Ligand-receptor engagement

Thus far, the discussion of G-proteins and effector enzymes has assumed that a ligand has engaged with its surface receptor. There is however, an important example of an alternative mechanism to activate an effector without the direct involvement of G-protein complex. NO is a local hormone, a neurotransmitter and part of the cell s armoury of oxidizing agents called free radicals. 

The dynamic contact of receptors on a moving cell with ligands on other cells warrants specific considerations for the study of activation of intracellular signaling pathways. The small number and the specific distribution of the receptors engaged by membrane ligands lead to a highly localized response with equally local regulation. Often, the fast dynamics of the receptor activation... 

CC Chemokine Receptor 2 (CCR2) is a member of the G protein-coupled receptor (GPCR) superfamily that serves as the receptor for monocyte chemoattractant proteins 1-4 (MCP-1 to -4), a group of pro-inflammatory chemotactic cytokines (chemokines). CCR2 is the primary chemokine receptor on inflammatory monocytes, and is also expressed on T-cells, dendritic cells, and endothelial cells. Upon ligand engagement, CCR2 mediates both cellular movement and activation. 

Receptors exhibit structural complementarity with their ligand in the same way that enzymes are complementary to their substrate. Often the actual binding of the hormone to its receptor involved just a small portion of both molecules. The peptide ACTH secreted by the pituitary gland contains 39 amino acids, but only about 12 of these near the N-terminal are required to engage the receptor. Furthermore, and as noted in Section 4.4.1, LH, FSH, TSH and hCG all share a common a subunit and their receptors recognize only the [3 unit. 

Figure 4.20 Tyrosine kinase-linked receptors (a) those with inherentTK activity insulin receptor. The intracellular tail and cytoplasmic proteins become phosphoryLated following hormone engagement with the ligand-binding domain...

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