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 ``TPN 1 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.