# Structural Biochemistry/Enzyme Catalytic Mechanism/Protein Function

Proteins are important macromolecules in living organisms because they are structurally. Therefore, they can take on essential roles in a wide variety of biological processes and functions. Protein structure can be described on several different levels. The primary structure of protein refers to the sequence of amino acids in the polypeptide chains. Different amino acids contain different functional groups. The secondary structure of protein deals with the fact that polypeptide chains fold into a regularly repeating structure, such as an alpha-helix and beta-sheet. The tertiary structure of proteins gives the overall structural arrangement of one single subunit polypeptide chain. The quaternary structure of protein refers to the arrangement and interaction of several subunit polypeptide chains to form a protein molecule. The structural diversity and complexity of proteins enable them to perform a diverse variety of functions. A protein is made up of many amino acids through peptide bonds between a carboxyl group and an amino group of another adjacent amino acid. This makes the protein form long chains. Some proteins are known to function alone; however, there are many proteins that work together to form complexes (for example: ribosomes, lipids, nucleic acids, etc.). This creates functionality of cells, and organisms all together. The main function of proteins depends on the amino acids that make up the protein as well as the way it folds.
Some of these functions are given below:\
Antibodies are proteins that participate in the immune response by defending the body against antigens (foreign invaders). Antibodies travel through the blood stream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. Certain antibodies destroy antigens by immobilizing them so that the white blood cells can destroy the antigens. An antibody is made up of 2 heavy and 2 light chains. The two chains are connected by disulfide bonds. There is a variable region and constant region. The variable region is the region where the antigens bind to. Because its variable, different antigens can bind to these regions.
Contractile proteins are the proteins that involved in movement. They include myosin and actin, which participate in muscle contractions and movement. Actin filaments are the major components of the network. Other contractile proteins interact with these filaments in order to create structural rigidity and movement. Contractile proteins's structure and function are striated muscles and well characterized; thus,they contribute a great example of nonmuscle cells. Moreover, the interaction of contractile proteins of various cells may be unique. The study of contractile proteins in cells other than muscle has distinct difficulties. For example, the proteins are presented in a lower concentration than in muscle, and only a few cell types are obtainable for study in quantities comparable to muscle. Also, proteolysis and other detriments may be more severe in nonmuscle cells. Other example can be that the organization of contractile proteins is difficult to define in nonmuscle cells. Nevertheless, the ubiquity of contractile proteins and the importance of their interactions presages increase relevancy for physiology and medicine.
Structural proteins are the proteins that are generally fibrous and stringy. They are the most abundant class of proteins in nature. Their main function is to provide mechanical support. Examples of structural proteins can be keratin, collagen, and elastin. Keratins are found in hair, quills, feathers, horns, and beaks. Collagens and elastin are found in connective tissues such as tendons and ligaments. Collagen is recognized as the most abundant mammalian protein. Structural proteins such as collagen, fibronectin and laminin are utilized in cell culture applications as attachment factors. Sigma offers the most comprehensive collection of structural proteins for extracellular matrix and cytoskeletal research as well as tools for cell culture and material science applications.
Enzymes are the proteins that regulate biochemical processes. They are often called catalysts because they function to lower the activation energy of the reaction and thereby increases the rate of the reaction. Essentially, enzymes are able to do so because they can stabilize the transition state. Lactase and pepsin are examples of enzymes. Lactase is involved in the breakdown of lactose, which are present in milk. Pepsin, on the other hand, helps break down proteins in food. Enzymes are biological catalysts or assistants. Enzymes consist of various types of proteins that work to drive the chemical reaction required for a specific action or nutrient. The chemicals that are transformed with the help of enzymes are called substrates. In the absence of enzymes, these chemicals are called reactants.
Hormonal proteins are the proteins that act as signaling proteins, which help regulate biological activities in the body. Insulin, oxytocin, and somatotropin are examples of hormonal proteins. Insulin is involved in the regulation of glucose metabolism by controlling the blood-sugar concentration. Oxytocin is responsible for stimulating contractions in women during childbirth. Lastly, somatotropin stimulates protein production in muscle cells.
Storage proteins are the proteins that act as storage for amino acids or specific ligands, such as biologically important metal ions. They include ovalbumin and casein. The former is present in egg whites while the latter is found in milk. Another example is myoglobin, which function as the storage of oxygen for tissues.
Transport proteins are the proteins that are responsible for moving molecules from one place to another. For example, the protein hemoglobin is responsible for the transport of oxygen in the blood. Another example is cytochromes, which acts as electron carrier proteins in the electron transfer chain.
Membrane proteins are the proteins that are found in biological membranes. They can either be peripheral or integral. They may act as biological markers or regulatory channels for ions and molecules.