Oxy-haemoglobin

Normal horse haemoglobin is a mixture of two distinct haemoglobins which arc present in approximately equal proportions. The nature of the chemical difference between the two components seems to reside in a single peptide. Crystallographically the two components appear to be identical and no differences can be ob.serv ed in the intensities of the X-ray reflections (36). At a resolution of 5.5 A the electron density maps of the two components should be identical. Horse oxy haemoglobin ciystallized... 

Figure 1.6 The structure of the haem residue in deoxy- and oxy-haemoglobins. In deoxy-Hb the bonding of the iron is pyramidal whilst in oxy-Hb it is octahedral...

A comparison of the deoxy- and oxy-haemoglobin structures reveals a number of important differences. Whereas in the T (deoxy) state the Fe atom is out of the haem plane, on oxygenation it moves into the plane of the now undomed porpyrin, pulling the proximal His F8 and the F-helix, to which it is attached, with it (Figure 13.6),... 

FIGURE 13.7 The ai—p2 interface in (a) human deoxy haemoglobin and (b) oxy-haemoglobin. (Adapted from Voet Voet, 2004.)... 

The absorption in tissue is dominated by oxy-haemoglobin, deoxy-haemo-globin, lipids, and water [121]. The extinction coefficients of the tissue constituents are shown in Fig. 5.41, left. Absorption spectra of tissue measured in vivo are shown right. There is an absorption window from approximately 650 to 900 nm. Therefore, NIR light can be transmitted and detected through tissue layers as thick as 10 cm. Absorption coefficients for various types of tissue are given in [367]. 

Second, carbon dioxide combines directly with amino groups near the N-terminus forming carbamates more readily with deoxyhaemoglobin than oxy-haemoglobin ... 

The time-scale of this haem conversion is related to the antioxidant status of the LDL and that of the erythrocyte lysate. The incorporation of lipid-soluble antioxidants, such as tocopherol and butylated hydroxy-toluene (BHT) at specific time points during the LDL-erythrocyte interaction, prolongs the lag phase to oxidation, eliminates the oxy to ferryl conversion of the haemoglobin and delays the oxidative modification of the LDL. 

The findings here surest that, after an initial slow phase corresponding to the antioxidant capacity of the LDL, hydroperoxides can interact with haemoglobin in a similar manner to hydrogen peroxide, forming ferryl haemoglobin, which is then rapidly reduced to mixtures consisting mainly of oxy- and met- forms, possibly by the synproportionation reaction, as proposed in the studies... 

The pulse oximeter is a non-invasive device used to monitor the percentage saturation of haemoglobin (Hb) with oxygen (Spo2). The underlying physical principle that allows this calculation to take place is that infrared light is absorbed to different degrees by the oxy and deoxy forms of Hb. 

Case, Huynh and Karplus have performed Parriser-Parr-Pople SCF calculations with configuration interaction and X-a calculations on oxy- and carboxy-haemoglobin, and have compared their results with those obtained from EHMO and ab initio calcula-tions They find configuration interaction to be very important, and suggest that the ground state may conveniently be described as a mixture of Fe (low spin d ) -02( Ag) and (Fe (S = 1, t2g Cg) - 02( Sg). The results all show an interaction of the in-plane orbital with dyz and of the out-of-plane orbital rg (i) with d, . The authors discuss in detail the spectroscopic data available, and their agreement with theoretical calculations. 

Neutron experiments were first made on haemoglobin [98,99,147,166,167] and were extended to myoglobin [44,168], lysozyme [169] and catalase [170] as models of typical globular proteins. In parallel with X-ray scattering, the haemoglobin work (mainly in H20) identified a conformational change between the oxy- and deoxy-forms which was reflected in an difference of 0.054 nm in H20 buffers. Scattering curve comparisons to <2 = 3 nm with the crystal structures verified this. 

The constraints which clearly distinguish the deoxy-forms from the oxy-structure include the salt bridges formed by the C-terminal residues and Perutz and Ten Eyck S have studied haemoglobins modified at these groups. 

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