The Chlorophyll Biosynthetic Heterogeneity and Chloroplast Bioengineering - Chlorophyll Biosynthesis and Technological Applications - page 337

complexity of thylakoid membranes is rooted in a multibranched, heterogeneous
Chl biosynthetic pathway (Rebeiz et al. 1999 ).
It was also proposed that Chlorophyll biosynthetic heterogeneity referred either
(a) to spatial biosynthetic heterogeneity, (b) to chemical biosynthetic heterogeneity,
or (c) to a combination of spatial and chemical biosynthetic heterogeneities
(Rebeiz et al. 2003b ). Spatial biosynthetic heterogeneity was defined as the biosyn-
thesis of an anabolic tetrapyrrole or end product by identical sets of enzymes, at
several different locations of the thylakoid membrane. On the other hand, chemical
biosynthetic heterogeneity was defined as the biosynthesis of an anabolic tetrapyr-
role or end product at several different locations of the thylakoid membrane, via
different biosynthetic routes, each involving at least one different enzyme.
Figures 6.3 , 6.4 and 6.5 of Chap. 6 organizes all known biosynthetic reactions
into a logical scheme made up of various different biosynthetic routes.
16.5.2 Thylakoid Apoprotein Biosynthesis
The biosynthesis of thylakoid apoproteins is a very complex phenomenon.
Some apoproteins are coded for by nuclear DNA, are translated on cytoplasmic
ribosomes and are transported to developing chloroplasts. Other apoproteins are
coded for by plastid DNA and are translated on chloroplast ribosomes. A detailed
discussion of chloroplast apoprotein biosynthesis is beyond the scope of this
discussion. The reader is referred to reference (Sundqvist and Ryberg 1993 ) for a
comprehensive discussion of this topic. For the purpose of this discussion it suffices
to say that a PSU is an extremely complex structure that consists of many highly
folded thylakoid and soluble proteins as well as membrane-bound pigment protein
complexes having different functions in the light and dark steps of photosynthesis.
An early visualization of a linear model of a PSU in the unfolded state is depicted
in Fig. 16.3 .
16.5.2.1 Assembly of Chl-Protein Complexes
Success in the bioengineering of smaller PSUs resides in a thorough understanding of
how the Chl and thylakoid apoprotein biosynthetic pathways are coordinated to
generate a specific functional Chl-protein complex. It is known for example that an
apoprotein formed in the cytoplasm or in the chloroplast has to bind Chl molecules,
has to fold properly, and has to wind up in the right place on the thylakoid. This
process has to take place in order for the Chl-apoprotein to become a functional
Chl-protein complex having a specific role in photosynthesis. What is unknown
however is how an apoprotein formed in the cytoplasm or in the chloroplast becomes
associated with Chl to become a specific Chl-protein complex of PSI, PSII or a light
harvesting Chl-protein complex, having a specific function in photosynthesis.