Lysine and Arginine Side Chains in Glycosaminoglycan−Protein Complexes Investigated by NMR, Cross-Linking, and Mass Spectrometry: A Case Study of the Factor H−Heparin Interaction

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• 作者：Brbel S. Blaum ; Jon A. Deakin ; Conny M. Johansson ; Andrew P. Herbert ; Paul N. Barlow ; Malcolm Lyon ; Duan Uhrn
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：May 12, 2010
• 年：2010
• 卷：132
• 期：18
• 页码：6374-6381
• 全文大小：316K
• 年卷期：v.132,no.18(May 12, 2010)
• ISSN：1520-5126

文摘

We have used the interaction between module 7 of complement factor H (CFH7) and a fully sulfated heparin tetrasaccharide to exemplify a new approach for studying contributions of basic side chains to the formation of glycosaminoglycan (GAG)−protein complexes. We first employed HISQC and H₂CN NMR experiments to monitor the side-chain resonances of lysines and arginines in ¹⁵N, ¹³C-labeled protein during titrations with a fully sulfated heparin tetrasaccharide under physiological conditions. Under identical conditions and using ¹⁵N-labeled protein, we then cross-linked tetrasaccharide to CFH7 and confirmed the 1:1 stoichiometry by FT-ICR-MS. We subsequently characterized this covalent protein−GAG conjugate by NMR and further MS techniques. MALDI-TOF MS identified protein fragments obtained via trypsin digestion or chemical fragmentation, yielding information concerning the site of GAG attachment. Combining MS and NMR data allowed us to identify the side chain of K405 as the point of attachment of the cross-linked heparin oligosaccharide to CFH7. On the basis of the analysis of NMR and MS data of the noncovalent and cross-linked CFH7−tetrasaccharide complexes, we conclude that the K446 side chain is not essential for binding the tetrasaccharide, despite the large chemical shift perturbations of its backbone amide ¹⁵N and ¹H resonances during titrations. We show that R444 provides the most important charge−charge interaction within a C-terminal heparin-binding subsite of CFH7 whereas side chains of R404, K405, and K388 are the predominant contributors to an N-terminal binding subsite located in the immediate vicinity of residue 402, which is implicated in age-related macular degeneration (AMD).