 In this video I will describe the building blocks, structure, and functions of proteins, both fibrous and globular proteins, and I will explain how acidity and high temperature affect the functioning of globular proteins. Proteins are major molecules in biology. They are important for regulating the majority of the functions of a cell. We will learn about lots of examples of important proteins as we go through this semester, but here we are going to develop the concept of what a protein is at the molecular level. A protein is made from one or more polymers, which are known as polypeptides, and each of those polypeptides is a long chain of monomers known as amino acids. In the illustration here we can see two amino acids being joined together through dehydration synthesis. The peptide bond is formed between the two amino acids to join them together into a dipeptide, or a very short peptide of only two amino acids length. However, much longer polypeptides can be produced by continuing dehydration synthesis to add more amino acids to form a longer chain. There are 20 amino acids that are used in order to make polypeptides. The specific sequence of the amino acids in a polypeptide chain is what we call the primary structure of the polypeptide or resulting protein. Proteins also have secondary and tertiary levels of structure. The secondary structure of the polypeptide is a folded up shape that results from hydrogen bonding between nearby amino acids. These amino acids can fold up to form an alpha helix spiral shaped secondary structure or they can fold into a pleated sheet secondary structure shape. In either case this structure is stabilized by hydrogen bonding between adjacent amino acids. A longer distance interaction between amino acids will create another level of folding at a larger size scale known as the tertiary structure of proteins. The tertiary structure of proteins can result from side chains of the amino acids that are hydrophilic facing out to form hydrogen bonds with the surrounding watery solution and side chains of amino acids that are hydrophobic pushing inwards tied away from the surrounding water. There are also covalent bonds that can form between some of the amino acids in order to add another level of stability to the tertiary structure. Finally proteins have quaternary structure if there are multiple polypeptides that join together to form the final protein. The example shown in the illustration here is the protein hemoglobin. Hemoglobin is formed from four polypeptide subunits. Notice that it says globular protein under the name hemoglobin because hemoglobin has a roughly round shape. Hemoglobin is an important protein found inside of our red blood cells where its function is to transport oxygen. Fibers proteins are long protein strands that provide structural support to cells and tissues. Collagen is the most abundant protein in the body and provides the structural support for the skin for example. The dermis, the majority of the skin is strengthened by fibers of collagen. The collagen is found in our bones and joints. Most of the tissues that support our body have the structural support from the collagen proteins. Globular proteins have a complex shape and the function of those proteins is heavily dependent upon the complex folding that creates the globular protein's shape. There's a very wide variety of functions for globular proteins but here we can see a few examples. Growth hormone is a type of globular protein that functions as a chemical message. Growth hormone is secreted from the anterior pituitary gland and signals cells all through the body to stimulate growth. Antibodies are proteins secreted by white blood cells of our immune system that help to defend against specific infections. When you are vaccinated and develop immunity to a specific pathogen, your body will start to produce antibodies that can protect you against that specific infection. In the bottom here we see carbonic anhydrase an example of an enzyme. Enzymes are proteins typically although they can be other molecules such as nucleic acids but most enzymes are proteins and enzymes are biological catalysts that is enzymes speed up chemical reactions by lowering the activation energy. The chemical reaction that's catalyzed by carbonic anhydrase is the inner conversion of carbon dioxide and water with carbonic acid. Carbon dioxide and water can be the reagents that are converted into carbonic acid but this is a reversible reaction so carbonic acid can be converted into carbon dioxide and water. Denaturation is when a globular protein becomes misfolded disrupting its function because globular proteins have functions that are depended upon the complex folded structure. An example that we see in the picture here is denaturation of the protein ovalbumin which is the primary protein found in the egg white. You're probably familiar with the idea that when you first crack the egg the egg white is clear but as it cooks it becomes an opaque white color. This opaque white color change is the result of a change in the shape of the protein as the protein is heated that extreme heat causes vibrations that will essentially shake the protein out of its complex folded structure and change its structure which can lead to disruption of the function of the protein. For example an enzyme like carbonic anhydrase when it becomes denatured will no longer be able to catalyze a chemical reaction. Other extreme environmental conditions can lead to denaturation. For example extreme pH can cause denaturation. If the pH becomes too low the excessive concentration of hydrogen ions can disrupt the pattern of hydrogen bonding necessary to stabilize the protein structure.