Role of Phenylalanine B10 in Plant Nonsymbiotic Hemoglobins

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• 作者：Benoit J. Smagghe ; Suman Kundu ; Julie A. Hoy ; Puspita Halder ; Theodore R. Weiland ; Andrea Savage ; Anand Venugopal ; Matthew Goodman ; Scott Premer ; Mark S. Hargrove
• 刊名：Biochemistry
• 出版年：2006
• 出版时间：August 15, 2006
• 年：2006
• 卷：45
• 期：32
• 页码：9735 - 9745
• 全文大小：423K
• 年卷期：v.45,no.32(August 15, 2006)
• ISSN：1520-4995

文摘

All plants contain an unusual class of hemoglobins that display bis-histidyl coordination yetare able to bind exogenous ligands such as oxygen. Structurally homologous hexacoordinate hemoglobins(hxHbs) are also found in animals (neuroglobin and cytoglobin) and some cyanobacteria, where they arethought to play a role in free radical scavenging or ligand sensing. The plant hxHbs can be distinguishedfrom the others because they are only weakly hexcacoordinate in the ferrous state, yet no structuralmechanism for regulating hexacoordination has been articulated to account for this behavior. Plant hxHbscontain a conserved Phe at position B10 (Phe^B10), which is near the reversibly coordinated distal His^E7.We have investigated the effects of Phe^B10 mutation on kinetic and equilibrium constants forhexacoordination and exogenous ligand binding in the ferrous and ferric oxidation states. Kinetic andequilibrium constants for hexacoordination and ligand binding along with CO-FTIR spectroscopy, midpointreduction potentials, and the crystal structures of two key mutant proteins (F40W and F40L) reveal thatPhe^B10 is an important regulatory element in hexacoordination. We show that Phe at this position is theonly amino acid that facilitates stable oxygen binding to the ferrous Hb and the only one that promotesligand binding in the ferric oxidation states. This work presents a structural mechanism for regulatingreversible intramolecular coordination in plant hxHbs.