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hopanoid biosynthesis associated radical SAM protein HpnH The sequences represented by this model are members of the radical SAM superfamily of enzymes (pfam04055). These enzymes utilize an iron-sulfur redox cluster and S-adenosylmethionine to carry out diverse radical mediated reactions. The members of this clade are frequently found in the same locus as squalene-hopene cyclase (SHC, TIGR01507) and other genes associated with the biosynthesis of hopanoid natural products. The linkage between SHC and this radical SAM enzyme is strong; one is nearly always observed in the same genome where the other is found. A hopanoid biosynthesis locus was described in Zymomonas mobilis consisting of the genes HpnA-E and SHC (HpnF). Continuing past SHC are found a phosphorylase enzyme (ZMO0873, i.e. HpnG, TIGR03468) and this radical SAM enzyme (ZMO0874) which we name here HpnH. Granted, in Z. mobilis, HpnH is in a convergent orientation with respect to HpnA-G, but one gene beyond HpnH and running in the same convergent direction is IspH (ZM0875, 4-hydroxy-3-methylbut-2-enyl diphosphate reductase), an essential enzyme of IPP biosynthesis and therefore essential for the biosynthesis of hopanoids. One of the well-described hopanoid intermediates is bacteriohopanetetrol. In the conversion from hopene several reactions must occur in the side chain for which a radical mechanism might be reasonable. These include the four (presumably anaerobic) hydroxylations and a methyl shift. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",