Ligand

From Palaeos

In chemistry, ligand refers to a functional group in a molecule. The term usually refers to something relatively small and straightforward, e.g., a carboxyl group, a phenyl group.

In biology, the term ligand usually refers to a small organic molecule that binds with high affinity (normally through covalent bonding and hydrogen bonding) to a much larger protein called a receptor. The ligand-receptor binding causes the receptor protein to change shape in a manner enabling recruitment of additional bound receptors (forming dimers) as well as other tissue-dependent polypeptides, forming a still larger molecular complex. Such large complexes are known as transcription factors. The transcription factor, in turn, binds to a DNA response elements. If the transcription factor binding to DNA initiates gene transcription, the ligand is called an agonist. If the transcription factor binding to DNA does not initiate transcription, but still occupies the DNA response element, the ligand is called an antagonist.

In general, genes that code for such active endogenous ligands as well as the genes that code for the receptor proteins to which the ligands bind are highly conserved through evolution. For example, all mammalian hormones bind to receptor proteins that belong to the nuclear receptor superfamily. Members of the nuclear receptor superfamily have very similar (homologous) amino acid sequences across mammalian species. Together , the superfamily regulates the mammalian endocrine system from inception of life, through developemt, until death. Molecular phylogenetics studies trace the origins of this protein superfamily to Precambrian time.

Credits

ATW? Edit - RGP