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Limited proteolysis of bovine α-lactalbumin: Isolation and characterization of protein domains
- PATRIZIA POLVERINO DE LAURETO, ELENA SCARAMELLA, MARTA FRIGO, FRANCESCA GEFTER WONDRICH, VINCENZO DE FILIPPIS, MARCELLO ZAMBONIN, ANGELO FONTANA
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- Journal:
- Protein Science / Volume 8 / Issue 11 / November 1999
- Published online by Cambridge University Press:
- 01 November 1999, pp. 2290-2303
- Print publication:
- November 1999
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The partly folded states of α-lactalbumin (α-LA) exposed to acid solution at pH 2.0 (A-state) or at neutral pH upon EDTA-mediated removal of the single protein-bound calcium ion (apo form) have been probed by limited proteolysis experiments. These states are nowadays commonly considered to be molten globules and thus protein-folding intermediates. Pepsin was used for proteolysis at acid pH, while proteinase K and chymotrypsin at neutral pH. The expectations were that these proteolytic probes would detect sites and/or chain regions in the partly folded states of α-LA sufficiently dynamic, or even unfolded, capable of binding and adaptation to the specific stereochemistry of the protease's active site. A time-course analysis of the proteolytic events revealed that the fast, initial proteolytic cuts of the 123-residue chain of α-LA in its A-state or apo form by the three proteases occur at the same chain region 39–54, the actual site(s) of cleavage depending upon the protease employed. This region in native α-LA encompasses the β-sheets of the protein. Subsequent cleavages occur mostly at chain regions 31–35 and 95–105. Four fragment species of α-LA have been isolated by reverse-phase high-performance liquid chromatography, and their conformational properties examined by circular dichroism and fluorescence emission spectroscopy. The single chain fragment 53–103, containing all the binding sites for calcium in native α-LA and cross-linked by two disulfide bridges, maintains in aqueous buffer and in the presence of calcium ions a folded structure characterized by the same content of α-helix of the corresponding chain segment in native α-LA. Evidence for some structure was also obtained for the two-chain species 1–40 and 104–123, as well as 1–31 and 105–123, both systems being covalently linked by two disulfide bonds. In contrast, the protein species given by fragment 1–34 connected to fragment 54–123 or 57–123 via four disulfide bridges adopts in solution a folded structure with the helical content expected for a native-like conformation. Of interest, the proteolytic fragment species herewith isolated correspond to the structural domains and subdomains of α-LA that can be identified by computational analysis of the three-dimensional structure of native α-LA (Siddiqui AS, Barton GI, 1995, Protein Sci 4:872–884). The fast, initial cleavages at the level of the β-sheet region of native α-LA indicate that this region is highly mobile or even unfolded in the α-LA molten globule(s), while the rest of the protein chain maintains sufficient structure and rigidity to prevent extensive proteolysis. The subsequent cleavages at chain segment 95–105 indicate that also this region is somewhat mobile in the A-state or apo form of the protein. It is concluded that the overall domain topology of native α-LA is maintained in acid or at neutral pH upon calcium depletion. Moreover, the molecular properties of the partly folded states of α-LA deduced here from proteolysis experiments do correlate with those derived from previous NMR and other physicochemical measurements.
Incorporation of noncoded amino acids into the N-terminal domain 1–47 of hirudin yields a highly potent and selective thrombin inhibitor
- VINCENZO DE FILIPPIS, ILARIA RUSSO, ALESSANDRO VINDIGNI, ENRICO DI CERA, STEFANO SALMASO, ANGELO FONTANA
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- Journal:
- Protein Science / Volume 8 / Issue 10 / October 1999
- Published online by Cambridge University Press:
- 01 October 1999, pp. 2213-2217
- Print publication:
- October 1999
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Hirudin is an anticoagulant polypeptide isolated from a medicinal leech that inhibits thrombin with extraordinary potency (Kd = 0.2–1.0 pM) and selectivity. Hirudin is composed of a compact N-terminal region (residues 1–47, cross-linked by three disulfide bridges) that binds to the active site of thrombin, and a flexible C-terminal tail (residues 48–64) that interacts with the exosite I of the enzyme. To minimize the sequence of hirudin able to bind thrombin and also to improve its therapeutic profile, several N-terminal fragments have been prepared as potential anticoagulants. However, the practical use of these fragments has been impaired by their relatively poor affinity for the enzyme, as given by the increased value of the dissociation constant (Kd) of the corresponding thrombin complexes (Kd = 30–400 nM). The aim of the present study is to obtain a derivative of the N-terminal domain 1–47 of hirudin displaying enhanced inhibitory potency for thrombin compared to the natural product. In this view, we have synthesized an analogue of fragment 1–47 of hirudin HM2 in which Val1 has been replaced by tert-butylglycine, Ser2 by Arg, and Tyr3 by β-naphthylalanine, to give the BugArgNal analogue. The results of chemical and conformational characterization indicate that the synthetic peptide is able to fold efficiently with the correct disulfide topology (Cys6–Cys14, Cys16–Cys28, Cys22–Cys37), while retaining the conformational properties of the natural fragment. Thrombin inhibition data indicate that the effects of amino acid replacements are perfectly additive if compared to the singly substituted analogues (De Filippis V, Quarzago D, Vindigni A, Di Cera E, Fontana A, 1998, Biochemistry 37:13507–13515), yielding a molecule that inhibits the fast or slow form of thrombin by 2,670- and 6,818-fold more effectively than the natural fragment, and that binds exclusively at the active site of the enzyme with an affinity (Kd,fast = 15.4 pM, Kd,slow = 220 pM) comparable to that of full-length hirudin (Kd,fast = 0.2 pM, Kd,slow = 5.5 pM). Moreover, BugArgNal displays absolute selectivity for thrombin over the other physiologically important serine proteases trypsin, plasmin, factor Xa, and tissue plasminogen activator, up to the highest concentration of inhibitor tested (10 μM).