Bacteriochlorophyll
Chemical compound
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Structure
Bacteriochlorophylls a, b, and g are bacteriochlorins, meaning their molecules have a bacteriochlorin macrocycle ring with two reduced pyrrole rings (B and D). Bacteriochlorophylls c, d, e, and f are chlorins, meaning their molecules have a chlorin macrocycle ring with one reduced pyrrole ring (D).
Bacteriochlorophylls c to f occur in the form of closely related homologs with different alkyl groups attached to pyrrole rings B and C and are illustrated above in their simplest versions, esterified with the sesquiterpene alcohol farnesol. Most of the variation occurs in the 8 and 12 positions and can be attributed to methyltransferase variation. BChl cS is a term for 8-ethyl,12-methyl homolog of BChl c.
Bacteriochlorophyll g has a vinyl group in ring (A), at position 8.
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Biosynthesis
There are a large number of known bacteriochlorophylls but all have features in common since the biosynthetic pathway involves chlorophyllide a (Chlide a) as an intermediate.
Chlorin-cored BChls (c to f) are produced by a series of enzymatic modifications on the sidechain of Chlide a, much like how Chl b, d, e are made. The bacteriochlorin-cored BChls a, b, g require a unique step to reduce the double bond between C7 and C8, which is performed by chlorophyllide a reductase (COR).
Isobacteriochlorins, in contrast, are biosynthesised from uroporphyrinogen III in a separate pathway that leads, for example, to siroheme, cofactor F430 and cobalamin. The common intermediate is sirohydrochlorin.