Evolving the [Myoglobin, Cytochrome b5] Complex from Dynamic toward Simple Docking: Charging the Electron Transfer Reactive Patch

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• 作者：Ethan N. Trana ; Judith M. Nocek ; Amanda K. Knutson ; Brian M. Hoffman
• 刊名：Biochemistry
• 出版年：2012
• 出版时间：October 30, 2012
• 年：2012
• 卷：51
• 期：43
• 页码：8542-8553
• 全文大小：448K
• 年卷期：v.51,no.43(October 30, 2012)
• ISSN：1520-4995

文摘

We describe photoinitiated electron transfer (ET) from a suite of Zn-substituted myoglobin (Mb) variants to cytochrome b₅ (b₅). An electrostatic interface redesign strategy has led to the introduction of positive charges into the vicinity of the heme edge through D/E 鈫?K charge-reversal mutation combinations at 鈥渉ot spot鈥?residues (D44, D60, and E85), augmented by the elimination of negative charges from Mb or b₅ by neutralization of heme propionates. These variations create an unprecedentedly large range in the product of the ET partners鈥?total charges (鈭? < 鈭?i>q_Mbq_b5 < 40). The binding affinity (K_a) increases 1000-fold as 鈭?i>q_Mbq_b5 increases through this range and exhibits a surprisingly simple, exponential dependence on 鈭?i>q_Mbq_b5. This is explained in terms of electrostatic interactions between a 鈥渃harged reactive patch鈥?(crp) on each partner鈥檚 surface, defined as a compact region around the heme edge that (i) contains the total protein charge of each variant and (ii) encompasses a major fraction of the 鈥渞eactive region鈥?(Rr) comprising surface atoms with large matrix elements for electron tunneling to the heme. As 鈭?i>q_Mbq_b5 increases, the complex undergoes a transition from fast to slow-exchange dynamics on the triplet ET time scale, with a correlated progression in the rate constants for intracomplex (k_et) and bimolecular (k₂) ET. This progression is analyzed by integrating the crp and Rr descriptions of ET into the textbook steady-state treatment of reversible binding between partners that undergo intracomplex ET and found to encompass the full range of behaviors predicted by the model. The generality of this approach is demonstrated by its application to the extensive body of data for the ET complex between the photosynthetic reaction center and cytochrome c₂. Deviations from this model also are discussed.