, we have no reason to argue against a certain degree of direct conversion of MV Chlide a to MV Chl a-phytol in green DMV-LDV-LDMV plants as was observed in spinach chloroplasts (Stoll et al, 1983, and greening cucumber etiochloroplasts (Daniell and Rebeiz, 1984).

    Since under continuous illumination, most of the synthesized Chl consists of MV Chl a, and b located in antenna Chl-protein complexes, (Alberte et al, 1972, Akoyunoglou, 1978, Akoyunoglou et al, 1981), we propose that the MV Chl a formed via route 10 is destined to LHCII and other antenna Chl-protein complexes. We also propose that MV Chl b formed via route 10, is convertible to MV Chl a as the photosynthetic apparatus is reorganized during acclimation to various light environments (Ohtsuka et al, 1997).

    e. Biosynthesis of MV Chl b from MV Pchlide b via Route 11 in Greening DMV-LDV-LDMV Plants during the Light Phases of the Photoperiod

    In route 11, MV Chl b is formed from MV Chlide b which in turns is formed from MV Pchlide a and MV Pchlide b in green DMV-LDV-LDMV plants. The conversion of MV Pchlide b to MV Chlide b is suggested by the detection of MV Pchlide b in barley, a DMV-LDV-LDMV plant Kolossov and Rebeiz, 2003, and the lack of precursor product relationship between MV Chlide a and MV Chlide b ingreening corn seedlings during lengthy (15 h) illuminations (Ioannides, 1993). This observation argues against the formation of MV Chl b from MV Chlide a and MV Chlide b during lengthy light phases of the photoperiod. A specific precursor-product relationship remains to be established however between MV Pchlide b and MV Chlide b and MV Chl b in green DMV-LDV-LDMV plant tissues. As in the case of other biosynthetic routes that operate during the light phases of the photoperiod, our guess would be that the putative MV Chl b formed via route 11 is destined to LHCII and other antenna Chl-protein complexes.

    z. Biosynthesis of MV Chl b from MV Chl a via Route 12 and from MV Chlide a via Route 14 in Etiolated DMV-LDV-LDMV Plants after Exposure to Light, and in Greening DMV-LDV-LDMV Plants during the Initial Dark Phases of the Photoperiod

    We have repeatedly observed that in DMV-LDV-LDMV plant tissues such as etiolated barley seedlings subjected to brief illumination then returned to darkness, MV Chl b accumulation, monitored by sensitive fluorescence techniques (Rebeiz, 2002) is observed after 15 to 30 min of dark-incubation. We propose that the biosynthesis of this MV Chl b originates in routes 12 and 14.

    In route 12, MV Chl b would be formed from MV Chl a which is formed in turn from MV Pchlide a and MV Chlide a (Fig. 1). In route 14, MV Chl b would be formed by esterification of nascent MV Chlide b which is formed in turn from MV Chlide a (Fig. 1). Esterification of MV Chlide b with GG followed by stepwise hydrogenation of MV Chl b-GG has been reported in etiolated oat leaves (Benz and Rudiger, 1981) a DMV-LDV-LDMV plant tissue (Abd-El-Mageed et al, 1997).

    Since during the initial phases of greening of etiolated tissues, most of the Chl consists of MV Chl, located in PSI and PSII (Alberte et al, 1972, Akoyunoglou, 1978, Akoyunoglou et al, 1981), we propose that the MV Chl b formed via routes 12, and 14 is destined to PSI and/or PSII inner antenna Chl-protein complexes. It is not certain at this stage whether the MV Chl b formed via routes 12, 14 during the very early phases of greening is convertible to MV Chl a or not as has been reported for later stages of greening (Ito et al, 1994; 1996; Ito and Tanaka, 1996).

    b. Biosynthesis of DV Chlorophyll b

    The occurrence of DV Chl b has not been observed in normal higher plants. However, DV Chl b is the major Chl b that accumulates in the Nec 7 corn mutant and in the prochlorophyte picoplankton of the subtropical waters of the North Atlantic as well as in the picoplankton of the euphotic zone of the world tropical and temperate oceans, and the Mediterranean sea (Chisholm et al, 1988; 1992; Goerike and Repeta, 1992; )

    a. Conversion of DV Chl a to DV Chl b via Route 1

    Conversion of DV Chl a to DV Chl b via DV Pchlide a and DV Chlide a (Fig. 1, route 1) probably takes place in the Nec 2 corn mutant and in the prochlorophyte picoplankton (see above) which form and accumulate only DV Chl a and DV Chl b (Chisholm et al, 1988; 1992; Goerike and Repeta, 1992; ). Precursor- product relationships between DV Chl a and DV Chl b in vivo and in vitro is required however, to validate this hypothesis.

    b. Conversion of DV Chlide b to DV Chl b via Route 6

    Conversion of DV Chlide b to DV Chl b via route 6 may also take place in the Nec 2 corn mutant and in the prochlorophyte picoplankton (Chisholm et al, 1988; 1992; Goerike and Repeta, 1992; ) which form and accumulate only DV Chl a and DV Chl b. Precursor- product relationships between DV Chlide b and DV Chl b in these organisms in vivo and in vitro is required however, to validate this hypothesis.

    E. References



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