Biomedical subjects

Penelope E Stein

Publications and source records attributed to Penelope E Stein.

4 recordsLinked to original sources

PubMed2006

Polymerization of human angiotensinogen: insights into its structural mechanism and functional significance.

In the present study, we have investigated the in vitro polymerization of human plasma AGT (angiotensinogen), a non-inhibitory member of the serpin (SERine Protease INhibitor) family. Polymerization of AGT is thought to contribute to a high molecular mass form of the protein in plasma that is increased in pregnancy and pregnancy-associated hypertension. The results of the present study demonstrate that the polymerization of AGT occurs through a novel mechanism which is primarily dependent on non-covalent linkages, while additional disulfide linkages formed after prolonged incubation are not essential for either formation or stability of polymers. We present the first analyses of AGT polymers by electron microscopy, CD spectroscopy, stability assays and sensitivity to proteinases and we conclude that their structure differs from the 'loop-sheet' polymers typical of inhibitory serpins. Histidine residues within the unique N-terminal extension of AGT appear to influence polymer formation, although polymer formation can still take place after their removal by renin. At a functional level, we show that AGT polymers are not substrates for renin, so polymerization of AGT in plasma would predictably lead to decreased formation of AngI (angiotensin I) with blood pressure lowering. Polymerization may therefore be an appropriate response to hypertension. The ability of AGT to protect its renin cleavage site through polymerization may explain why the AngI decapeptide has remained linked to the large and apparently inactive serpin body throughout evolution.

Angiotensinogen↗

PubMed2004

How small peptides block and reverse serpin polymerisation.

Many of the late-onset dementias, including Alzheimer's disease and the prion encephalopathies, arise from the aberrant aggregation of individual proteins. The serpin family of serine protease inhibitors provides a well-defined structural example of such pathological aggregation, as its mutant variants readily form long-chain polymers, resulting in diseases ranging from thrombosis to dementia. The intermolecular linkages result from the insertion of the reactive site loop of one serpin molecule into the middle strand (s4A) position of the A beta-sheet of another molecule. We define here the structural requirements for small peptides to competitively bind to and block the s4A position to prevent this intermolecular linkage and polymerisation. The entry and anchoring of blocking-peptides is facilitated by the presence of a threonine which inserts into the site equivalent to P8 of s4A. But the critical requirement for small blocking-peptides is demonstrated in crystallographic structures of the complexes formed with selected tri- and tetrapeptides. These structures indicate that the binding is primarily due to the insertion of peptide hydrophobic side-chains into the P4 and P6 sites of s4A. The findings allow the rational design of synthetic blocking-peptides small enough to be suitable for mimetic design. This is demonstrated here with a tetrapeptide that preferentially blocks the polymerisation of a pathologically unstable serpin commonly present in people of European descent.

Alzheimer Disease↗

PubMed2003

BmSPN2, a serpin secreted by the filarial nematode Brugia malayi, does not inhibit human neutrophil proteinases but plays a noninhibitory role.

The filarial nematode, Brugia malayi, is a causative agent of lymphatic filariasis. Bm-spn-2, one of two serpin genes identified in B. malayi, is expressed only in humans where the encoded protein, BmSPN2, is secreted by blood-dwelling microfilariae. Previous work reported that BmSPN2 could inhibit the activities of elastase and cathepsin G from human neutrophils, despite an atypical amino acid sequence. This did not fit with accepted theories as to the sequence requirements of serpins for proteinase inhibition. We have cloned and expressed Bm-spn-2 in Escherichia coli and characterized the structural and functional properties of recombinant BmSPN2. Sequence alignment, circular dichroism spectroscopy, and susceptibility to cleavage by proteinases all suggest that BmSPN2 shares the tertiary structure typical of the serpin family including an accessible reactive center loop. However, we have found that BmSPN2 has no effect on the activity of neutrophil elastase or cathepsin G and does not form SDS-stable complexes with these proteinases. We provide evidence that BmSPN2 cannot undergo the characteristic stressed to relaxed transition required for proteinase inhibition by serpins. We conclude that BmSPN2 is not an atypical inhibitor but is a new noninhibitory serpin, in keeping with its sequence.

Amino Acid Sequence↗

PubMed2003

Serpin polymerization is prevented by a hydrogen bond network that is centered on his-334 and stabilized by glycerol.

Polymerization of serpins commonly results from mutations in the shutter region underlying the bifurcation of strands 3 and 5 of the A-sheet, with entry beyond this point being barred by a H-bond network centered on His-334. Exposure of this histidine in antithrombin, which has a partially opened sheet, allows polymerization and peptide insertion to occur at pH 6 or less when His-334 will be predictably protonated with disruption of the H-bond network. Similarly, thermal stability of antithrombin is pH-dependent with a single unfolding transition at pH 6, but there is no such transition when His-334 is buried by a fully closed A-sheet in heparin-complexed antithrombin or in alpha(1)-antitrypsin. Replacement of His-334 in alpha(1)-antitrypsin by a serine or alanine at pH 7.4 results in the same polymerization and loop-peptide acceptance observed with antithrombin at low pH. The critical role of His-334 and the re-formation of its H-bond network by the conserved P8 threonine, on the full insertion of strand 4, are relevant for the design of therapeutic blocking agents. This is highlighted here by the crystallographic demonstration that glycerol, which at high concentrations blocks polymerization, can replace the P8 threonine and re-form the disrupted H-bond network with His-334.

Antithrombins↗