Title: A Radical Solution for C(sp3)–C(sp3) Bond Formation during the Biosynthesis of Macrocyclic Membrane Lipids

Archaea synthesize isoprenoid-based ether-linked membrane lipids, which enable them to withstand 

extreme environmental conditions, such as high temperatures, high salinity, and low or high pH 

values. In some archaea, such as Methanocaldococcus jannaschii, these lipids are further modified 

by forming carbon–carbon bonds between the termini of two lipid tails within one 

glycerophospholipid to generate the macrocyclic archaeol or forming two carbon– carbon bonds 

between the termini of two lipid tails from two glycerophospholipids to generate the macrocycle 

glycerol dibiphytanyl glycerol tetraether (GDGT). GDGT contains two 40-carbon lipid chains 

(biphytanyl chains) that span both leaflets of the membrane, providing enhanced

stability to extreme conditions. How these specialized lipids are formed has puzzled scientists for 

decades. The reaction necessitates coupling two completely inert sp3-hybridized carbon centers, 

which has not been observed in nature. Here we use X-ray crystallography, high-resolution mass 

spectrometry, chemical synthesis, and biochemical analyses to show that the gene product of mj0619 

from M. jannaschii, which encodes a radical S-adenosylmethionine enzyme, is responsible for 

biphytanyl chain formation during synthesis of both the macrocyclic archaeol and GDGT membrane

lipids