P chains, of hemoglobin

Sickle-cell anemia is the classic example of an inherited disease that is caused by a change in a protein s amino acid sequence. Linus Pauling proposed in 1949 that it was caused by a defect in the hemoglobin molecule he thus coined the term molecular disease. Seven years later Vernon Ingram showed that the disease was caused by a single mutation, a change in residue 6 of the P chain of hemoglobin from Glu to Val. 

The genetic defects known as thalassemias result from the partial or total absence of one or more a or P chains of hemoglobin. Over 750 different mutations have been identified, but only three are common. Either the a chain (alpha thalassemias) or P chain (beta thalassemias) can be affected. A superscript indicates whether a subunit is completely absent (a or p ) or whether its synthesis is reduced (a or P ). Apart from marrow transplantation, treatment is symptomatic. 

A long-tailed messenger. Another thalassemic patient had a mutation leading to the production of an mRNA for the P chain of hemoglobin that was 900 nucleotides longer than the normal one. The poly (A) tail of this mutant mRNA was located a few nucleotides after the only AAUAAA sequence in the additional sequence. Propose a mutation that would lead to the production of this altered mRNA. 

Sickle cell anemia results from the nonconservative substitution of valine for glutamate at the sixth residue of the P-chain of hemoglobin. 

Consider the human genetic disease sickle cell anemia. In the normal p-chain of hemoglobin, the sixth amino acid is glutamic acid. In the p-chain of sickle cell hemoglobin, the sixth amino acid is valine. How did this amino acid substitution arise The answer lies in examination of the codons for glutamic acid and valine ... 

Answer the following questions about what was revealed when DNA encoding the gene for the P-chain of hemoglobin and the mRNA for the P-chain were compared. 

Fig. 6.5 The genetic defect in sickle-cell hemoglobin. As the result of a mutation in the p chain gene, the glutamic acid residue normally present in the 6-position of the P chain of hemoglobin A is replaced by a valine residue. This replacement results in the loss of one negative charge in each of two p chains.

The P-chain of hemoglobin is a protein that contains 146 amino acid residues. What minimum number of nucleotides must be present in a strand of mRNA that is coded for this... 

Figure 7.2. Single residue hydrophobicity plots for the a- and P-chains of hemoglobin and for myoglobin. Note the marked decrease in polar residues (the negative deflections) in the plots for the a- and p-...

As examples of the folding of globular proteins. Fig. 1.28 shows schematically the course of the peptide chains in the P-chain of hemoglobin, in triosephosphate isomerase and carboxypeptidase. Other protein conformations are shown in the following figures ... 

Fig. 1.28. Tertiary structures (schematic spiral a-helix, arrow pleated sheet) of the p-chain of hemoglobin (a), of triosephosphate isomerase (b) and carboxypeptidase (c). (according to Walton, 1981)...

Let us assume that the rate at which spontaneous mutation results in detectable changes in a protein is 0.5 X 10- /locus/generation. Let us assume we can monitor for 20 different proteins (some associated, like the a and p chains of hemoglobin), and that we wish to be able, at the 0.05 and 0.01 probability limits, to detect a 50 percent increase in the frequency of mutation. 

The next phase of the study has been concerned with the effects of heme on the synthesis and assembly of the a and p chains of hemoglobin (Vanderhoff et cd., 1967 Tavill et al., 1967). Reticulocytes of iron-deficient rabbits or of rabbits made anemic by acetylphenyl-hydrazine (APH) were incubated with isotopically labeled amino acids in the presence and absence of added hemin. When globin was prepared from the ribosome-free hemolysate which had undergone no further purification, the specific activities (S.A.) of the separated a and p chains were almost equal and the cc/p ratio was approximately... 

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