Factor VIIa: Difference between revisions

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Jolanta Amblo, David Canner, Alexander Berchansky, Michal Harel

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 The charge relay system explains the action of Ser344 as part of resonance forms Asp-CO2-/H-His/Ser-OH and Asp-CO2H/His-H/Ser-O which involve proton transfers between Ser344 to His193 and His193 to Asp242. However this two-proton transfer mechanim requires that the pKa of His344 is lower than pKa of Asp242. The argument for one-proton transfer mechanism, supported by 1H NMR experiments, observed the pKa of free enzyme and acylenzyme His344 to be ~7 whereas that of tetrahedral intermediate to be more than 10. This argues for electrostatic stabilization of protonated His344 by the negatively charged Asp242 or a one-proton transfer mechanims<ref>PMID:12475199</ref>.
-====Low barrier hydrogen bonds====
+====Low barrier hydrogen bonds (LBHB)====
-The hydrogen bond between Asp242 and His344 has been suggested to fulfill many criteria for low barrier hydrogen bond. This criteria includes a <2.6Ǻ distance between Asp242 and His344,an unusually downfield signal of the Nδ1 proton of His344 in 1H NMR, an observed deuterium isotope effect, and a low D/H fractionation factor of the Nδ1 proton<ref>PMID:7661899</ref>.
+The hydrogen bond between Asp242 and His344 has been suggested to fulfill many criteria for low barrier hydrogen bond. This criteria includes a <2.6Ǻ distance between Asp242 and His344,an unusually downfield signal of the Nδ1 proton of His344 in 1H NMR, an observed deuterium isotope effect, and a low D/H fractionation factor of the Nδ1 proton<ref>PMID:7661899</ref>.The arguments in opposition to LBHB include unmatched pKas of His344 and Asp242, localization of the proton mainly on His57 as determined by 15N-H spin couplings and 15N chemical shifts, lack of sheilding of the Nδ1 proton from water and polar environment, and multiple hydrogen interactions of both Asp242 and His344 which would weaken the Oδ1-Nδ1-H hydrogen bond<ref>PMID:9353195</ref>.