netLibrary - eBook Summary Structure-based Drug Design by ...

Document
Page 336
P 1' in human and mouse angiotensinogens. Accordingly residue 213 is leucine in human renin and valine
in mouse renin. The S 1' pockets of chymosin, pepsin, and endothiapepsin have an aromatic side chain at
residue 189 while the renins have amino acids with smaller side chains (valine in human and serine in
mouse renins). This would be expected to make the pocket larger in renins. However the structure of the
mouse renin complex shows that the substrate moves closer to the enzyme in renins as a result of the
smaller residue at 189 and the pocket is made even more compact due to a compensatory change in the
position and composition of the polyproline loop (residues 290–297). Thus, the specificity difference at
this site arises not only from a compensatory movement of a secondary structure, in this case a loop
region, but also from the substitution of an enzyme residue that allows the substrate to come closer to
the body of the enzyme.
Elaboration of loops on the periphery of the binding cleft in renins also influences the specificity. This is
most marked at P 3' and P 4', for which it has been particularly difficult to obtain complexes with welldefined
conformations for other aspartic proteinases. In endothiapepsin, which has been the subject of
the greatest number of studies, different conformations are adopted at P 3' and the residue at P 4' is
generally disordered. In contrast these residues are clearly defined in mouse renin. This is mainly a
consequence of the polyproline loop, illustrated in Figure 6, which occurs uniquely in renins. The x-ray
analysis of the mouse renin complex shows that the S 3' and S 4' subsites are formed by the polyproline
loop together with residues of the flap, and a similar situation is likely to occur in human renin. The welldefined
interactions of P 3' described in the mouse renin complex explains the significant affinity when
inhibitors have phenylalanine or tyrosine at P 3' as well as the importance of a P 3' residue for catalytic
cleavage of a substrate by renin [50].
Hydrogen bonds between the side chains of the inhibitor and the enzyme do not play a major role in
most specificity pockets. However, S 2 is an exception. This subsite is large and contiguous with S 1', so
that in human renin the S-methyl cysteine (SMC) side chain of P 2 is oriented towards the S 1' pocket,
which is only partly filled by the isopropyloxy group of the putative P 1' residue. The carbonyl oxygen of
P 2 accepts a hydrogen from the O γ of Ser76, which is unique to human renin; residue 76 is a highly
conserved glycine in all the other aspartic proteinases, including mouse renin. In mouse renin the P 2
histidyl group has a different orientation and forms a hydrogen bond with the O γ of Ser222. If such a
conformation were adopted by the human angiotensinogen in complex with human renin, the two
imidazole nitrogens would be hydrogen bonded to the O γ of both Ser76 and Ser222. The observed
reduction in the rate of cleavage of a human angiotensinogen analog containing a 3-methyl histidine
substituent at P 2 [51] could be explained on the basis of the hydrogen bonding scheme proposed above.
http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_336.html [4/5/2004 5:26:17 PM]