ALA using the C-5 pathway (reviewed in Refs. 7 and 9). This pathway starts with the activation of glu-
tamate by a transfer RNA (tRNA Glu) molecule. This complex is then reduced to glutamate 1-semialde-
hyde, which is transaminated to -ALA (Figure 1) (reviewed in Refs. 7 and 9). In photosynthetic eukary-
otes, all the enzymes catalyzing the formation of protoporphyrin IX (Proto-IX) appear to be localized only
in chloroplasts (reviewed in Ref. 9). From the finding of an Fe^2 -chelatase in chloroplasts, it was deduced
that chloroplastic hemes are synthesized there [10], whereas mitochondrial hemes are synthesized in mi-
tochondria. Therefore, the Fe^2 or Mg^2 insertion inside Proto-IX constitutes the reaction at which the
pathways yielding hemes and open tetrapyrroles and those yielding Chl and bacteriochlorophylls diverge
(reviewed in Refs. 7 and 9). After insertion of Mg^2 , Mg-Proto-IX undergoes several specific chemical
modifications ending with Chl or Bchl formation. The most important steps are (1) the formation of the
isocyclic ring, typical of Bchl and Chl; (2) the reduction of Pchlide to Chlide; and (3) the esterification of
Chlide to Chl.
All the reactions of the Chl biosynthetic pathway are catalyzed by enzymes that can be called “nor-
mal” in the sense that they transform their substrate to a product when they are in contact. There is, how-
266 SCHOEFSFigure 1 Outline of the Chl biosynthetic pathway.