Proteins

Proteins

As mentioned in a previous minilecture, proteinsLarge molecules made of amino acids with various functions in the body. are so big that they can fold over and bond with themselves. Yes, proteins are huge moleculesGroups of atoms bonded together. with very complex structures. These structures are usually specific and meant to be used for only one purpose. I have some of the common uses of proteins here. You may be aware of proteins as your muscles, your hair, or even as your enzymesProteins that speed up chemical reactions in the body.. They are also the basis for your antibodies and some hormones. To us in this class, the most important uses are as ion channelsProtein passages in the cell membrane that allow specific molecules to pass through. and as receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter. We will explore these applications in detail in this class.

Amino Acids

There are 20 total amino acids in this world to make all the different proteins. That is a small alphabet and a lot of words. A protein is two or more amino acids bonded together. They are bonded by a covalent bondStrong chemical bonds formed by the sharing of electrons between atoms., but this bond gets a more specific name: peptide bond. A polypeptide is a string of amino acids bonded by peptide bondsBonds that link amino acids in proteins.. This is not officially a protein, but it is the start of a protein. These amino acids are bonded together in dehydrationA condition in which fluid loss exceeds intake, leading to a decrease in total body water. reactions in which waterThe universal solvent essential for life. is removed to build a molecule.

Protein Structure

I think of protein structure like those folded paper fortune tellers we made in second grade. What’s your favorite color? Favorite number? And then you open up the fortune teller, and it tells you who you’re going to marry. I married Joe. Mr. Sherman. Despite your potential spouse, the fortune teller is important since it is folded specifically to create the final product. In factA statement based on direct observation that is repeatedly confirmed., there are different levels of folding to create this final product. Proteins are similar in that they are folded three times to create a final, complex, and very specific structure. These four levels are known as primary, secondary, tertiary, and quaternary. Every time the protein is folded, hydrogen bondsWeak attractions between hydrogen and electronegative atoms like oxygen or nitrogen. hold the folds in place. We referred to hydrogen bonds in a previous minilecture as intramolecular bonds… well… here it is! I hope that you are starting to realize that EVERYTHING in my course is connected. By design or accident, yes.

Levels of Structure

The secondary level of protein structure creates two common shapes: an alpha helixA common spiral structure in proteins, stabilized by hydrogen bonds. and a beta-pleated sheet. An alpha helix is a region of a protein that is twisted into a tight spiral. The coils of the helix are held together by hydrogen bonds between nearby amino acids. Other regions of a protein fold up like a paper fan. This type of structure is referred to as a beta sheet. The alpha helices and the beta sheets together comprise the secondary structure of a protein. A given protein might have three alpha helices and four beta sheets. Another protein might have eight alpha helices and two beta sheets. Protein structures can vary widely. Most proteins have at least some of each.

Once all the helices and sheets are coiled, they are folded and locked into place. The polypeptide takes on a distinct three-dimensional shape. This is the third or tertiary level of protein structure: the overall three-dimensional shape of a protein. The fourth level of protein structure describes interactions among different polypeptides. Not all proteins have a quaternary structure. However, the big ones usually do. Some complex proteins consist of multiple polypeptide chains. These chains bond together to create a sort of super protein.

Protein Textures

You know, I’m always disappointed at the naming of biological structures. I just feel like there’s this great opportunity to name them something great and beautiful. Instead, we end up with words like globular and fibrous. These two terms mean exactly what you think they mean. Globular proteins are a glob. I guess you could roughly say they are spherical. Most globular proteins are water-soluble, which means that they are soluble in your blood. Most globular proteins have roles that are very specific. They may help to make ATPThe energy currency of cells used for muscle contraction. or function as enzymes. Opposite to globular are fibrous proteins, which are strand-like. You might be familiar with collagenA structural protein in the dermis that provides strength and elasticity. or keratinA strong, fibrous protein that forms the structure of skin, hair, and nails.. These are mostly structural proteins. Rather than catalyzing a reaction, these proteins contribute more to the structural building of the human body. They do not serve as a channel for ionsCharged atoms or molecules. to flow through.

Denaturing Proteins

We see how the protein structure is made through these levels of folding, but we can also deconstruct a protein. This process of changing the protein shape is called denaturing. Denaturing usually involves a chemical or heat. I love me some eggs. Scrambled. When I cook the eggs, I’m denaturing the storage proteins. You are denaturing meat when you add a marinade, especially an acidicA solution with a pH below 7, having a higher concentration of H⁺ ions. one. We can actually denatureThe process in which a protein loses its shape and function due to heat or pH changes. proteins, making them safe for consumption without using heat. Fish exposed to acids is called sous vide and is safe to eat. So are eggs that are exposed to lemon juice.