JB Accepts, published online ahead of print on 30 October 2009

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J. Bacteriol. doi:10.1128/JB.01271-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Competitive inhibitions of the chlorophyll synthase of Synechocystis sp. PCC6803 by bacteriochlorophyllide a and the bacteriochlorophyll synthase of Rhodobacter sphaeroides by chlorophyllide a

Eui-Jin Kim and Jeong K. Lee^*

From the Department of Life Science and Basic Science Institute for Cell Damage Control, Sogang University, Seoul 121-742, Korea

^* To whom correspondence should be addressed. Email: jgklee{at}sogang.ac.kr.

The photosynthetic growth of Synechocystis sp. PCC6803 is hampered by the exogenously added bacteriochlorophyllide a (Bchlide a) in a dose dependent manner. The growth inhibition caused by Bchlide a, however, is relieved by an increased level of exogenously added chlorophyllide a (Chlide a). The results are explained by the competitive inhibition of chlorophyll synthase by Bchlide a with K_I of 0.3 mM and 1.14 mM in the presence of sufficient geranylgeranyl pyrophosphate (GGPP) and phytyl pyrophosphate (PPP), respectively. Surprisingly, the bacteriochlorophyll synthase of Rhodobacter sphaeroides is inhibited competitively by Chlide a with K_I of 0.54 mM and 0.77 mM in the presence of sufficient GGPP and PPP, respectively. Consistently, the exogenously added Chlide a inhibits the metabolic conversion of the exogenously added Bchlide a to bacteriochlorophyll a by an R. sphaeroides bchFNB-bchZ mutant that neither synthesizes nor metabolizes Chlide a. The metabolic inhibition by Chlide a, however, is relieved by the elevated level of Bchlide a. Thus, the chlorophyll synthase of Synechocystis sp. PCC6803 and the bacteriochlorophyll synthase of R. sphaeroides, both of which perform ping-pong type reactions, are inhibited by Bchlide a and Chlide a, respectively. Although neither of inhibitors is catalyzed by target enzyme, inhibitions in the competitive mode suggest a structural similarity between their active sites.