macromolecules in living organisms; they are what act out the duties that are encoded in genes. In humans they help our bodies to repair, regulate, and protect themselves. Proteins help in the building and repair of tissues, and in body processes such as water balancing, nutrient transport, and muscle contractions. Many essential enzymes and hormones are proteins. Proteins are basically essential for life. The reason that proteins can carry out such a diverse set of functions is because they are able to bind to other proteins specifically and tightly. Their binding ability can be contributed to their tertiary structure that creates a binding or active site; the chemical properties of the surrounding amino acids' side chains also have a large influence on the binding ability of proteins.
Proteins acting as enzymes are probably their most important function. Enzymes are the biological catalysts that are essential for almost all the biological systems in our bodies to work, they are what catalyze reactions in processes like metabolism, DNA replication, and digestion. Enzymes are extremely specific and will only catalyze certain reactions. The high specificity is related to the structure of the substrate and the enzyme. The enzyme will bind only to an active site only in the substrates which is complementary to its structure, like a key in a lock. Protein-protein interactions regulate this enzymatic activity.
Proteins are also essential for cell signaling and molecular transport systems. Because a protein produced by one cell can bind with a protein from another cell, they provide good cell signal and molecular transport pathways. An example of a protein that acts in this fashion is hemoglobin. Hemoglobin binds iron molecules and transports them from the lungs, through the blood stream, to all the essential organs and tissues. This examples shows how essential proteins are in living systems.
There are also structural proteins such as actin and tubulin that polymerize to form the cytoskeleton of a cell. The essential motor proteins such as myosin, kinesin,and dynein are also structural proteins.