The gene product Sll1127 is a predicted 1,4-dihydroxy-2-naphthoyl-CoA synthase catalyzing an intramolecular Claisen condensation in the phylloquinone biosynthesis of the cyanobacterium Synechocystis sp.PCC 6803.This predicted catalytic function has been verified and the enzyme has been characterized for the first time with kcat = 0.013 s-1 and KM = 9μM.Its catalytic activity is found to strictly depend on externally added bicarbonate with an apparent KD = 0.60 mM.In addition,this enzyme is inhibited by its 1,4-dihydroxy-2-naphthoyl-CoA product through high-affinity binding,which causes a 18 nm shift of the inhibitor absorption at 392 to 410 nm and engenders a new absorption peak at 345 nm.All these properties of the cyanobacterial enzyme are closely similar to those of the Escherichia coli orthologue from the menaquinone biosynthetic pathway.These results provide additional supporting evidence for the essential role of bicarbonate as a catalytic base in the enzymatic reaction and the eubacterial origin of the enzymes in the cyanobacterial biosynthesis of phylloquinone.
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