Main Role of protein in body.

Enzymes Main article: Enzyme The best-known role of proteins in the cell is as enzymes, which catalyzechemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in metabolism, as well as manipulating DNA in processes such as DNA replication, DNA repair, and transcription. Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification. About 4,000 reactions are known to be catalyzed by enzymes. [ 28 ]The rate acceleration conferred by enzymatic catalysis is often enormous—as much as 1017-fold increase in rate over the uncatalyzed reaction in the case of orotate decarboxylase(78 million years without the enzyme, 18 milliseconds with the enzyme). [ 29 ] The molecules bound and acted upon by enzymes are called substrates. Although enzymes can consist of hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with the substrate, and an even smaller fraction—three to four residues on average—that are directly involved in catalysis. [ 30 ]The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site. Dirigent proteinsare members of a class of proteins which dictate the stereochemistry of a compound synthesized by other enzymes. Cell signaling and ligand binding Ribbon diagramof a mouse antibody against cholerathat binds a carbohydrateantigen Many proteins are involved in the process of cell signalingand signal transduction. Some proteins, such as insulin, are extracellular proteins that transmit a signal from the cell in which they were synthesized to other cells in distant tissues. Others are membrane proteinsthat act as receptorswhose main function is to bind a signaling molecule and induce a biochemical response in the cell. Many receptors have a binding site exposed on the cell surface and an effector domain within the cell, which may have enzymatic activity or may undergo a conformational changedetected by other proteins within the cell. [ 31 ] Antibodiesare protein components of an adaptive immune systemwhose main function is to bind antigens, or foreign substances in the body, and target them for destruction. Antibodies can be secretedinto the extracellular environment or anchored in the membranes of specialized B cellsknown as plasma cells. Whereas enzymes are limited in their binding affinity for their substrates by the necessity of conducting their reaction, antibodies have no such constraints. An antibody's binding affinity to its target is extraordinarily high. [ 32 ] Many ligand transport proteins bind particular small biomoleculesand transport them to other locations in the body of a multicellular organism. These proteins must have a high binding affinity when their ligandis present in high concentrations, but must also release the ligand when it is present at low concentrations in the target tissues. The canonical example of a ligand-binding protein is haemoglobin, which transports oxygenfrom the lungsto other organs and tissues in all vertebratesand has close homologsin every biological kingdom. [ 33 ] Lectinsare sugar-binding proteins which are highly specific for their sugar moieties. Lectinstypically play a role in biological recognitionphenomena involving cells and proteins. [ 34 ] Receptorsand hormonesare highly specific binding proteins. Transmembrane proteinscan also serve as ligand transport proteins that alter the permeabilityof the cell membrane to small moleculesand ions. The membrane alone has a hydrophobiccore through which polaror charged molecules cannot diffuse. Membrane proteins contain internal channels that allow such molecules to enter and exit the cell. Many ion channelproteins are specialized to select for only a particular ion; for example, potassiumand sodiumchannels often discriminate for only one of the two ions.