 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.Several major human diseases are caused by the obligate intracellular protozoan parasites in the phylum Apicomplexa.