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Publication Abstract from PubMed

Lamprey angiotensinogen (l-ANT) is a hormone carrier in the regulation of blood pressure, but it is also a heparin-dependent thrombin inhibitor in lamprey blood coagulation system. The detailed mechanisms on how angiotensin is carried by l-ANT and how heparin binds l-ANT and mediates thrombin inhibition are unclear. Here we have solved the crystal structure of cleaved l-ANT at 2.7 angstrom resolution and characterized its properties in heparin binding and protease inhibition. The structure reveals that l-ANT has a conserved serpin fold with a labile N-terminal angiotensin peptide and undergoes a typical stressed-to-relaxed conformational change when the reactive center loop is cleaved. Heparin binds l-ANT tightly with a dissociation constant of ~10 nM involving ~8 monosaccharides and ~6 ionic interactions. The heparin binding site is located in an extensive positively charged surface area around helix D involving residues Lys148, Lys 151, Arg155 and Arg380. Although l-ANT by itself is a poor thrombin inhibitor with a second order rate constant of 500 M-1s-1, its interaction with thrombin is accelerated 90-fold by high-molecular-weight heparin following a bell-shaped dose dependent curve. Short heparin chains of 6-20 monosaccharide units are insufficient to promote thrombin inhibition. Furthermore, an l-ANT mutant with the P1 Ile mutated to Arg inhibits thrombin nearly 1500-fold faster than the wild type which is further accelerated by high-molecular-weight heparin. Taken together, these results suggest that heparin binds l-ANT at a conserved heparin binding site around helix D and promotes the interaction between l-ANT and thrombin through a conserved template mechanism of vertebrates.

Heparin binds lamprey angiotensinogen and promotes thrombin inhibition through a template mechanism.,Wei H, Cai H, Wu J, Wei Z, Zhang F, Huang X, Ma L, Feng L, Zhang R, Wang Y, Ragg H, Zheng Y, Zhou A J Biol Chem. 2016 Sep 28. pii: jbc.M116.725895. PMID:27681598^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.