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Highlights Protein Structure II
1. Globular proteins have a folded structure arising from turns between regions of secondary structure. Tertiary structure arises from interactions between amino acids that are NOT close in primary structure.
2. Globular proteins have many folds and turns than arrange the helices and sheets into unusual patterns. The process whereby the protein assumes is final shape is called 'folding.'
3. Disruption of forces that stabilize protein structure cause folded proteins to unfold. Unfolded proteins are not functional. We describe them as denatured. Denaturing agents include heat, detergent, acid or base.
4. Collagen is a fibrous protein that is unusual. It has three intertwined helices and the helices are rich in proline, hydroxyproline, and glycine.
5. Addition of hydroxyl groups to proline requires vitamin C. Deficiency of vitamin C results in weak collage and the condition known as scurvy.
6. Hydroxyls on hydroxyproline can react with each other, splitting out water and form a covalent "bridge" between the prolines with oxygen covalently bound in the middle. These cross links help to give collagen strength.
7. Globular proteins have tertiary structure arising from folding. Tertiary structure arises from interactions between amino acids that are not close in primary sequence.
8. Myoglobin is a globular oxygen binding protein that acts like a 'battery', storing oxygen in muscles until it is needed. It is related to the oxygen transport protein called hemoglobin, but does not have a signicant function in transporting oxygen - only storing it.
9. Myoglobin contains a functional group called 'heme'. Heme is composed of a ring called Protoporphyrin IX plus an iron atom. Heme is closely related to chlorophyll. Carbon monoxide can bind to the iron of the heme group.
10. Binding of oxygen by the iron of heme causes the iron to move slightly. Correspondingly, the histidine attached to the iron also moves slightly causing the entire polypeptide chain to alter its shape.
11. Hemoglobin differs from myoglobin in containing four polypeptide subunits instead of one. Interactions between multiple polypeptide subunits of a protein are called quaternary structure.
12. In the lungs, the oxygen concentration is high, so hemoglobin easily gets loaded up with oxygen. In tissues, where oxygen concentration is low, one of the oxygens comes off of hemoglobin and the reversal of what happened above occurs. Loss of one oxygen by hemoglobin favors loss of the others and oxygen is dumped where it is needed.