Without proteins, living things would not exist. Proteins are involved in every aspect of every living thing. Many proteins provide structure to cells; others bind to and carry important molecules throughout the body. Some proteins are involved in reactions in the body when they serve as enzymes. Still others are involved in muscle contraction or immune responses.
Amino acid chains
All proteins are made up of amino acids. Think of amino acids as train cars that make up an entire train called a protein. Proteins are formed by amino acids, which are produced based on the genetic information in a cell. Then, the amino acids that are created in the cell are linked together in a certain order. Each protein is made up of a unique number and order of amino acids. The protein that is created has a specific job to do or a specific tissue (such as muscle tissue) to create.
The structure of amino acids is fairly simple. Each amino acid has an amino group at its core with a carboxyl group and a side chain attached. The side chain (a chemical compound) that is attached determines which amino acid it is.
An enzyme is a protein used to speed up the rate of a chemical reaction. Because they regulate the rate of chemical reactions, they also are called catalysts. There are many, many, many different types of enzymes, because for each chemical reaction that occurs, an enzyme specific to that reaction must be made.
Metabolic processes don’t just automatically happen; they need enzymes. And, a reason that you must consume protein is so that you can make more enzymes so that your processes will occur. Chemical reactions control the metabolism and life of living things.
Proteins are long chains of polypeptides, and thus, so are enzymes. However, some enzymes contain parts that are not made up of proteins but assist the enzyme in its function. These are called coenzymes. Vitamins often act as coenzymes. The name of an enzyme usually reflects the name of the chemical on which the enzyme acts (that is, the chemical substrate). For example, an enzyme that acts on a fat (fat being the substrate) is called a lipase (remember, lip = fat).
To act on a substrate, an enzyme must contain an active site. The active site is the area on the enzyme that allows the substrate and enzyme to fit together (like puzzle pieces). The way that enzymes and substrates fit together is often compared to the way a key fits a lock; the way enzymes kick-start reactions often is referred to as the lock-and-key model. Once the substrate and enzyme are connected, the enzyme can get to work.
During an enzymatic reaction, the substrate is changed during the reaction, and new products are formed during the reaction, but the enzyme comes out of the whole thing unchanged. Then, the enzyme leaves the reaction to form a complex with a different substrate and catalyze another reaction. The products of the reaction continue on in their pathway.
Enzymes are able to catalyze reaction after reaction millions of times before they start to wear out. Then, the body creates more enzymes by synthesizing the proper protein chains from the correct amino acids.
Collagen
Collagen is the most abundant protein in animals with a backbone (that is, vertebrate animals). Between 25 percent and 33 percent of your body weight is comprised of collagen. It is a fibrous (structural) protein that is found in connective tissue, which is all the tissue that joins muscles to bones to allow movement and forms skin that protects the muscle tissue.
Connective tissue includes ligaments, tendons, cartilage, bone tissue, and even the cornea of the eye. It provides support in the body, and it has a great capability to be flexible and resistant to stretching.
The lower layer of the skin (called the dermis) largely is made up of collagen. When skin is removed from an animal (think about removing the skin from a chicken breast), the collagen allows the skin to be pulled away without tearing the muscle tissue underneath. Collagen (and other fibrous proteins) is arranged in long polypeptide chains that form sheets.
Hemoglobin
Hemoglobin is an example of the other major type of proteins: globular proteins. Globular proteins serve a larger variety of functions than the fibrous proteins. For example, the globular proteins include such useful proteins as enzymes, antibodies, and transport proteins.
These proteins, as their name implies, are globular. Many globular proteins can change their shape to fit into very small areas (like an antibody would have to do to go after a virus), cross cell membranes (as a transport protein would have to do), and be involved at the cellular level in chemical reactions (as an enzyme would be).
Hemoglobin is a transport protein found in red blood cells: It carries oxygen around the body. A hemoglobin molecule is shaped kind of like a 3-D four-leaf clover without a stem. Each leaf of the clover represents a certain chain of protein. In the center of the clover, but touching each protein chain, lies a heme group. At the very center of a heme group is an atom of iron.
When gas exchange occurs between the lungs and a blood cell, it is the iron that binds (attaches to) the oxygen. Then, the iron-oxygen complex releases from the hemoglobin molecule in the red blood cell so that the oxygen can cross cell membranes and get inside any cell of the body.
However, the atom of iron and the hemoglobin are not used just once. The iron and hemoglobin usually carry carbon dioxide back to the lungs and deposit it there so it can be exhaled. When the red blood cell that hemoglobin calls “home” is ready to die, the iron either is recycled and gets picked up by another red blood cell to be incorporated into another hemoglobin molecule, or it is excreted as cellular waste.