Proteins Are Combinations of Amino Acids
How many amino acids are found in proteins?
There are numerous amino acids found in nature, but only twenty one are incorporated into the proteins found in most living things (see Amino Acids). This means that these twenty amino acids are the basis of protein found in birds, lizards, plants, bacteria, fungi, yeast, and so on. This is a very profound and also convenient situation. First, it allows us to further appreciate that, despite the obvious structural and functional differences between the different life-forms on this planet, there is common protein ground and more than likely common ancestry. Second, it somewhat simplifies human nutrition as we are able to obtain all of the amino acids we need to make our body proteins by eating the proteins of other life-forms Some proteins contain just a few amino acids linked together, while others contain hundreds of amino acids. Scientists often refer to the links of amino acids in the following manner: Peptides are two to ten amino acids including dipeptides, tripeptides, etc.Polypeptides are 11 to 100 amino acids.Proteins are > 100 amino acids. Other scientists will describe protein size based upon the weight of the protein molecule (molecular weight) and sometimes use the term Daltons as a unit of weight. When we discuss proteins in this book we will only refer to protein size and design only if its helps us understand a protein’s unique function.
What do proteins look like?
As mentioned above, peptides and proteins are comprised of links of amino acids. Some smaller proteins will exist as a somewhat straight chain of amino acids; however, most proteins will exist in a complex three-dimensional design (see Protein 3D). Links of amino acids will contort themselves based upon the specific sequencing of the amino acids. How links of amino acids contort depends on the interaction between the side groups “R” group on the different amino acids. For instance, some amino acids are attracted to other amino acids in the chain while others are repulsed. This is due to either opposing or similar charges. An analogy would be children holding hands to form a chain. As you can imagine, within a short period of time the chain would bend in a manner specific to the children. Some children would want to be closer (or further away) from others. As amino acid chain bends, twists, and warps about three dimensionally, some amino acids will form bonds with other amino acids. This helps stabilize the three-dimensional design. It will be the final structure that determines the functional properties of a protein. It is interesting that many proteins are actually all globbed up, somewhat like crumpled paper or loosely packed yarn. In fact, the names of some proteins, such as hemoglobin and immunoglobin, reflect their globbed (globular) nature. On the contrary, many proteins have more of a filament design, meaning that they are much longer than they are wide. Many of these proteins are like stretched out coils. This is the case with collagen. In fact, numerous collagen proteins come together, side by side, to form a ropelike fibrous super-protein. Further still, it is possible for a protein to demonstrate both globular and filament attributes as is the case with muscle proteins actin and myosin.
What role do proteins play in the human body?
Much of structure and function of our body is based on proteins. Thus, protein and individual amino acids must function in our body in a number of ways. For instance, proteins can function as: enzymes (regulate chemical reactions),structural proteins (yield form to cells and tissue),contractile proteins (provide basis for muscle contraction),antibodies (help protect us from foreign entities),transport proteins (help transport substances in our blood),protein hormones (i.e., insulin, glucagon, and growth hormone),clotting factors (allow our blood to clot to stop a hemorrhage), andreceptors (allow hormones and neurotransmitters to function). Individual amino acids can be used to make certain hormones and neurotransmitters such as epinephrine, serotonin, norepinephrine, and thyroid hormone. In fact, most neurotransmitters are derived from amino acids. Amino acids are also used to make other important substances such as creatine, choline, carnitine, nucleic acids, and the vitamin niacin. Last, amino acids can be used by some tissue as an energy source or can be converted to glucose or fat depending upon our current nutritional/metabolic state (i.e., fasting, fed, exercise).