Two Biologically Active Ferredoxins from the Aerobic Nitrogen-fixing Bacterium, Azotobacter vinelandii

Abstract

Abstract The recently discovered, biologically active, ferredoxin component of the aerobic nitrogen fixer, Azotobacter vinelandii, has been found to comprise two distinct, low potential proteins (Fd I, Em = -420 mv; Fd II, Em = -460 mv) which were about equally effective as electron carriers for the nitrogenase system and in NADP reduction by illuminated chloroplasts. The absorption spectrum of Fd I showed a shoulder between 380 and 420 nm, whereas Fd II had a broad peak with a maximum at 388 nm. The 400:280 absorbance ratio for Fd I was 0.58, and the 388:280 absorbance ratio of Fd II was 0.68. Fd I which was found to be the main ferredoxin component in A. vinelandii cells (about 5 times more abundant than Fd II) was crystallized and further characterized. Fd I yielded crystals of unusually large size, up to 1 mm in length and 0.01 mm in thickness. Fd I resembles clostridial ferredoxins in having eight non-heme iron and eight labile sulfide groups per mole but its molecular weight is more than twice as great (14,400 versus 6,000 daltons). The sulfur groups of Fd I, unlike those of other ferredoxins, were only slowly and incompletely titrable by p-chloromercuribenzoate. Fd I also differed from other ferredoxins in being significantly more stable in 4 m guanidine-HCl. The possible significance of some of these properties to the role of ferredoxin as a reductant for a nitrogenase system that functions in an aerobic organism is discussed.