Haemoglobin subunits

Fig. 4.7 A haemoglobin subunit [16] and its porphyrin core (left, hydrogen atoms removed for clarity)...

Fig. 23 (74). Acrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate and mercaptoethanol. a) Cytochrome c (mol. wt. 12,000) b) haemoglobin subunit (mol. wt. 15,500) c) ovalbumin (mol. wt. 46,000) d) catalase (mol. wt. 60,000) and e) erj throcuprein. All reference compounds (cone. 2 mg/ml) and erythrocuprein were incubated in the presence of 0.1 M sodium phosphate buffer (pH 7.2), 1% 2-mercaptoethanol. 4 M urea and 1% sodium dodecyl sulphate for 1 h at 45°. Aliquots of 10 p (approx. 20 fig protein) were subjected to gel electrophoresis for 2 h at 100 V and 200 mA. The buffer of the upper electrode chamber contained 0.1% sodium dodecyl sulphate in 0.1 M sodium phosphate buffer (pH 7.2). The lower electrode chamber contained 0.1 M sodium phosphate (pH 7.2).

M. L. Connolly, Biopolymers, 25, 1229 (1986). Shape Complementarity at the Haemoglobin], Subunit Interface. 

The association of the haemoglobin subunits has an appreciable effect on the 02-binding properties of the protein so that release of O2 occurs much more readily than with myoglobin. A further consequence of the quaternary structure is that, as described elsewhere, the structure and properties of the molecule undergo modifications in response to environmental changes. 

The oxygen binding protein of heart and skeletal muscle. Its molecular weight is one quarter that of haemoglobin and it resembles a haemoglobin subunit. It is released from muscle in crushing injuries and subsequently appears in the urine (myoglobinuria) where it imparts a dark colour. It can be identified in urine by its electrophoretic mobility or by its spectral characteristics. Radioimmunoassays have also been developed. 

Figure Bl.2.11. Biologically active centre in myoglobin or one of the subunits of haemoglobin. The bound CO molecule as well as the proximal and distal histidines are shown m addition to the protohaeme unit. From Rousseau D L and Friedman J M 1988 Biological Applications of Raman Spectroscopy vol 3, ed T G Spiro (New York Wiley). Reprinted by pennission of John Wiley and Sons Inc.

H2O2 The chemical symbol for hydrogen peroxide Hag Haemagglutinin Hag-1, Hag-2 Cleaved haemagglutinin subunits-1, -2 H E Haematoxylin and eosin hIL Human interleukin Hb Haemoglobin... 

Related behaviour to that just described occurs in haemoglobin, although the latter is a tetramer consisting of four subunits each containing an iron porphyrin. The latter are located within hydrophobic pockets in the globin portion of the molecule (which, in this case, is composed of four linked chains - consisting of two a and two / chains, which differ in their respective amino acid compositions). 

Even though the iron atoms are separated in haemoglobin by about 25 A, communication between them is still able to occur and this has been postulated to involve a trigger mechanism (Perutz, 1971). The trigger is the movement of the proximal histidine as dioxygen binds to (or is released from) the Fe(n) and results in interconversion between the T and R structures. This movement causes a conformational change which is transmitted through the protein to the other iron sites. X-ray studies indicate that relative shifts of up to 6 A at subunit interfaces occur between the T and R states (Perutz, 1978). 

The haemoglobin molecule (Hb) has a molecular weight of about 64000. Four subunits can be identified, each consisting of a polypeptide chain to which is attached a porphyrin group (Fig. 8.6), with an iron atom near its centre. The iron atom is in the II oxidation state (high-spin d6) and is further bonded to a nitrogen atom from an amino-acid residue below the porphyrin ring. The conformation of the polypeptide chain prohibits... 

Quaternary protein structures are the three dimensional protein structures formed by the noncovalent associations of a number of individual peptides and polypeptide molecules. These individual peptide and polypeptide molecules are known as subunits. They may or may not be the same. Haemoglobin, for example, consists of four subunits, two a- and two P-units held together by hydrogen bonds and salt bridges. 

Figure 1.32 A schematic representation of the gene responsible for the control of the production of the P-subunit of haemoglobin...

Haemoglobin is composed of four similar subunits to give an a2 2 structure. The a and P structures are closely related to that of myoglobin. As illustrated in Fig. 4.7,... 

This is the equation for the hyperbolic curve shown in figure above. Haemoglobin with its four subunits has more complex behaviour it approximately follows the equation... 

The replacement of native haem with metal-substituted porphyrin can be performed in two ways. In the first, iron ions are removed from the protein by treating native protein with anhydrous HF, followed by insertion of the appropriate metal to metal-free protein [57-59]. In the second, haem is removed either chemically or by recombination (preparing a proper recombinant protein), and then protein is reconstituted with metal-substituted porphyrins [60-64]. The Zn-substituted metal-loproteins such as cytochrome c [65-67], myoglobin [59,61, 62, 64, 68, 69], and haemoglobin [64, 68,70] have been extensively used to study photoinduced ET (PET) between modified proteins and their physiological redox partners. Interestingly, in haemoglobin with a and /3 subunits it was possible to determine ET parameters for... 

Naito NR, Huang H, Sturgess AW, Nocek JM, Hoffman BM. Binding and electron transfer between cytochrome bs and the haemoglobin a- and (1-subunits through the use of [Zn, Fe] hybrids. J Am Chem Soc 1998 120 11256-62. 

Quaternary structure is confined to those proteins which are made up of a number of protein subunits (Fig. 3.15). For example, haemoglobin is made up of four protein molecules—two identical alpha subunits and two identical beta subunits (not to be confused with the alpha and beta terminology used in secondary structure). The quaternary structure of haemoglobin is the way in which these four protein units associate with each other. 

---