Siroheme: an essential component for life on earth

Abstract

Life on earth is dependent on sulphur (S) and nitrogen (N). In plants, the second step in the reduction of sulphate and nitrate are mediated by the enzymes sulphite and nitrite reductases, which contain the iron (Fe)-containing siroheme as a cofactor. It is synthesized from the tetrapyrrole primogenitor uroporphyrinogen III in the plastids via three enzymatic reactions, methylation, oxidation and ferrochelatation. Without siroheme biosynthesis, there would be no life on earth. Limitations in siroheme should have an enormous effect on the S- and N-metabolism, plant growth, development, fitness and reproduction, biotic and abiotic stresses including growth under S, N and Fe limitations, and the response to pathogens and beneficial interaction partners. Furthermore, the vast majority of redox-reactions in plants depend on S-components, and S-containing compounds are also involved in the detoxification of heavy metals and other chemical toxins. Disturbance of siroheme biosynthesis may cause the accumulation of light-sensitive intermediates and reactive oxygen species, which are harmful, or they can function as signaling molecules and participate in interorganellar signaling processes. This review highlights the role of siroheme in these scenarios.

Keywords: iron; nitrogen; plant/microbe interaction; redox; siroheme; sulphur.

Figure 1

Tetrapyrrole biosynthesis pathway leading to chlorophylls, siroheme and phytochromes. Crucial enzymes are in red: HemA, Glu tRNA synthase, POR, protochlorophyllide oxidoreductase. Chelexed ions are in green. Yellow boxes: Enzymes and subtrates, which require siroheme and heme, respectively. Blue error: feedback regulation of HemA enzyme activity.

Figure 2

Siroheme biosynthesis from uroporphyrinogen III . The plant, yeast and bacterial enzymes are indicated. For details, see text.

Figure 3

Role of Fe, Fe-S clusters and siroheme in the metabolism of a plant cell. The synthesis of siroheme and heme from uroporphyrinogen III and the uptake and assimilation of S, N and Fe is shown. Dashed lines show the requirement of these compounds as cofactors and their involvement in biological pathways (yellow boxes). ProFC, protochlorophyllide ferrochelatase; IR T1, inducible iron transporter 1, FRO2, ferric-chelate reductase 2.