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Papers In Press, published online ahead of print February 27, 2004
Chemistry, Princeton University, Princeton, NJ 08544
Corresponding Author: spiro{at}princeton.edu
CooA is a dimeric CO-sensing heme protein from Rhodospirillum rubrum. The heme iron in reduced CooA is six-coordinate; the axial ligands are His77 and Pro2. CO displaces the Pro2 ligand and induces a conformation change that allows CooA to bind DNA and activate transcription of coo genes. Equilibrium CO binding is cooperative, with a Hill coefficient n = 1.4, P50 = 2.2 mM, and fitted Adair constants K1 = 0.16 and K2 = 1.3 mM-1 at pH 7.4, 25°C. However, the rates of CO binding and release are both strongly biphasic, with roughly equal amplitudes for the fast and slow phases. The association rates show a hyperbolic dependence on [CO], consistent with Pro2 dissociation being rate limiting. The kinetic characteristics of the transiently formed five-coordinate heme have been probed via flash photolysis. These observations are integrated into a kinetic model, in which CO binding to one subunit decreases the rate of Pro2 rebinding in the second subunit, leading to a net increase in affinity for the binding of the second CO. In addition, the CO adduct exists in slowly interconverting “open” and “closed” forms. This interconversion probably involves the large-scale motions required to bring the DNA-binding domains into proper orientation for regulation of transcription. The combination of low CO affinity, slow CO binding, and slow conformational transitions ensures that activation of CooA only occurs at high (micromolar) and sustained (= 1 minute) levels of CO. However, when micromolar levels do occur, positive cooperativity allows efficient activation over a narrow range of CO concentrations.
J. Biol. Chem, 10.1074/jbc.M400613200
Submitted on January 20, 2004
Revised on February 25, 2004
Accepted on February 27, 2004
Dynamics of carbon monoxide binding to CooA
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