Editing Protein (section)

===Cell signaling and ligand binding===

{{See also|Glycan-protein interactions}}

[[File:Mouse cholera antibody.png|thumb|upright|[[Ribbon diagram]] of a mouse antibody against [[cholera]] that binds a [[carbohydrate]] antigen]]

Many proteins are involved in the process of [[cell signaling]] and [[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 [[biological tissue|tissues]]. Others are [[membrane protein]]s that act as [[receptor (biochemistry)|receptors]] whose 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 change]] detected by other proteins within the cell.<ref name = "Brandon_1999" />{{rp|251–81}}

[[Antibodies]] are protein components of an [[adaptive immune system]] whose main function is to bind [[antigen]]s, or foreign substances in the body, and target them for destruction. Antibodies can be [[secrete]]d into the extracellular environment or anchored in the membranes of specialized [[B cell]]s known as [[plasma cell]]s. 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.<ref name = "Van_Holde_1996" />{{rp|275–50}}

Many ligand transport proteins bind particular [[Small molecule|small biomolecules]] and transport them to other locations in the body of a multicellular organism. These proteins must have a high binding affinity when their [[ligand]] is present in high concentrations, and 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 [[oxygen]] from the [[lung]]s to other organs and tissues in all [[vertebrate]]s and has close homologs in every biological [[kingdom (biology)|kingdom]].<ref name = "Van_Holde_1996" />{{rp|222–29}} [[Lectins]] are [[Glycan-protein interactions|sugar-binding proteins]] which are highly specific for their sugar moieties. [[Lectins]] typically play a role in biological [[Molecular recognition|recognition]] phenomena involving cells and proteins.<ref name=Rudiger2000/> [[Receptor (biochemistry)|Receptors]] and [[hormone]]s are highly specific binding proteins.

[[Transmembrane protein]]s can serve as ligand transport proteins that alter the [[Semipermeable membrane|permeability]] of the cell membrane to [[small molecule]]s and ions. The membrane alone has a [[hydrophobic]] core through which [[Chemical polarity|polar]] or charged molecules cannot [[diffusion|diffuse]]. Membrane proteins contain internal channels that allow such molecules to enter and exit the cell. Many [[ion channel]] proteins are specialized to select for only a particular ion; for example, [[potassium]] and [[sodium]] channels often discriminate for only one of the two ions.<ref name = "Brandon_1999" />{{rp|232–34}}