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Total chemical synthesis of human matrix Gla protein

¹ Department of Biochemistry, Cardiovascular Research Institute Maastricht, University Maastricht, The Netherlands
² Department of Biomedical Engineering, Eindhoven University of Technology, The Netherlands

Reprint requests to: Tilman M. Hackeng, Ph.D., Department of Biochemistry, Cardiovascular Research Institute Maastricht, University Maastricht, P.O. Box 616, 6200 MD Maastricht, The Netherlands; e-mail: t.hackeng{at}bioch.unimaas.nl; fax: 31-43-388-4159.

Human matrix Gla protein (MGP) is a vitamin K–dependent extracellular matrix protein that binds Ca²⁺ ions and that is involved in the prevention of vascular calcification. MGP is a 10.6-kD protein (84 amino acids) containing five -carboxyglutamic acid (Gla) residues and one disulfide bond. Studies of the mechanism by which MGP prevents calcification of the arterial media are hampered by the low solubility of the protein (<10 µg/mL). Because of solubility problems, processing of a recombinantly expressed MGP-fusion protein chimera to obtain MGP was unsuccessful. Here we describe the total chemical synthesis of MGP by tBoc solid-phase peptide synthesis (SPPS) and native chemical ligation. Peptide Tyr¹-Ala⁵³ was synthesized on a derivatized resin yielding a C-terminal thioester group. Peptide Cys⁵⁴-Lys⁸⁴ was synthesized on Lys-PAM resin yielding a C-terminal carboxylic acid. Subsequent native chemical ligation of the two peptides resulted in the formation of a native peptide bond between Ala⁵³ and Cys⁵⁴. Folding of the 1–84-polypeptide chain in 3 M guanidine (pH 8) resulted in a decrease of molecular mass from 10,605 to 10,603 (ESI-MS), representing the loss of two protons because of the formation of the Cys⁵⁴-Cys⁶⁰ internal disulfide bond. Like native MGP, synthetic MGP had the same low solubility when brought into aqueous buffer solutions with physiological salt concentrations, confirming its native like structure. However, the solubility of MGP markedly increased in borate buffer at pH 7.4 in the absence of sodium chloride. Ca²⁺-binding to MGP was confirmed by analytical HPLC, on which the retention time of MGP was reduced in the presence of CaCl₂. Circular dichroism studies revealed a sharp increase in -helicity at 0.2 mM CaCl₂ that may explain the Ca²⁺-dependent shift in high-pressure liquid chromatography (HPLC)-retention time of MGP. In conclusion, facile and efficient chemical synthesis in combination with native chemical ligation yielded MGP preparations that can aid in unraveling the mechanism by which MGP prevents vascular calcification.

Keywords: Matrix Gla protein; vascular calcification; solid phase peptide synthesis; native chemical ligation; protein folding; electrospray ionization mass spectrometry

Abbreviations: Boc, tert-butoxycarbonyl • DIEA, N,N,-diisopropylethylamine • DMF, N,N,-Dimethyl-formamide • Dnp, 2,4-Dinitrophenyl • ESI-MS: electrospray ionization mass spectrometry • m/z, mass to charge ratio • HBTU, 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate • SPPS, solid phase peptide synthesis • TAMPAL, trityl-associated mercaptopropionic acid leucine • TFA, trifluoroacetic acid

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