Protein Structure III

This course is part of a series taught by Kevin Ahern at Oregon State University on General Biochemistry. For more information about online courses go to http://ecampus.oregonstate.edu/    • Плейлист  

1. Another type of fibrous protein is collagen, the most abundant protein in your body. It contains three intertwined helices comprised of abundant repeating units of glycine, proline, and hydroxylproline

2. Hydroxylation of proline is a post-translational modification (occurs after the protein is made) and the hydroxyls are placed there in a reaction that uses vitamin C.

3. The hydroxyls of hydroxyproline can react with other, forming covalent cross-links that make the collagen fibers more sturdy.

4. Tertiary structure relates to interactions between amino acids in a protein that are not close in primary sequence. These interactions are made possible by folding to the protein chain to bring the distant amino acids closer together.

5. Tertiary structure is stabilized by disulfide bonds, ionic interactions, hydrogen bonds, hydrophilic, and hydrophobic interactions. Disulfide bonds are the strongest forces holding tertiary structure together, as they are covalent bonds.

6. Most proteins that are in cells are globular in nature.

7. Myoglobin is protein that acts as an oxygen 'battery', storing oxygen in muscles for when it is needed. Myoglobin contains a heme group that contains iron. Heme is a 'prosthetic group', which refers to a non-amino acid containing group that binds to a protein and augments its function.

8. Amino acid residues in myoglobin are arranged such that hydrophilic (and what your book calls ionic) amino acids are arranged on the outside and hydrophobic amino acids are largely arranged on the inside.

9. Porin is a membrane protein. Proteins embedded in membranes often have external amino acids that are hydrophobic so they can interact with the non-polar portions of membranes. Porin, in addition, has a hole in the center that allows water to pass through it. The amino acids in porin are arranged with non-polars outside and polars inside.

10. Quaternary structure of proteins relates to the interactions between separate polypeptide chains within the protein. The word polypeptide refers to a polymer of amino acids. A protein may contain one or more polypeptides and is folded and may be covalently modified.

11. Hemoglobin (and many other proteins) have multiple polypeptide subunits. Interactions between the subunits include disulfide bonds, ionic interactions, hydrogen bonds, hydrophilic, and hydrophobic interactions. Modification of the quaternary structure of a protein may have the same effects as modification of its tertiary structure - alteration of its function/activity.

12. Ribonuclease is an enzyme that degrades RNA. It is unsually stable. For example, it can be heated up to break its hydrogen bonds, but when cooled down, the enzyme still is active, meaning it has assumed its original shape.

13. Denaturation is a word that means the tertiary and/or quaternary structure of a protein is disrupted. RNase has disulfide bonds that help it to remain resistant to denaturation.

14. Some chemicals, such as mercaptoethanol, can reduce the disulfides (between cysteine residues) in proteins to sulfhydryls. In the process of transferring electrons to the cysteines, the sulfhydryls of mercaptoethanol become converted to disulfides. Treatment of RNase with mercaptoethanol reduces RNAse's disulfides to sulfhydryls. Subsequent treatment of RNase with urea disrupts hydrogen bonds and allows the protein to be denatured.

15. Interestingly, removal of the mercaptoethanol and urea from the solution allows RNase to refold slightly, and regaining activity. Clearly, the primary sequence of this protein is sufficient for it to be able to refold itself to the proper configuration. (Note many proteins do NOT do this, however) More interesting, though about ribonuclease is the fact that is one denatures it with urea and mercaptoethanol and then removes the urea, but leaves a tiny amount of mercptoethanol, a lot more enzyme is converted to the active form. The question I asked in class was why. The answer is as follows - RNase has many disulfide bonds and these appear to be key to helping the enzyme to fold. If one removes the urea and the mercaptoethanol simultaneously, the disulfides come back together randomly. Only a small percentage of these random joinings are the right ones that lead to correctly folded RNase. On the other hand, if one keeps the disulfides reduced with a little mercaptoethanol, the hydrogen bonds can reform first, starting part of the folding process that results in the proper alignment of cysteine residues and proper formation of disulfides. Consequently, much more properly folded protein is produced.