Enzyme specificity renin

Figure 2.3 shows the binding of aliskiren to renin in relation to the enzyme specificity pockets, S4-S2. Superimposition of the peptide inhibitor CGP 38560A reveals how very different the binding interactions of aliskiren are compared to the older peptide. 

Another example of a nitrile conversion without enzyme specification is the biotransformation of (i ,5)-3-phenyllactonitrile (phenylacetaldehyde cyanohydrin) to (5)-3-phen-yUactic acid, a precursor for the synthesis of pharmacophores including renin inhibitors, protease inhibitors, and anti-HIV reagents, by Pseudomonas sp. BC-18 [19]. The enantiomeric excess of 75% after biotransformation was increased to 99.8% by repeated crystallization, and the production was enhanced by the addition of 2% (w/v) CaCl2 to the reaction mixture for the precipitation of the (5)-acid. In a mutant strain, the specific activity and the final accumulation were enhanced 16- and 1.2-fold, respectively, compared to the parent strain. A product yield of 70% after biotransformation was obtained due to the... 

Mammals, fungi, and higher plants produce a family of proteolytic enzymes known as aspartic proteases. These enzymes are active at acidic (or sometimes neutral) pH, and each possesses two aspartic acid residues at the active site. Aspartic proteases carry out a variety of functions (Table 16.3), including digestion pepsin and ehymosin), lysosomal protein degradation eathepsin D and E), and regulation of blood pressure renin is an aspartic protease involved in the production of an otensin, a hormone that stimulates smooth muscle contraction and reduces excretion of salts and fluid). The aspartic proteases display a variety of substrate specificities, but normally they are most active in the cleavage of peptide bonds between two hydrophobic amino acid residues. The preferred substrates of pepsin, for example, contain aromatic residues on both sides of the peptide bond to be cleaved. 

Urata H, Kinoshita A, Misono KS, Bumpus FM. Husain A Identification of a highly specific chy-mase as the major angiotensin Il-forming enzyme in the human heart. J Biol Chem 1990 265 22348. Silver RB, Reid AC, Mackins CJ, Askwith T, Schaefer U, Herzlinger D, Levi R Mast cells a unique source of renin. Proc Natl Acad Sci USA 2004 101 13607. Mackins CJ, Kano S, Sevedi N, Schafer U, Reid AC, Machida T, Silver RB, Levi R Cardiac mast cell-derived renin promotes local angiotensin formation, norepinephrine release, and arrhythmias in ischemia/reperfusion. J Clin Invest 2006 116 1063. 

A rather new development is the orally available renin inhibitor aliskiren. It was approved by the U.S. Food and Drug Administration in 2007 for the treatment of hypertension. As mentioned above renin is a protease released on various stimuli from the jux-taglomerula apparatus in the kidney. Its release is the limiting step in the whole renin-angiotensin cascade. Since renin is highly substrate-specific its inhibition can be expected to have very little unspecific side effects. The result of an effective blockade of this enzyme is a reduced angiotensin I and angiotensin II formation. In contrast to ACE-inhibition or ATi-receptor blockade, the plasma concentrations of both peptides stay low. No interaction with other systems like the Kallikrenin-Bradykinin system seems to take place. 

If the main-chain hydrogen bonding of substrates is conserved among aspartic proteinases, how are the differences in specificities achieved Table 1 defines the enzyme residues that line the specificity pockets for both mouse and human renin. In modeling exercises (e.g., Reference 4) it was assumed that specificities derive from differences in the sizes of the residues in the specificity pockets (Sn) and their ability to complement the corresponding side chains at positions Pn in the substrate/inhibitor. A detailed analysis now shows that this simple assumption only partly accounts for the steric basis of specificity. 

Under the proper stimulus renin is released into the circulating blood where it can be identified, especially in that from the renal vein. It acts rapidly upon its specific substrate, splitting the protein into peptides, one or more of which have been called hypertensin or angiotonin. Hypertensin has been concentrated but not obtained in pure form. It is the effector substance of renin, constricting arterioles and raising blood pressure. The action of hypertensin is abolished by hypertensinase, an enzyme found in blood and renal extracts. Fortunately, the latter is destroyed by heat and alkalinity. 

Figure 1.5. The renin-angiotensin system, a) Angiotensinogen is cleaved site-specifically by renin to yield angiotensin I. The latter is converted by another specific protease (angiotensin con-vertase or converting enzyme) to angiotensin 11. b) Angiotensin effects vasoconstriction by acting on a G protein-conpled receptor that is fonnd on smooth mnscle cells. This nltimately leads to increased availability of free Ca in the cytosol and contraction of the smooth mnscle cells.

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