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Fourier transform infrared (FTIR) spectra, "light" minus "dark" difference FTIR spectra, and time-resolved step-scan (TRS 2) FTIR spectra are reported for carbonmonoxy aldoxime dehydratase. Two C-O modes of heme at 1945 and 1964 cm-1 have been identified and remained unchanged in H2O/D2O exchange and in the pH 5.6-8.5 range, suggesting the presence of two conformations at the active site. The observed C-O frequencies are 5 and 16 cm-1 lower and higher, respectively, than that obtained previously (Oinuma, K.-I. et al. FEBS Lett.2004, 568, 44-48). We suggest that the strength of the Fe-His bond and the neutralization of the negatively charged propionate groups modulate the ν(Fe-CO)/ν(CO) back-bonding correlation. The "light" minus "dark" difference FTIR spectra indicate that the heme propionates are in both the protonated and deprotonated forms, and the photolyzed CO becomes trapped within a ligand docking site (ν(CO) = 2138 cm-1). The TRS2-FTIR spectra show that the rate of recombination of CO to the heme is k1945 cm-1 = 126 ± 20 s -1 and k1964 cm-1 = 122 ± 20 s -1 at pH 5.6, and k1945 cm-1 = 148 ± 30 s-1 and k1964 cm-1 = 158 ± 32 s -1 at pH 8.5. The rate of decay of the heme propionate vibrations is on a time scale coincident with the rate of rebinding, suggesting that there is a coupling between ligation dynamics in the distal heme environment and the environment sensed by the heme propionates. The implications of these results with respect to the proximal His-Fe heme environment including the propionates and the positively charged or proton-donating residues in the distal pocket which are crucial for the synthesis of nitriles are discussed. © 2011 American Chemical Society. 115 44 13012 13018 Cited By :7

We have identified a ligand (CO) accommodation cavity in the signal transducer sensor protein HemAT (heme-based aerotactic transducer) that allows us to gain single-molecule insights into the mechanism of gas sensor proteins. Specific mutations that are distal and proximal to the heme were designed to perturb the electrostatic field near the ligand that is bound to the heme and near the accommodated ligand in the cavity. We report the detection of a second site in heme proteins in which the exogenous ligand is accommodated in an internal cavity. The conformational gate that directs the ligand-migration pathway from the distal to the proximal site of the heme, where the ligand is trapped, has been identified. The data provide evidence that the heme pocket is the specific ligand trap and suggest that the regulatory mechanism may be tackled starting from more than one position in the protein. Based on the results, we propose a dynamic coupling between the two distinct binding sites as the underlying allosteric mechanism for gas recognition/discrimination that triggers a conformational switch for signaling by the oxygen sensor protein HemAT. © 2006 by The National Academy of Sciences of the USA. 103 40 14796 14801 Cited By :26

The determination of ligand binding properties is a key step in our understanding of gas sensing and discrimination by gas sensory proteins. HemAT is a newly discovered signal transducer heme protein that recognizes O 2 and discriminates against other gases such as CO and NO. We have used FTIR spectroscopy on CO- and NO-bound sensor domain HemAT and sensor domain distal mutants Y70F, T95A, R91A, and L92A to gain insight into the structure of the iron-bound ligand at ambient temperature. These mutations were designed to perturb the electrostatic field near the iron-bound gaseous ligand and also allow us to investigate the communication pathway between the distal residues of the protein and the heme. We show the formation of both H-bonded and non-H-bonded conformations in the CO-bound forms. In addition, we report the presence of multiple conformations in the NO-bound forms. Such distal H-bonding is crucial for ligand binding and activation by the heme. The comparison of the O2, NO, and CO data demonstrates that Thr95 and Tyr70 are crucial for ligand recognition and discrimination and, thus, for specific sensing of gases, and L92 is crucial for controlling the conformational changes of the Thr95 and Tyr70 residues upon NO binding. © 2006 American Chemical Society. 45 25 7763 7766 Cited By :18