Valine 77 of heterocystous ferredoxin FdxH2 in Anabaena variabilis strain ATCC 29413 is critical for its oxygen sensitivity

Ferredoxins are small iron sulfur proteins necessary for electron donation. FdxH1 and FdxH2 are associated with two different nif gene clusters where they transfer electrons for the reduction of nitrogenase complex. FdxH1 was observed to be stable towards oxygen, whereas, FdxH2 was relatively unstab le. We had identified the amino acid involved in oxygen sensitivity of ferr edoxin protein using protein modeling. The exchange of valine to leucine at position 77 was critical for ferredoxin proteins in relation to its oxygen sensitivity. This exchange leads to a longer side chain, which inhibits th e accessibility of oxygen to the iron sulfur cluster. Site directed mutagen esis and in vitro experiments confirms that valine indeed is involved in th e oxygen sensitivity. The exchange of leucine to valine in FdxH1 makes it o xygen unstable. Thus, from the above results we can conclude that the posit ion of leucine at position 77 is critical for oxygen sensitivity of ferredo xin and protein modeling can be used to identify specific amino acids in ot her oxygen-sensitive proteins.