The Chl a Carboxylic Biosynthetic Routes: Protochlorophyllide a - Chlorophyll Biosynthesis and Technological Applications - page 225

Mpe reductase (4VMpeR) in greening barley etiochloroplasts (Kolossov and
Rebeiz 2010 ). Such etiochloroplasts can actively convert MV Mpe to MV Pchlide
a (Tripathy and Rebeiz 1986 ).
Biosynthesis of MV Pchlide a in DMV-LDV-LDMV Plant Species via Routes 12
MV Pchlide a is formed via biosynthetic route 12 fromMV Mg-Proto and MV Mpe
(Fig. 8.8 ). Indeed in green DMV-LDV-LDMV plants MV Pchlide a formation is
very active (Abd-El-Mageed et al. 1997 ) and barley etiochloroplasts are capable of
strongly converting MV Mpe to MV Pchlide a (Tripathy and Rebeiz 1986 ).
8.2 Pchlide-Protein Complexes
Further metabolism of DV and MV Pchlide a takes place only if the Pchlide is
complexed to an apoprotein. The Pchlide-apoprotein complex is referred to as a
Pchl-holochrome (Pchl-Hs). Various Pch-Hs holochromes will be discussed in
detail below.
8.2.1 Heterogeneity of Pchlide a-Protein Complexes
As we have mentioned previously, Pchlides a are chemically heterogeneous. That
heterogeneity is expressed chemically as DV, and MV substitutions at the level of
the tetrapyrrole chromophore at position 4 of the macrocycle. It is also expressed at
the level of the esterifying group at position 7 of the macrocycle as will be discussed
in Chap. 9 , and the chromophore-protein complexes referred to as Pchl-Hs. Since in
most cases the chromophore of Pchl-Hs consist mostly of Pchlide a and much
smaller amounts of Pchlide a ester (Chap. 9 ), it is appropriate to refer to these
holochromes as Pchl(ide)-Hs. The overall heterogeneity is also expressed by the
detection of multiple resonance excitation energy transfer bands between Pchl(ide)-
Hs and various Chl-protein complexes (Table 6.1 , Chap. 6 ) .
Discussion of the heterogeneity of Pchl(ide)-Hs will focus (a) on the possible
nature of the chromophore-apoprotein association, (b) on the spectroscopic
properties of various Pchl(ide) a -Hs in situ , and (c) on the properties of purified
Pchl(ide)-Hs.
8.2.1.1 Nature of the Chromophore-Protein Association of Pchl(ide)-Hs
In etiolated tissues, Pchlide a is the most abundant tetrapyrrole (90-95 %) followed
by less abundant Pchlide a ester (Pchlide a E) (5-10 %) (Rebeiz et al. 1970 ).
Pchlide a and its ester will be referred to collectively as Pchl(ide) a . Upon binding