Evolution
Ferredoxin NADP+ reductases are present in many organisms, including plants, bacteria, and the mitochondria of eukaryotes. However, these proteins belong to two unrelated protein families and are an example of convergent evolution. The plant-type FNRs include the plastidic FNRs seen in plants and the bacterial FNRs. The glutathione-reductase-type FNRs are seen in the mitochondria of eukaryotes.
In the plant-like family of FNRs, selective evolutionary pressure has led to differences in the catalytic efficiency of FNRs in photosynthetic and nonphotosynthetic organisms. Electron transfer by FNR is a rate limiting step in photosynthesis, so the plastidic FNR in plants have evolved to be highly efficient. These plastidic FNRs are 20–100 fold more active than bacterial FNRs. This higher catalytic efficiency of the transfer of electrons from FAD to NADP+ is related to structural changes in the active site that reduce the distance between the N5 in FAD and the C4 in NADP+.
The plastidic FNRs in plants have also evolved to have a high degree of substrate specificity for NADP+ over NAD+; studies of amino acid mutations have shown that the terminal tyrosine residue in plastidic FNRs plays a key role in this substrate specificity. In contrast, some nonphotosynthetic FNRs do not preferentially bind NADP+ and lack this tyrosine residue.
Read more about this topic: Ferredoxin-NADP+ Reductase
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