1992
DOI: 10.1016/0014-5793(92)80375-q
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Alteration of the proximal bond energy in the unliganded form of the homodimeric myoglobin from Nassa mutabilis Kinetic and spectroscopic evidence

Abstract: CO binding kinetics to the homodimeric myoglobin (Mb) from Nassa mut#bilis has been investigated between pH 1.9 and 7.0. Protonation of the proximal imidazole at low pH ('~ 3.0) and the consequent cleavage of the HisFgNE2-Fe proximal bond brings about a ~ 20-fold increase of the second.order rate constant for CO binding. This process displays a pK~ = 4.0 ± 0.2, significantly higher than that observed in all other deoxygenated hemoproteins investigated up to now. Such a feature underlies a decreased energy for … Show more

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Cited by 11 publications
(16 citation statements)
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“…1). This second protonation is reminiscent of that described previously in human adult Hb (10), as well as in other monomeric and dimeric hemoproteins (9,29,30), and it may be referred to the protonation of the N ⑀ atom of the His F8 imidazole ring, and the consequent cleavage (or severe weakening) of the proximal His F8 -Fe bond. This interpretation seems supported by the observation that the transient absorption spectrum of unliganded C. kumu Hb in the visible region displays features characteristic of a tetracoordinated heme (see Fig.…”
Section: Ph Dependence Of Subunit Heterogeneity In Fish Hbsupporting
confidence: 55%
“…1). This second protonation is reminiscent of that described previously in human adult Hb (10), as well as in other monomeric and dimeric hemoproteins (9,29,30), and it may be referred to the protonation of the N ⑀ atom of the His F8 imidazole ring, and the consequent cleavage (or severe weakening) of the proximal His F8 -Fe bond. This interpretation seems supported by the observation that the transient absorption spectrum of unliganded C. kumu Hb in the visible region displays features characteristic of a tetracoordinated heme (see Fig.…”
Section: Ph Dependence Of Subunit Heterogeneity In Fish Hbsupporting
confidence: 55%
“…An additional piece of information may come from the pH dependence of CO binding to both Hs-Nb and Mt-Nb, which does not show any enhancement over the 2.2-7.0 pH range ( Figure 5A). This behavior is drastically different from what observed in most of the other hemoproteins, wherefore at pH < 5.0 a relevant increase of the CO binding rate constant is observed with variable pK a values [56,[61][62][63][64][65][66][67]. This feature, which was attributed to the cleavage (or severe weakening) of the heme-Fe-His proximal bond in the unliganded form, as demonstrated by the spectroscopic features, is characterized by the blue-shift of the absorption spectrum in the Soret region and the appearance of two peaks at 525 and 565 nm [56,61,62].…”
Section: Heme-proteincontrasting
confidence: 89%
“…Actually, the much slower CO binding rate constants for Hs-Nb and Mt-Nb (Table 1), which display an activation free energy (∆G ‡ = 43.7 kJ/mol for Hs-Nb and 45.0 kJ/mol for Mt-Nb) much higher than that of Ec-Mb (∆G ‡ = 39.2 kJ/mol) and Pc-Mb (∆G ‡ = 39.7 kJ/mol), might stem from crowding of H 2 O molecules in the distal side of the heme pocket, as observed from X-ray structures of Fe(III) Nbs [26], which would raise the free energy barrier for ligand binding to the heme's Fe atom. However, an additional contribution might arise from a higher energy barrier for the in-plane movement of the unliganded heme-Fe-His proximal bond to bind CO [56,[61][62][63][64][65][66][67]. The strain, exerted on the proximal His-Fe(II) bond by this movement, may be due to the clustering of amino acid side chains in the proximal side of the heme pocket, which might be also responsible for the resistance of the proximal bond even at very low pH values ( Figure 5).…”
Section: Discussionmentioning
confidence: 99%
“…A very strong ⌬H, larger than that of mammalian Hbs (22), was measured in A. orianae Hb at pH 8.0, in the absence and presence of chloride and ATP. However, lowering the pH to 7.0 brought about a dramatic decrease of the oxygen-binding enthalpy, which became almost zero in the presence of effectors (Table II).…”
Section: Resultsmentioning
confidence: 99%