Investigation of the haem-nicotinate interaction in leghaemoglobin. Role of hydrogen bonding.
A strategic assessment of the contributions of two active-site hydrogen bonds in the binding of nicotinate to recombinant ferric soybean leghaemoglobin a (rLb) was carried out by mutagenic replacement of the hydrogen-bonding residues (H61A and Y30A variants) and by complementary chemical substitution of the carboxylate functionality on the nicotinate ligand. Dissociation constants, Kd (pH 5.5, mu = 0.10 M, 25.0 +/- 0.1 degrees C), for binding of nicotinate to ferric rLb, H61A and Y30A were 1.4 +/- 0.3 microM, 19 +/- 1 microM and 11 +/- 1 microM, respectively; dissociation constants for binding of nicotinamide were, respectively, 38 +/- 1 mM, 50 +/- 2 mM and 12 +/- 1 mM, and for binding of pyridine were 260 +/- 50 microM, 4.5 +/- 0.5 microM and 66 +/- 8 microM, respectively. Binding of cyanide and azide to the H61A and Y30A variants was unaffected by the mutations. The pH-dependence of nicotinate binding for rLb and Y30A was consistent with a single titration process (pKa values 6.9 +/- 0.1 and 6.7 +/- 0.2, respectively); binding of nicotinate to H61A was independent of pH. Reduction potentials for the rLb and rLb-nicotinate derivatives were 29 +/- 2 mV (pH 5.40, 25.0 degrees C, mu = 0.10 M) and - 65 +/- 2 mV (pH 5.42, 25.0 degrees C, mu = 0.10 M), respectively. The experiments provide a quantitative assessment of the role of individual hydrogen bonds in the binding process, together with a definitive determination of the pKa of His61 and unambiguous evidence that titration of His61 controls binding in the neutral to alkaline region.
Patel
N
N
Department of Chemistry, University of Leicester, UK.
Jones
D K
DK
Raven
E L
EL
D001483:Base Sequence; D017931:DNA Primers; D004563:Electrochemistry; D006418:Heme / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007874:Leghemoglobin / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D009525:Niacin / Q000737:chemistry* / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism
D017931:DNA Primers; D007874:Leghemoglobin; D008024:Ligands; D011994:Recombinant Proteins; D009525:Niacin; D006418:Heme