ß-Globin Prefolded Model Teacher Notes©

This mini toober model of the ß-globin protein is composed of 3 differently-colored mini toobers that are joined together and shaped to approximate the “topology” of the ß-globin protein. It represents an alpha-carbon backbone model of the protein. Beginning at the N-terminal end of the protein, the blue fragment represents amino acids 1-42. The green fragment represents residues 43-95 and the C-terminal red fragment represents residues 96-146. The secondary structure of this protein consists of 8 segments of alpha helix. It does not contain any beta sheet.

The side chains of 15 amino acids are displayed in this model. These side chains were chosen to illustrate important structural features of the protein and to emphasize the basic laws of chemistry that drive protein folding. (See the Amino Acid Starter Kit© for an activity that establishes these laws.)

BGM Teacher Notes

BGM Technical Specifications

The ß-Globin Prefolded Protein Model

 

General Information

This toober-based model of the ß-globin protein is composed of three differently-colored mini toobers that are joined together and shaped to approximate the “topology” of the ß-globin protein. It represents an alpha-carbon backbone model of the protein. Beginning at the N-terminal end of the protein, the blue fragment represents amino acids 1- 42. The green fragment represents residues 43-95 and the C-terminal red fragment represents residues 96-146. The secondary structure of this protein consists of eight segments of alpha helix. It does not contain any beta sheet.

The side chains of 15 amino acids are displayed in this model. These side chains were chosen to illustrate important structural features of the protein, and to emphasize the basic laws of chemistry that drive the folding of proteins. (See the Amino Acid Starter Kit© for an activity that establishes these laws.)

  • Sickle Cell Anemia: One amino acid – glutamic acid 6 – is mutated to valine in sickle cell anemia. The model includes one extra side chain – valine labeled with a yellow 6 -- which can be substituted for glutamic acid with a red 6 to demonstrate the sickle cell anemia mutation.
  • Charged Side Chains: Three pairs of amino acids – glutamic acid 7 and Lysine 132, lysine 17 and glutamic Acid 121, and glutamic acid 90 and lysine 144 – represent pairs of oppositely-charged side chains that form “salt bridges” on the surface of the protein. (Glutamic acid side chains are negatively charged while lysine side chains are positively charged.)
  • Hydrophobic Side Chains: Six amino acids – tyrosine 35, phenylalanine 71, phenylalanine 85, phenylalanine 103, leucine 106 and valine 137 – have hydrophobic side chains that are oriented toward the interior of the folded protein, where they are protected from water.
  • Heme Group: Two amino acids – histidine 63 and histidine 92 – are positioned on either side of the planar heme group. Histidine 63 is known as the “proximal histidine” and directly binds to the iron atom of the heme group. Histidine 92 is known as the “distal histidine” and is positioned on the opposite side of the planar heme group. It does not directly bind heme. Instead, molecular oxygen (O2) is bound to the heme iron on this side of the heme group.
  • Hydrophilic Side Chains: One amino acid – tyrosine 35 – is shown as an example of a non-charged but polar side chain that is positioned on the surface of the protein, where it can hydrogen bond with water.

The following key features of this protein are present in your model:

1. You should see three “salt bridges” on the surface of your protein. Salt bridges are composed of pairs of positively and negatively charged side chains (basic and acidic).  The three salt bridges on your model include:

 Glu7 – Lys132       Lys17 – Glu121     

    Glu90 – Lys144

2. You should see a hydrophobic pocket (yellow) created by the side chains Phe71, Phe85, Phe103, Leu106 and Val137.  The hydrophobic edge of the heme group should be buried in this hydrophobic pocket.

3. The iron of the heme group is coordinated by histidine side chains from amino acids 63 and 92. These two side chains should be pointing toward the heme iron.

4. The side chain of Glu6 should be exposed on the surface of the protein. In sickle cell anemia, this negatively-charged side chain is replaced by a valine side chain. What effect do you think this change would have on the solubility of the ß-globin protein?

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