Chemical reactivity of <Emphasis Type="Italic">Synechococcus</Emphasis> sp. PCC 7002 and <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 hemoglobins: covalent heme attachment and bishistidine coordination |
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Authors: | Henry J Nothnagel Matthew R Preimesberger Matthew P Pond Benjamin Y Winer Emily M Adney Juliette T J Lecomte |
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Institution: | (1) T.C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218, USA; |
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Abstract: | In the absence of an exogenous ligand, the hemoglobins from the cyanobacteria Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002 coordinate the heme group with two axial histidines (His46 and His70). These globins also form a covalent linkage
between the heme 2-vinyl substituent and His117. The in vitro mechanism of heme attachment to His117 was examined with a combination
of site-directed mutagenesis, NMR spectroscopy, and optical spectroscopy. The results supported an electrophilic addition
with vinyl protonation being the rate-determining step. Replacement of His117 with a cysteine demonstrated that the reaction
could occur with an alternative nucleophile. His46 (distal histidine) was implicated in the specificity of the reaction for
the 2-vinyl group as well as protection of the protein from oxidative damage caused by exposure to exogenous H2O2. |
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