Carbon monoxide binding kinetics in "capped" porphyrin compounds.

Abstract

The rate constants for CO binding to the fivecoordinate ferrous iron complexes of 5,10,15,10,15, meoitoyl(etrakis-oypropowyphenyl)]porphyrin have been measured and compared with the corresponding rate constants for other hemes and hemoproteins. The second-order rate constant Is independent ofcap size and is comparable to that ofhigh-affinity state hemoglobin (k5 4 X 106 M-'s-'). Therefore, these capped porphyrins provide no steric hindrance to CObincing. In addition, a kdnetic scheme involving an unusual seven-coordiate porphyrin species is described.The effect of protein structure on the reactivity of the heme prosthetic group has been the focus ofnumerous investigations. It is known that ligand binding kinetics of Hb are affected by protein nformation and that the CO binding rate for the lowaffinity m state is 1/60th to 1/20th that for the high-affinity (R) state (1). Among the mechanisms that have been proposed to control the reactivity of the heme prosthetic group (2-5) is the suggestion that steric effects on the distal side ofthe heme pocket influence ligand binding (6)(7)(8). This steric hindrance apparently causes the stable linear Fe-C--O to be either bent or slanted and is supported by x-ray and IR studies (9). The binding properties of various model compounds have been studied in an effort to obtain information on the importance of distal-side steric effects in ligand biding to hemes (10-15).We now probe the influence of distal steric effects on CO binding rates through use ofthe model compounds 5,10,15,20-[pyromellitoyl(tetrakis-o -

Intramolecular Electrostatic Effects on O2, CO2, and Acetate Binding to a Cationic Iron Porphyrin

“…In general, the enthalpic parameters for O 2 binding to [Fe II ( o -TMA)­(AcO)] 3+ are close to values reported for sterically hindered “picket-fence” and “capped” porphyrins. ,− This was initially surprising because all of these metalloporphyrins are neutral complexes. The ligand motifs in these various complexes, such as cations, hydrogen-bond donors, axial ligands, or protected faces, have remarkably little effect on O 2 binding.…”

A “Sliding Scale Rule” for Selectivity among NO, CO, and O2 by Heme Protein Sensors

“…In Fig. 1, the K D values for NO, CO, and O 2 binding to 5c (Fe(II)PP(1-MeIm)) (14–16), Mb (17–19), sGC (8, 9, 20–22), cytochrome c′ (cyt c′) (23–27) and HemAT (28) were taken from the literature, and those for sGC containing the I145Y mutation in the β subunit (αβI145Y sGC) (9), and Ns H-NOX (13) were determined in our laboratory (Table 1). NO always shows the highest affinity (lowest K D ) due to its radical nature and some back bonding, CO shows an intermediate affinity due to extensive back bonding, and O 2 shows the lowest affinity because only a sigma bond can be formed with the iron atom (29–31).…”

“…1, 6, and 7). Studies of ligand binding parameters for synthetic model hemes designed to modulate the steric and electrostatic interaction seen in proteins are sporadic and incomplete (14–16, 68–72) (Table S5). We could not find another model heme compound, containing a proximal imidazole ligand, for which binding kinetic studies had been conducted for all three gases.…”

Regulation of Insulin-Like Growth Factor-Binding Protein-1 by Nitric Oxide under Hypoxic Conditions1

“…NO or CO) bind to the oxygen sensor with higher affinities than O 2 (37,38), the sensor protein locks into a relaxed oxy form configuration, whereas without ligands the sensor keeps the deoxy form, and the hypoxic signal transduction pathway is activated. NO or CO) bind to the oxygen sensor with higher affinities than O 2 (37,38), the sensor protein locks into a relaxed oxy form configuration, whereas without ligands the sensor keeps the deoxy form, and the hypoxic signal transduction pathway is activated.…”