Abstract
This chapter focuses on tetrapyrrole metabolism as a general process that occurs in all chloroplast-containing organisms, but stresses results that have been obtained with Chlamydomonas as the experimental system, and results that may be unique to Chlamydomonas. Conceptually, chlorophyll a biosynthesis pathway can be divided into several sections. 5-Aminolevulinic acid (ALA), the first universal tetrapyrrole precursor, can be formed by two different pathways. This is described broadly with the functioning of Glu-tRNA synthetase (GTS), Glu-tRNA reductase (GTR), GSA aminotransferase (GSAT) and their interactions. This chapter individually covers the steps from uroporphyrinogen iii to protoporphyrin IX, steps specific to chlorophyll formation, biosynthesis of chlorophyll b, chlorophyll degradation and interconversion of chlorophylls a and b. The branch from protoporphyrin IX to hemes and heme catabolism, the regulation of specific biosynthetic steps that cause changes in the amounts of end products produced are also described. Because regulatory responses and mechanisms are not as highly conserved between Chlamydomonas and higher plants as are the biosynthetic steps themselves, this section concentrates on Chlamydomonas.
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