Marchany-Rivera Darya, Smith Clyde A, Rodriguez-Perez Josiris D, López-Garriga Juan
Department of Chemistry, P.O. Box 9000, University of Puerto Rico, Mayagüez Campus, 00681, Puerto Rico.
Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
J Inorg Biochem. 2020 Jun;207:111055. doi: 10.1016/j.jinorgbio.2020.111055. Epub 2020 Mar 7.
Lucina pectinata live in high concentrations of hydrogen sulfide (HS) and contains one hemoglobin, Hemoglobin I (HbI), transporting HS and two hemoglobins, Hemoglobin II (HbII) and Hemoglobin (HbIII), transferring dioxygen to symbionts. HbII and HbIII contain B10 tyrosine (Tyr) and E7 glutamine (Gln) in the heme pocket generating an efficient hydrogen bonding network with the (HbII-HbIII)-O species, leading to very low ligand dissociation rates. The results indicate that the oxy-hemeprotein is susceptible to pH from 4 to 9, at acidic conditions, and as a function of the potassium ferricyanide concentration, 100% of the met-aquo derivative is produced. Without a strong oxidant, pH 5 generates a small concentration of the met-aquo complex. The process is accelerated by the presence of salts, as indicated by the crystallization structures and UV-Vis spectra. The results suggest that acidic pH generates conformational changes associated with B10 and E7 heme pocket amino acids, weakening the (HbII-HbIII)-O hydrogen bond network. The observation is supported by X-ray crystallography, since at pH 4 and 5, the heme-Fe tends to oxidize, while at pH 7, the oxy-heterodimer is present. Conformational changes also are observed at higher pH by the presence of a 605 nm transition associated with the iron heme-Tyr interaction. Therefore, pH is one crucial factor regulating the (HbII-HbIII)-O complex hydrogen-bonding network. Thus, it can be proposed that the hydrogen bonding adjustments between the heme bound O and the Tyr and Gln amino acids contribute to oxygen dissociation from the (HbII-HbIII)-O system.
栉孔扇贝生活在高浓度硫化氢（HS）环境中，含有一种血红蛋白，即血红蛋白I（HbI），用于运输HS，还有两种血红蛋白，即血红蛋白II（HbII）和血红蛋白III（HbIII），用于向共生体传递双原子氧。HbII和HbIII在血红素口袋中含有B10酪氨酸（Tyr）和E7谷氨酰胺（Gln），与（HbII-HbIII）-O物种形成高效的氢键网络，导致配体解离速率非常低。结果表明，氧合血红素蛋白在酸性条件下，pH值为4至9时易受影响，并且作为铁氰化钾浓度的函数，会产生100%的高铁水合衍生物。在没有强氧化剂的情况下，pH值为5时会产生少量的高铁水合络合物。如结晶结构和紫外可见光谱所示，盐的存在会加速这一过程。结果表明，酸性pH值会导致与B10和E7血红素口袋氨基酸相关的构象变化，削弱（HbII-HbIII）-O氢键网络。X射线晶体学支持了这一观察结果，因为在pH值为4和5时，血红素铁倾向于氧化，而在pH值为7时，氧合异二聚体存在。在较高pH值下，通过与铁血红素-Tyr相互作用相关的605nm跃迁也观察到了构象变化。因此，pH值是调节（HbII-HbIII）-O络合物氢键网络的一个关键因素。因此，可以提出，血红素结合的O与Tyr和Gln氨基酸之间的氢键调整有助于氧从（HbII-HbIII）-O系统中解离。
