Globin composition

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). 

In the blood pigment, haemoglobin, the coloured component which is separated as haemin in the manner described above is combined with a complicated protein component known as globin. According to Hans Fischer, haemin has the composition C31H3204N4FeCl. This compound in the form of the so-called Teichmann crystals serves for the microscopic detection of blood. 

In eukaryotes, the existence of compositional biases is unclear and most attempts to detect the replication origins from strand compositional asymmetry have been inconclusive. Several studies have failed to show compositional biases related to replication, and analysis of nucleotide substitutions in the region of the -globin replication origin in primates do not support the existence of mutational bias... 

Figure 27-10 Organization of the globin genes on human chromosomes 11 and 16. The composition of the various embryonic, fetal, and adult hemoglobins is also indicated. Closed boxes indicate active genes and open boxes pseudogenes. The triangles ( ) indicate Alu repetitive sequences and their orientation. The shaded boxes indicate Kpn repeat sequences and the half-arrows their respective orientation. The Kpn sequence between the e and 7° genes in fact consists of two tandemly linked Kpn repeats. From Karlsson and Nienhuis196 and Proudfoot.197...

The poly(A) tracts contained in these presumptive mRNA molecules are generally 150-200 nucleotides in length and have a base composition of greater than 85 adenine. Evidence has also been presented for the occurrence of an adenylate-rich sequence in 10 S rabbit reticulocyte globin mRNA (Lim and Canellakis, 1970). RNA transcribed from vaccinia virus DNA has also been shown to contain a large poly( A) sequence localized at the 3 end of the molecule (Kates, 1970). Poly (A) tracts... 

The compositions of the polypeptide monomers of the various human hemoglobins thus far studied and designated as a, y, are given in Table VI. If one gene governs a sequence of one polypeptide then, in humans, three genes a, y are involved in globin synthesis. 

Regulation at the translational level can also affect Hb synthesis. It is known that globin chain synthesis is reduced when heme and its precursors (ferrum) are absent. The addition of ferrum to reticulocytes stimulates Hb synthesis. Moreover, the a-chains are not released from the polysomes until the jS-chains are synthesized (Baglioni, 1966 Colombo and Baglioni, 1966). Also, the composition of methionine and leucine tRNA populations differs in embryonic and adult reticulocytes of chickens. It is possible that the transition from y to P polypeptide synthesis occurs at the translation level and depends upon the composition of the tRNA pop-... 

Although the globins are chemically specific in their interaction with heme and enabling the latter to combine reversibly with molecular oxygen, their amino acid compositions show rather large variation among the species (Roche et al., 1934, 1941, 1942). Indeed even for the same... 

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