The Iron and Magnesium Branches of the Porphyrin Biosynthetic Pathway - Chlorophyll Biosynthesis and Technological Applications - page 182

Fig. 6.1 Protoheme
Fig. 6.2 Conversion of Proto to protoheme
6.2 The Mg-Branch of the Porphyrin Biosynthetic Pathway
Most of the Chl in nature is formed from divinyl (DV) Proto via divinyl (DV) and
monovinyl (MV) Mg-carboxylic biosynthetic routes. These routes are referred to as
carboxylic routes because the Mg-tetrapyrrole intermediates all have one or two
free carboxylic groups. Furthermore, most of these routes are heterogeneous. That
is the biosynthesis of most of the intermediates can proceed via more than one path.
This phenomenon is a manifestation of the overall Chl biosynthetic heterogeneity
(Rebeiz et al. 2003 ) that permeates the whole Chl biosynthetic pathway. Chloro-
phyll biosynthetic heterogeneity was discovered when it was realized that most of
the carboxylic and fully esterified tetrapyrrole pools of plants consist of DV and
MV components. The biological significance of this phenomenon is becoming
clearer as the Chl biosynthetic pathway is increasingly viewed in the context of
the structural and functional heterogeneity of photosynthetic membranes.
Since some of the early biochemical work was done before discovery of the DV
and MV Chl biosynthetic heterogeneity, and before development of appropriate
analytical methodologies (Rebeiz 2002 ), it is not certain whether the investigated
reactions of this early biochemical work involved only DV or both DV and MV
tetrapyrroles. To emphasize this ambiguity, the MV and DV connotation will be
omitted from the discussion of the early work. In other words, in this context,
Mg-Proto would refer either to DV Mg-Proto, to MV Mg-Proto or to a mixture of
both. On the other hands, DV and MV Mg-Proto would refer specifically to the DV
and MV tetrapyrrole species respectively. The Mg-tetrapyrrole intermediates of the
various Chl biosynthetic routes will be discussed in various section of this
monograph.