A Comparison of Functional and Structural Consequences of the Tyrosine B10 and Glutamine E7 Motifs in Two Invertebrate Hemoglobins (Ascaris suum and Lucina pectinata)

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• 作者：Eric S. Peterson ; Shuocai Huang ; Jiaqian Wang ; Lisa M. Miller ; Gediminas Vidugiris ; Andrew P. Kloek ; Daniel E. Goldberg ; Mark R. Chance ; Jonathan B. Wittenberg ; and Joel M. Friedman
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：October 21, 1997
• 年：1997
• 卷：36
• 期：42
• 页码：13110 - 13121
• 全文大小：192K
• 年卷期：v.36,no.42(October 21, 1997)
• ISSN：1520-4995

文摘

The architecture of the distal heme pocket inhemoglobins and myoglobins can play an importantrole in controlling ligand binding dynamics. The size and polarityof the residues occupying the distalpocket may contribute steric and dielectric effects. In vertebratesystems, the distal pocket typically containsa "distal" histidine at position E7 and a leucine at position B10.There are several invertebrate organismsthat have hemoglobins or myoglobins that display a pattern in whichresidues E7 and B10 are a glutamineand tyrosine, respectively. These proteins often have very highoxygen affinities stemming from veryslow ligand off rates. In this study, two such hemoglobins, onefrom the nematode Ascaris suum and theother from the sulfide-fixing clam Lucina pectinata, arecompared with respect to conformational andfunctional properties. Ultraviolet resonance Raman spectroscopyand visible resonance Raman spectroscopyare used to probe, respectively, the ligand-dependent hydrogen bondingpattern of the tyrosine residuesand the proximal heme pocket interactions. Fourier transforminfrared absorption spectroscopy is usedto probe the dielectric properties of the distal heme pocket throughthe stretching frequency of carbonmonoxide bound to the heme. Functionality is probed through thegeminate rebinding of both CO andO₂. The findings reveal two very different patternsindicative of two different mechanisms for achievinglow oxygen off rates. In Hb Ascaris, a hydrogen bonding networkthat includes the E7 Gln, B10 Tyr,and oxygen bound to the heme results in a tight cage for the oxygen.Dissociation of the O₂ requires alarge amplitude conformational fluctuation that results both in aspontaneous dissociation of the oxygenthrough the loss of hydrogen bond stabilization and in an enhancedprobability for ligand escape thoughthe transient disruption and opening of the tight distal cage. Inthe case of the Hb from Lucina, there isno evidence for a tight cage. Instead the data support a model inwhich the hydrogen bonding networkis far more tenuous and the equilibrium state of distal pocket is farmore open and accessible than is thecase in Ascaris. The results explain why Hb Ascaris hasone of the highest oxygen affinities known(P₅₀~ 10^-3 Torr) while Hb Lucina II has anoxygen affinity comparable to that of Mb (P₅₀= 0.13 Torr) eventhough both of these Hbs contain the B10 Tyr and E7 Gln motif anddisplay very low oxygen off rates.The roles of water and proximal strain are discussed.