Dynamics of carbon monoxide binding to protoheme

195

We present temperature-dependent kinetic measurements of ultrafast diatomic ligand binding to the ''bare'' protoheme (L 1-FePPIX-L 2, where L1 ‫؍‬ H2O or 2-methyl imidazole and L2 ‫؍‬ CO or NO). We found that the binding of CO is temperature-dependent and nonexponential over many decades in time, whereas the binding of NO is exponential and temperature-independent. The nonexponential nature of CO binding to protoheme, as well as its relaxation above the solvent glass transition, mimics the kinetics of CO binding to myoglobin (Mb) but on faster time scales. This demonstrates that the nonexponential kinetic response observed for Mb is not necessarily due to the presence of protein conformational substates but rather is an inherent property of the solvated heme. The nonexponential kinetic data were analyzed by using a linear coupling model with a distribution of enthalpic barriers that fluctuate on slower time scales than the heme-CO recombination time. Below the solvent glass transition (T g Ϸ 180 K), the average enthalpic rebinding barrier for H 2O-PPIX-CO was found to be Ϸ1 kJ/mol. Above T g, the barrier relaxes and is Ϸ6 kJ/mol at 290 K. Values for the first two moments of the heme doming coordinate distribution extracted from the kinetic data suggest significant anharmonicity above T g. In contrast to Mb, the protoheme shows no indication of the presence of ''distal'' enthalpic barriers. Moreover, the wide range of Arrhenius prefactors (10 9 to 10 11 s ؊1 ) observed for CO binding to heme under differing conditions suggests that entropic barriers may be an important source of control in this class of biochemical reactions.heme proteins

supporting

confidence: 70%

Temperature-dependent heme kinetics with nonexponential binding and barrier relaxation in the absence of protein conformational substates

Ionascu

Gruia

et al. 2007

195

“…In Fig. 3A, absorbance changes at 436 nm after photodissociation are shown for temperatures between 20 and 120 K. The kinetics are nonexponential and remarkably fast, comparable to those measured earlier on protoheme (26). We have fitted the data with a two-state model that invokes a static distribution of activation enthalpy barriers, g(H), between bound and photoproduct state, so that the survival probability in the (five-coordinate) deligated state is given by N͑t͒ ϭ ͵ g͑H͒exp͓Ϫk͑H, T͒t͔dH.…”

Section: Experimental Results and Interpretationmentioning

confidence: 51%

Ligand binding and protein dynamics in neuroglobin

Kriegl

Bhattacharyya

Nienhaus

et al. 2002

144

210

Neuroglobin (Ngb) is a recently discovered protein in vertebrate brain tissue that belongs to the globin family of proteins. It has been implicated in the neuronal response to hypoxia or ischemia, although its physiological role has been hitherto unknown. Ngb is hexacoordinate in the ferrous deoxy form under physiological conditions. To bind exogenous ligands like O2 and CO, the His E7 endogenous ligand is displaced from the sixth coordination. By using infrared spectroscopy and nanosecond time-resolved visible spectroscopy, we have investigated the ligand-binding reaction over a wide temperature range (3-353 K). Multiple, intrinsically heterogeneous distal heme pocket conformations exist in NgbCO. Photolysis at cryogenic temperatures creates a five-coordinate deoxy species with very low geminate-rebinding barriers. The photodissociated CO is observed to migrate within the distal heme pocket even at 20 K. Flash photolysis near physiological temperature (275-353 K) exhibits four sequential kinetic features: (i) geminate rebinding (t < 1 s); (ii) extremely fast bimolecular exogenous ligand binding (10 s < t < 1 ms) with a nontrivial temperature dependence; (iii) endogenous ligand binding (100 s < t < 10 ms), which can be studied by using flash photolysis on deoxy Ngb; and (iv) displacement of the endogenous by the exogenous ligand (10 ms < t < 10 ks). All four processes are markedly nonexponential, suggesting that Ngb fluctuates among different conformations on surprisingly long time scales.G lobins are proteins that bind dioxygen and other small ligands at the central iron of a heme prosthetic group embedded in a highly conserved ␣-helical ''globin'' fold. Hemoglobin (Hb) and myoglobin (Mb) are the most prominent members of this protein family (1). Interrelations among structure, dynamics, and function in globins have been investigated in great detail, most likely more thoroughly than for any other protein family. Mb especially has, for a long time, served as a paradigm in the biological physics of proteins (2, 3).Recently, two new members have joined the globin family, cytoglobin (4) and neuroglobin (Ngb) (5). The latter consists of a single chain of 151 aa. Ngb, which occurs in neurons, has less than 25% sequence identity with Mb or Hb but nevertheless displays all key determinants of the globin fold (6). It has moderate oxygen affinity and seems most closely related to the globin found in the glial cells of the annelid Aphrodite (7). The discovery of a six-coordinate heme in the deoxy form of Ngb (5), with the His E7 side chain occupying the sixth coordination, came somewhat as a surprise because it was believed for a long time that a pentacoordinate heme iron, with a vacant ligandbinding site, was a common characteristic of globins. In recent years, however, hexacoordinate globins also have been isolated from bacteria, unicellular eukaryotes, and plants [nonsymbiotic Hbs, nsHbs (8,9), and truncated Hbs, trHbs (10)]. In these proteins, the exogenous ligand has to compete with an intramolecular side chain for ...

“…One is that the ATP(out)-ADP(in) exchange catalyzed by the well-known ATP-ADP translocase may actually be electroneutral, but this is contrary to a great deal of direct and indirect evidence (3,9,10,(13)(14)(15)(16) Communicated by Britton Chance, August 11, 1978 ABSTRACT (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Most of this work was directed toward studying the rebinding of CO or 02 to Hb or myoglobin on time scales of microseconds or longer (8, 9, 11, 14-16, 18, 20).…”

mentioning

confidence: 44%

Spectroscopic studies of oxy- and carbonmonoxyhemoglobin after pulsed optical excitation.

Greene¹,

Hochstrasser²,

Weisman³

et al. 1978

161

107

The photolysis of HbO2 and HbCO has been investigated with picosecond laser techniques. Transient absorption spectra were measured in the Soret and visible regions after excitation with 353- or 530-nm pulses. The photoproducts appeared within 8 psec and exhibited considerably broadened deoxyhemoglobin-like spectra, which persisted to 680 psec. The altered spectra are attributed to the production of deoxyheme conformational and spin states that might result from the intense excitation.