What are the functions and properties of a ligand protein receptor?

The receptor is a protein that resides on the cell membrane surface and is bound to ligands, which then cause changes in the cell. The ligand is the first messenger, and the receptor is the one that causes the changes. The receptor lodges in one or more membranes, has an active site that allows it to bind to specific targets, and may have a domain or something else underneath it to begin spreading changes in the cell.

The receptor may have subunits beneath it that activate when the ligand attaches: they pop off and start a process. Certain receptors have recruitment stations (docking stations) where things attach (and, for example, get phosphorylated when the ligand binds). The receptor may also work directly with other molecules, second messengers that spread the message across the cell and cause a change.

This is the TLDR version of it, but it all begins with the receptor. An excellent example is when a macrophage sends a ligand to bind to the tumor necrosis factor receptor. This causes changes in the cell that lead to caspases activating each other: a proteolytic cascade. One caspase cleaves and activates another til the executioner caspase is activated and kills the cell.

Receptors can be membrane-bound or intracellular, and their properties include specificity, affinity, and saturation. Specificity is the ability of the receptor to bind only certain ligands; affinity is the strength of the binding between the ligand and the receptor; and saturation is the point at which all available receptors are bound to ligands. A ligand-protein receptor's function is to bind specific signaling molecules (ligands) and initiate a cellular response. This binding triggers a conformational change in the receptor protein, which activates intracellular signaling pathways.