Porphyrins are tetrapyrroles with a metal ion at their center. Accordingly, the haeme groups of cytochromes and globines contain iron while chlorophyll contains magnesium. The involvement of the first in redox reactions has already been discussed. That of the central function of chlorophyll in photosynthesis is still to follow.
Starting compounds for the biosynthesis of tetrapyrroles can be alpha-ketoglutarate and succinyl-CoA. The product of the condensation is always delta-aminolevulinic acid that is converted to porphobilinogen by ring-formation and splitting off of protons and water.
Four such units together form the porphyrine skeleton. Subsequently, the metal ions are added and single side chains modified. For example, a long alcohol (phytol) is bound to one of the porphyrine rings of the chlorophyll molecule.
The synthesis starts in chloroplasts with glutamate, in the
reactions shown below, delta-aminolevulininc acid is
produced.
Porphyrines have a biological significance as cofactors. Their biosynthesis is controlled by numerous extern factors. Depending on the species, their syntheses are located in different compartments. The haeme groups necessary for their production are synthesized at their future domains, in the mitochondria or the chloroplasts. The biosynthesis of chlorophyll takes place in the chloroplasts. The transformation of protochlorophyllide a into chlorophyllide a (an addition of hydrogen to ring IV) necessary for chlorophyll biosynthesis is dependent on light and occurs usually only if enough of the necessary carrier protein is present.
For a more detailed description of heme and chlorophyll a and b
biosynthetic pathways and their influence on the greening of plants
see the feature of C. A. REBEIZ (University of Illinois at
Urbana-Champaign)
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