Myoglobin complexes

Experimental values of —18.1 and —19.5 kcal mol-1 have also been reported for a six-coordinate myoglobin complex (151,152). 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

Figure 5.17 Structure of myoglobin complexed with a heme group. The two residues chosen for mutation, Y103 and L72, are shown. Left Side view of myoglobin. Right Top view of myoglobin. Color scheme Carbon (white), oxygen (red), nitrogen (blue), iron (pink).

Vickery L, Nozawa T, Sauer K (1976) Magnetic circular dichroism studies of myoglobin complexes. Correlations with heme spin state and axial ligation. J Am Chem Soc 98 343-350... 

The steady state, one-dimensional mass balance equations for oxygenated myoglobin complex (2) and free oxygen (1) are... 

Figure 6. The pH dependence of cat for cleavage of myoglobin by Co BU at 37°C. The bellshaped curve was obtained by analyzing the data by treating the Co BU-myoglobin complex as a diprotonic acid (pXai = 5.50, pXa2 = 8.68) and by assuming that the monoprotonated species is...

The fccat values thus measured at various pH values are illustrated in Fig. 6. If ionization of myoglobin or BU is disregarded, the pATa values (5.50, 8.68) estimated from analysis of the bell-shaped pH profile may be assigned to the ionization of water molecules coordinated to the Co(III) ion of the Co BU-myoglobin complex. Note that the catalyst is most active at the physiological pH. The fccat measured at pH 7.5 and 37°C corresponds to a half-life of 30 h. 

Cytochrome c peroxidase compound ES Myoglobin complexed with H2O2. 

P9.34 Assuming that one can identify the CO peak in the infrared spectrum of the CO-myoglobin complex, taking infrared spectra of each of the isotopic variants of CO-myoglobin complexes can show which atom binds to the haem group and determine the C=0 force constant. Compare isotopic variants to C 0 as the standard when an isotope changes but the vibrational frequency does not, then the atom whose isotope was varied is the atom that binds to the haem. See table below, which includes predictions of the wavenumber of all isotopic variants compared to that of i5( C 0). (As usual, the better the experimental results agree with the whole set of predictions, the more confidence one would have with the conclusion.)... 

Chance et al. have also investigated cytochrome c peroxidase Compound I (Compound ES) with EXAFS spectroscopy and found that it, too, has a short Fe-O bond of 1.67+0.04 A [145]. As discussed earlier, cytochrome c peroxidase Compound I is thought to contain an Fe =0 complex with a protein-centered radical [Fe =0]R [71]. Comparative crystallography of cytochrome c peroxidase Compound I and the native ferric enzyme [154] have been interpreted in terms of a short, strong Fe=0 double bond just as indicated by the results of EXAFS spectroscopy [145]. Thus, consensus has been reached that short Fe=0 bonds ( 1.65 A) are present for the Compound I derivative of horseradish peroxidase, the Compound I intermediate of cytochrome c peroxidase, and the related myoglobin complex (Table 2). 

Figure 5 A spectral hole in the absorption spectrum of myoglobin complexed with protoporphyrin IX at 7= 1.5 K.

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