Bradykinin structure

A reevaluation of the original sequence data established that natural bradykinin was indeed the nonapeptide shown. Here the synthesis of a peptide did more than confirm str-ucture synthesis was instrumental in determining structure. 

The mechanisms of the allergy-like reactions to RCM are still a matter of speculation (table 2). Anaphylaxis to RCM has been discussed to be due to a direct membrane effect possibly related to the osmolality of the RCM solution or the chemical structure of the RCM molecule (pseudo-allergy) [2], an activation of the complement system [27], a direct bradykinin formation [28], or an IgE-mediated mechanism [3]. 

Lopez JJ, Shukla AK, Reinhart C, Schwalbe H, Michel H, Glaubitz C (2008) The structure of the neuropeptide bradykinin bound to the human G-protein coupled receptor bradykinin B2 as determined by solid-state NMR spectroscopy. Angew Chem Int Ed Engl 47 1668-1671... 

Stimulation of free nerve endings known as nociceptors is the first step leading to the sensation of pain. These receptors are found in both somatic and visceral structures and are activated by mechanical, thermal, and chemical factors. Release of bradykinins, K1, prostaglandins, histamine, leukotrienes, serotonin, and substance P may sensitize and/or activate nociceptors. Receptor activation leads to action potentials that are transmitted along afferent nerve fibers to the spinal cord. 

The AT2 receptor has a structure and affinity for Ang II similar to those of the A receptor. In contrast, however, stimulation of AT2 receptors causes vasodilation that may serve to counteract the vasoconstriction resulting from ATi receptor stimulation. AT2 receptor-mediated vasodilation appears to be nitric oxide (NO)-dependent and may involve the bradykinin B2 receptor-NO-cGMP pathway. 

The pathway for the formation and metabolism of kinins is shown in Figure 17-4. Three kinins have been identified in mammals bradykinin, lysylbradykinin (also known as kallidin), and methionyllysylbradykinin. Each contains bradykinin in its structure. 

In the late 1980s when we began the pursuit of bradykinin receptor antagonists, information of relevance to medicinal chemists was scarce. For example, not one nonpeptide antagonist of this receptor was known, nor were any series upon which to base a structure-activity relationship. Moreover, all publications described bradykinin as being highly flexible in an aqueous environment, such that no structural mimetics could be rationalized. Of course the receptors had not been cloned at that time so nothing was known about the primary sequence of the receptor or the three-dimensional structure. 

The bradykinin receptor is a member of a family of receptors for which an intracellular interaction with a G-protein is a critical part of the signal transduction pathway following agonist binding. Structurally, these G-protein-coupled receptors extend from beyond the extracellular boundary of the cell membrane into the cytoplasm. The tertiary structure is such that the protein crosses the bilayer of the cell membrane seven times, thus forming three intracellular loops, three extracellular loops, and giving rise to cytoplasmic C-terminal and extra-cellular N-terminal strands. It is generally presumed that the transmembrane domains of these receptors exist as a bundle of helical strands. This assumption is derived primarily from the known structure of the trans-membrane portions of a structurally related protein, bacteriorhodopsin [40]. 

G-Protein-coupled receptors do not lend themselves to analysis by either NMR or x-ray crystallography due to their structural dependence on an intact cell membrane. In our laboratories we pursued this valuable structural information by utilizing a combination of structural homology modeling, molecular dynamics, systematic conformational searching methods, and mutagenesis experiments. The combination of these techniques led to a proposed model of bradykinin bound to the agonist site on its receptor [41]. 

There have been a variety of single alanine point mutations experimentally introduced into both rat and human bradykinin B2 receptors. Several of these have been shown to decrease the affinity of bradykinin to the receptor and have been implicated structurally near the agonist binding site. In contrast, at the time of this manuscript, there have been no mutations reported that adversely affect the ability of any peptide antagonists to bind to the receptor. Furthermore, antibodies raised against the certain extracellular domains of the kinin receptor compete with bradykinin for binding to the receptor but have no inhibitory... 

Not surprisingly, compound I showed divergent potency when assayed in different species homologues of the bradykinin B2 receptor. In particular, in a model of bradykinin-induced hypotension in rats and rabbits, it showed no activity. Likewise, it did not block bradykinin-induced contraction of the isolated guinea pig ileum. Since compound I is considerably smaller than previously reported decapeptide antagonists, subtle structural differences (which are... 

Table 4 Chemical Structure and Bradykinin B2 Affinity Dais for Compounds I and IA...

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