Catalase has a major role in the photorespiration reactions, as well as in the glycolate pathway, and
is involved in the protection of chloroplasts from free radicals produced during the water-splitting
reaction of photosynthesis. The reaction sequence of peroxidase shown above includes cell wall
peroxidases, which catalyze the polymerization of phenols to form lignin. Peroxidase activity is
noticeably depressed in roots of iron-deficient plants, and inhibited cell wall formation and
lignification, and accumulation of phenolic compounds have been reported in iron-deficient roots.
As well as being a constituent of the heme group, iron is required at two other stages in its manufacture.
It activates the enzymes aminolevulinic acid synthetase and coproporphorinogen oxidase. The
protoporphyrin synthesized as a precursor of heme is also a precursor of chlorophyll, and although iron
is not a constituent of chlorophyll this requirement, and the fact that it is also required for the conversion
of Mg protoporphyrin to protochlorophyllide, means that it is essential for chlorophyll biosynthesis
. However, the decreased chloroplast volume and protein content per chloroplast (11)
that chlorophyll might not be adequately stabilized as chromoprotein in chloroplasts under iron
deficiency conditions, thus resulting in chlorosis.
Along with the iron requirement in some heme enzymes and its involvement in the manufacture
of heme groups in general, iron has a function in Fe-S proteins, which have a strong involvement
with the light-dependent reactions of photosynthesis. Ferredoxin, the end product of
photosystem I, has a high negative redox potential that enables it to transfer electrons to a number
of acceptors. As well as being the electron donor for the synthesis of NADPH in photosystem I, it
can reduce nitrite in the reaction catalyzed by nitrite reductase and it is an electron donor for sulfite