Douglas T, Ripoll D R
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
Protein Sci. 1998 May;7(5):1083-91. doi: 10.1002/pro.5560070502.
Calculations to determine the electrostatic potential of the iron storage protein ferritin, using the human H-chain homopolymer (HuHF), reveal novel aspects of the protein. Some of the charge density correlates well with regions previously identified as active sites in the protein. The three-fold channels, the putative ferroxidase sites, and the nucleation sites all show expectedly negative values of the electrostatic potential. However, the outer entrance to the three-fold channels are surrounded by regions of positive potential, creating an electrostatic field directed toward the interior cavity. This electrostatic gradient provides a guidance mechanism for cations entering the protein cavity, indicating the three-fold channel as the major entrance to the protein. Pathways from the three-fold channels, indicated by electrostatic gradients on the inner surface, lead to the ferroxidase center, the nucleation center and to the interior entrance to the four-fold channel. Six glutamic acid residues at the nucleation site give rise to a region of very negative potential, surrounding a small positively charged center due to the presence of two conserved arginine residues, R63, in close proximity (4.9 A), suggesting that electrostatic fields could also play a role in the nucleation process. A large gradient in the electrostatic potential at the 4-fold channel gives rise to a field directed outward from the internal cavity, indicating the possibility that this channel functions to expel cations from inside the protein. The 4-fold channel could therefore provide an exit pathway for protons during mineralization, or iron leaving the protein cavity during de-mineralization.
利用人H链同聚物（HuHF）来确定铁储存蛋白铁蛋白静电势的计算，揭示了该蛋白的新特性。一些电荷密度与该蛋白中先前确定为活性位点的区域有很好的相关性。三重通道、假定的铁氧化酶位点和成核位点的静电势均呈现预期的负值。然而，三重通道的外部入口被正电势区域包围，形成了一个指向内部腔室的静电场。这种静电梯度为阳离子进入蛋白腔室提供了一种引导机制，表明三重通道是蛋白的主要入口。内表面的静电梯度所指示的来自三重通道的路径，通向铁氧化酶中心、成核中心以及四重通道的内部入口。成核位点的六个谷氨酸残基产生了一个非常负的电势区域，由于两个保守的精氨酸残基R63紧密相邻（4.9埃），围绕着一个带小正电荷的中心，这表明静电场在成核过程中也可能起作用。四重通道处静电势的大幅梯度产生了一个从内部腔室向外的场，这表明该通道可能起到从蛋白内部排出阳离子的作用。因此，四重通道可以在矿化过程中为质子提供一条出口路径，或者在脱矿化过程中为离开蛋白腔室的铁提供路径。
