Haemoglobin and myoglobin

An alternative explanation of the effect of the proximal base in determining R and T behaviour relies on strong hydrogen-bonding to the N-1 proton of imidazole which in effect causes deprotonation. Deprotonation of the imidazole base does have an effect on the binding rate of CO on Fe TPP complexes in toluene but this is in the opposite sense to what might be expected. It is unlikely that this mechanism is operative but the effect could be explained by relatively small changes in the state of protonation. 

The roles of the distal residues histidine and valine on substrate binding have also been investigated. They help maintain restraint in the T-state and prevent iron(n) oxidation. Neither residue has an effect on the bent FeOO chromophore but the distal histidine interacts with linearly bound CO. 

Iron(ii) porphyrins, like iron(n) phthalocyanin, interact reversibly with Oj in aqueous solution but subsequently dimerize and are oxidized to iron(ra). Sterically hindered o/ /A -analylamide derivatives of iron(ii) porphyrins, the picket fence porphyrins, prevent dimerization and hence oxidation and serve as models for the high-afl nity J -state of Hb(ii). Using as the proximal base 2-methylimidazoIe, which has a steric interaction with the porphyrin ring and holds the iron in its unliganded position below the ring, a low-affinity T-form of the complex can be produced. In the crystalline state, picket fence porphyrins show co-operativity very similar to that shown by haemoglobin in mechanistic and quantitative detail.  

The high-affinity R- and low-affinity T-states of fish haemoglobins are readily interconverted by changes in pH. At pH 6 and 20 °C, chain heterogeneity is detected in the reactions of the T-state with CO while under more alkaline conditions, pH 8.5, reactions of the i -state are monophasic. Experiments at intermediate pH are readily accounted for by a conformational stability constant which varies from 4 x 10 at pH 6.0 to 8 at pH 8.5. Light increases CO dissociation from both R- and T-states. The effects of pH on tadpole haemoglobin have also been investigated.  

Spin label studies reveal that the extent of deoxygenation of Hb shows no preference in the a- and /S-chains. The presence of IHP, however, causes deviations which imply that the a-subunit exhibits preferential binding. Inosine hexaphosphate can stabilize Hb with asparagine-102 replaced by threonine in a form with a deoxy-quaternary structure (T-state) even when bound with CO. The protein has a low CO affinity and is almost devoid of co-operativity with a second-order binding rate of 1.03 x 10 s. There is no evidence for 

7 (a) The structure of haemoglobin shown in a ribbon representation. The four subunits, each containing a haem unit, are shown in different colours, (b) The structure of the haem unit in its rest state. The Fe(II) centre is coordinated by a protoporph5Tin IX ligand and a histidine residue the non-terminated stick represents the connection to the protein backbone. Hydrogen atoms are omitted for clarity. Colour code Fe, green C, grey N, blue O, red. 

The Co(III) complex formed in reaction 28.4 can be considered to contain coordinated [02], but the presence of the axial base, L, is crucial to the formation of the monomeric product. In its absence, a dicobalt species with a Co—0—0—Co peroxo-bridge (i.e. analogous to those discussed in Section 21.10) is formed. 

A logical ligand to model the active sites in myoglobin and haemoglobin is one derived from porphyrin. Tetraphenyl-porphyrin (H2tpp, 28.8) is readily available, but the reaction of the Fe(II) complex Fe(tpp)2 with O2 leads to a peroxo-bridged Fe(III) complex (equation 28.5). 

---

Figure Bl.2.10. Structure of the protohaeme unit found in haemoglobin and myoglobin.

Fermi, G., Pemtz, M.F. Atlas of Molecular Structures in Biology. 2. Haemoglobin and Myoglobin. Oxford,... 

The 4 g of iron in the human body is normally com-partmented into its functional locations in the haem- and non-haem-containing, and iron-binding proteins and enzymes (Fig. 3.5). The majority (65%) of the iron is in the divalent state in haemoglobin and myoglobin, which are involved in the transport and storage of oxygen in erythrocytes and myocytes, respectively. The remainder is distributed between storage sites, predominantly in the... 

Hard, S. and Kanner, J. (1989). Haemoglobin and myoglobin as inhibitors of hydroxyl radical generation in a model system of iron redox cycle. Free Rad. Res. Commun, 6, 1-10. 

Table 9.1 gives a comparison of the respective properties of haemoglobin and myoglobin. 

Models for haemoglobin and myoglobin. It has been known for a long time that a range of synthetic Co(n) complexes are able to bind reversibly dioxygen in a manner related to the natural systems (Basolo, Hoffman Ibers, 1975 McLendon Martell, 1976). However, similar complexes of iron do not normally function as 02 carriers - rather oxidation to Fe(m)... 

G. Fermi and M. F. Perutz, Haemoglobin and Myoglobin, Atlas of Biological Structures 2. Oxford Univ. Press (Clarendon), London and New York, 1981. 

The porphyrin motif, four pyrrole rings linked by carbon atoms to give a planar cyclic compound, is found in many biomolecules. It is present in haemoglobin and myoglobin and its derivatives are also at the core of vitamin B12, hydrogenases, cytochrome c, the chlorophyll photosystem and similar essential proteins. 

The two most important metalloprotein, other than haemoglobin and myoglobin are haemerythrins and haemocyanin which act as natural oxygen carrier. 

Ortiz de Montellano PR, Catalano CE (1985) Epoxidation of styrene by haemoglobin and myoglobin. Transfer of oxygen equivalents to the protein surface. J Biol Chem 260 9265-9271... 

Q5 The average diet in the United Kingdom contains approximately 15 to 20 mg of iron only about 10% of this is absorbed. The main iron content of the diet is haem iron derived from the haemoglobin and myoglobin in red meat, and this is the form of iron which is most readily absorbed. Non-haem iron, for example that derived from cereal products, is less well absorbed. The majority of iron absorption occurs in the duodenum and first part of the jejunum. Non-haem iron absorption is very variable, and some types of food, such as bran and egg yolk, limit its absorption. Gastric acidity helps to keep iron in the ferrous state, which is more easily absorbed than the ferric form. Iron absorption is increased when iron stores are low and when there is increased erythropoietic activity. 

Various interesting biomedical applications of phenothiazines were investigated, such as the anti-inflammatory effects of fluphenazine and triflupro-mazine, the use of chloropromazine and trifluoperazine to determine the secondary and tertiary structure of haemoglobin and myoglobin, and the efficiency of the promethazine (+) enantiomer to inhibit the bone resorbing cells, which led to the development of new methods for the treatment of periodontitis and osteoporosis. Recently, it was shown that some phenothiazine derivatives inhibited the production of prions in cultured neuroblastoma cells, suggesting that these phenothiazines might be applied to treat CJD. 

---