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The effect of pH on the initial rate kinetics of the dimeric biliverdin‐IXα reductase from the cyanobacterium Synechocystis PCC6803
Author(s) -
Hayes Jerrard M.,
Mantle Timothy J.
Publication year - 2009
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2009.07149.x
Subject(s) - kinetics , biliverdin reductase , synechocystis , cyanobacteria , chemistry , biliverdin , biochemistry , biophysics , biology , heme , enzyme , bacteria , physics , gene , heme oxygenase , genetics , mutant , quantum mechanics
Biliverdin‐IXα reductase from Synechocystis PCC6803 (sBVR‐A) is a stable dimer and this behaviour is observed under a range of conditions. This is in contrast to all other forms of BVR‐A, which have been reported to behave as monomers, and places sBVR‐A in the dihydrodiol dehydrogenase/N‐terminally truncated glucose–fructose oxidoreductase structural family of dimers. The cyanobacterial enzyme obeys an ordered steady‐state kinetic mechanism at pH 5, with NADPH being the first to bind and NADP + the last to dissociate. An analysis of the effect of pH on k cat with NADPH as cofactor reveals a p K of 5.4 that must be protonated for effective catalysis. Analysis of the effect of pH on k cat / K m NADPH identifies p K values of 5.1 and 6.1 in the free enzyme. Similar p K values are identified for biliverdin binding to the enzyme–NADPH complex. The lower p K values in the free enzyme (p K 5.1) and enzyme–NADPH complex (p K 4.9) are not evident when NADH is the cofactor, suggesting that this ionizable group may interact with the 2′‐phosphate of NADPH. His84 is implicated as a crucial residue for sBVR‐A activity because the H84A mutant has less than 1% of the activity of the wild‐type and exhibits small but significant changes in the protein CD spectrum. Binding of biliverdin to sBVR‐A is conveniently monitored by following the induced CD spectrum for biliverdin. Binding of biliverdin to wild‐type sBVR‐A induces a P‐type spectrum. The H84A mutant shows evidence for weak binding of biliverdin and appears to bind a variant of the P‐configuration. Intriguingly, the Y102A mutant, which is catalytically active, binds biliverdin in the M‐configuration.