A Personal Historical Introduction to Photosystem I: Ferredoxin + FNR, the Key to NADP⁺ Reduction

Abstract

The system concerned with reduction of the pyridine nucleotide coenzymes depends on two proteins: a non-heme iron protein associated closely with the photochemical reaction center (Photosystem I) and a flavoprotein that functions as a reductase. The non-heme iron protein, previously known as the “methemoglobin reducing factor (MRF),” as “photosynthetic pyridine nucleotide reductase (PPNR),” and as the “TPN¹ reducing factor (TRF),” is recognized as a member of a class of proteins now called “ferredoxin (Fd).” The FAD-containing flavoprotein known now as “Fd:NADP⁺ oxidoreductase (FNR)” was known previously as “TPNH diaphorase,” as “pyridine nucleotide transhydrogenase,” and as “TPN reductase.” An abbreviated review of the investigations leading to the isolation and characterization of these two proteins that have extended over a number of years and in a number of different laboratories is presented herein. This has unavoidably led to a synonymy and I describe here my personal recollection of how this came about.