Abstract
Bacteriochlorophyll b has the most red-shifted absorbance maximum of all naturally occurring photopigments. It has a characteristic ethylidene group at the C8 position in place of the more common ethyl group, the product of a C8-vinyl reductase, which is carried by the majority of chlorophylls and bacteriochlorophylls used in photosynthesis. The subsequent and first step exclusive to bacteriochlorophyll biosynthesis, the reduction of the C7=C8 bond, is catalyzed by chlorophyllide oxidoreductase. It has been demonstrated that the enzyme from bacteriochlorophyll a-utilizing bacteria can catalyze the formation of compounds carrying an ethyl group at C8 from both ethyl- and vinyl-carrying substrates, indicating a surprising additional C8-vinyl reductase function, while the enzyme from organisms producing BChl b could only catalyze C7=C8 reduction with a vinyl substrate, but this product carried an ethylidene group at the C8 position. We have replaced the native chlorophyllide oxidoreductase-encoding genes of Rhodobacter sphaeroides with those from Blastochloris viridis, but the switch from bacteriochlorophyll a to b biosynthesis is only detected when the native conventional C8-vinyl reductase is absent. We propose a non-enzymatic mechanism for ethylidene group formation based on the absence of cellular C8-vinyl reductase activity.
Highlights
Bacteriochlorophyll (BChl) b, the most strongly red-shiftedchlorin used for light harvesting, was first discovered in 1963 as the major photosynthetic pigment of a Rhodopseudomonas sp. photosynthetic bacterium [1]
Due to the high sequence similarity between the chlorophyllide oxidoreductase (COR)-encoding genes from Rba. sphaeroides and Blc. viridis, and the high probability of introducing unwanted mutations during replacement via homologous recombination, the native bchX, bchY and bchZ genes were deleted in both WT and ΔbciA backgrounds prior to the integration of the Blc. viridis orthologs
The red shift in the Soret band of the pigment accumulated by ΔbciA/ΔbchXYZ, when compared to that of 3HE,8E Chlide, can be accounted for by the lack of both C8-vinyl reductase (8VR); an 8- to 10-nm shift in this band is consistent with 8V/8E substitutions of chlorins [34]
Summary
Bacteriochlorophyll (BChl) b, the most strongly red-shifted (bacterio)chlorin used for light harvesting, was first discovered in 1963 as the major photosynthetic pigment of a Rhodopseudomonas sp. photosynthetic bacterium [1]. A recent review proposed a re-engineered photosynthetic apparatus for energy capture in which one of the two photosystems found in plants and cyanobacteria was modified so that it could utilize far-red light, while the other photosystem retained the ability to absorb light in the 650- to 700-nm region [7]. Accomplishing such modifications requires control over pigment biosynthesis and photosystem assembly; in this work, we explore the potential of re-routing the biosynthesis of BChl a towards the formation of BChl b. Hunter / Biochimica et Biophysica Acta 1837 (2014) 1611–1616 never been detected in nature) will spontaneously isomerize to the more stable b-type
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