Globin fold structure

Figure 3.13 The hemoglobin molecule is built up of four polypeptide chains two a chains and two (3 chains. Compare this with Figure 1.1 and note that for purposes of clarity parts of the a chains are not shown here. Each chain has a three-dimensional structure similar to that of myoglobin the globin fold. In sicklecell hemoglobin Glu 6 in the (3 chain is mutated to Val, thereby creating a hydrophobic patch on the surface of the molecule. The structure of hemoglobin was determined in 1968 to 2.8 A resolution in the laboratory of Max Perutz at the MRC Laboratory of Molecular Biology, Cambridge, UK.

The globin fold has been used to study evolutionary constraints for maintaining structure and function. Evolutionary divergence is primarily constrained by conservation of the hydrophobicity of buried residues. In contrast, neither conserved sequence nor size-compensatory mutations in the hydrophobic core are important. Proteins adapt to mutations in buried residues by small changes of overall structure that in the globins involve movements of entire helices relative to each other. 

Answer Myoglobin is all three. The folded structure, the globin fold, is a motif found in all globins. The polypeptide folds into a single domain, which for this protein represents the entire three-dimensional structure. 

When more than 60% of the residues in a protein adopt a helical conformation, the protein is usually classified as a. The helices axe usually in contact with one another. The simplest antiparallel a structure, a four-helix bundle, looks like a bundle of sticks. It usually consists of four a helices, aligned to form a hydrophobic core (Figure 12.37). The helices that are adjacent in the amino-acid sequence are also adjacent (and antiparallel) in the three-dimensional structure. The total structure, which is cylindrical, has a left-handed twist of approximately 15° between the axes of the helices. Hemerythrin is one example of a protein that adopts this topology. The globin fold, so-named because it was found in myo-... 

Another important helical domain is the globin fold, found in the first protein for which the three-dimensional structure was determined, the oxygen storage protein, myoglobin (Figure 3.15b), from the muscle of the sperm... 

Globin fold The three-dimensional structure of the proteins that is common to myoglobin and the subunits of hemoglobin. 

The tertiary structure of myoglobin consists of eight a-helices connected by short coils, a structure that is known as the globin fold (see Fig. 7.11). This structure is unusual for a globular protein in that it has no (3-sheets. The helices create a hydrophobic O2 binding pocket containing tightly bonnd heme with an iron atom (Fe ) in its center. 

Figure 21 Two helical motifs found in protein structures (a) the four-helix bundle and (b) the globin fold. Note how the a helices tend to line up antiparallel to each other...

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