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COMMENTS Comment on the Misunderstanding of the BSA-SDS Complex Model: Concern about Publications of an Impractical Model Kunio Takeda* and Yoshiko Moriyama Department of Applied Chemistry, Okayama UniVersity of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan ReceiVed: October 17, 2006; In Final Form: NoVember 30, 2006 Recently, we have seen quite a few papers containing an impractical model of the protein-surfactant complex. A representative and typical combination of a protein-surfactant system is bovine serum albumin (BSA) and sodium dodecyl sulfate (SDS). Here, we introduce an example that appeared on p 16608 of ref 1. Although the model was cited from some reference in this paper, the schematic model of BSA polypeptide wrapping around three SDS micelles has been drawn frankly. Similar models have been also described in other papers. However, the authors of these papers completely ignore the fact that the polypeptide of intact BSA cannot behave as flexibly as they assume. First of all, an important fact is that BSA has 17 disulfide bridges, which are rather regularly located in the molecule.2 The point is that, in addition to the general folding nature of protein polypeptide, the flexibility of the BSA molecule is strongly restricted by these 17 disulfide bridges. This protein is composed of a single polypeptide chain with 583 amino acids. It is well-known that BSA resembles human serum albumin (HSA) with its 585 amino acids. Thus, it is easy to anticipate that the structure of the intact BSA is similar to that of HSA. The three-dimensional structure of HSA has been determined crystallographically to be heart-shaped.3 The impracticality of the model we have cited results from a misunderstanding of the necklace model proposed by Shirahama et al.4 An important point of the necklace model (the term “necklace bead model” is also used), which has been proposed for the BSA polypeptide, is that all of its disulfide bridges are reduced and blocked (carboxymethylated or carboxyamidomethylated). This model was tentatively proposed against the prolate ellipsoid model consisting of a polypeptide chain surrounded by the SDS shell.5 In the original necklace model,4 it is emphasized that micelle-like clusters are scattered along the polypeptide chain in the case of the disulfide-bridge-reduced and -blocked BSA. This suggests that micelle-like aggregates * Corresponding author. E-mail:
[email protected]. Phone and fax: 086-256-9553
are locally formed at some hydrophobic moieties of such a polypeptide. It is still doubtful that the long polypeptide of the reduced BSA behaves just like a string. Recent misunderstanding of the protein-surfactant complex model is produced by ignoring the presence of disulfide bridges in proteins. BSA is especially rich in the disulfide bridges.2 On the other hand, early works of the BSA-SDS interaction pointed out that BSA has about 10 stoichiometric binding sites for anionic and cationic surfactants and cooperatively binds rather many surfactants by the hydrophobic interaction. It should be noted that the binding of an ionic surfactant such as SDS occurs before the free concentration of surfactant reaches the critical micelle concentration. Therefore, an ordinal watersoluble protein, which can bind many surfactant ions in the monomer state, does not interact with micelles. Many investigators have studied the similarity, for example, between the BSASDS complex and the SDS micelle. The BSA-SDS complex behaves just like the SDS micelle. Such a complex cannot interact with the identically charged micelle. In conclusion, the polypeptides of the intact BSA and many other proteins do not wrap around the SDS micelles. Needless to say, such polypeptides cannot form a helical structure around the micelle. The BSA polypeptide conserves many secondary structures in the presence of SDS. In the SDS solution, the helicity of the intact BSA decreases down to 50%, while that of the disulfide-bridge-reduced BSA, which decreases through the reduction, increases up to 50%.6-8 Most of the papers containing the above-mentioned impractical model of the protein-surfactant complex have cited many references including the old ones. However, their contents or concepts have not been examined sufficiently. We are concerned about the spread of the misunderstood necklace model through the repeated uncritical citations. This misunderstanding must misrepresent the essential nature of protein-surfactant interaction and must cause other misunderstandings. References and Notes (1) Chakraborty, A.; Seth, D.; Setua, P.; Sarkar, N. J. Phys. Chem. B 2006, 110, 16607. (2) Brown, J. R. In Albumin Structure, Function, and Uses; Rosenoer, V. M., Oratz, M., Rothschild, M. A., Eds.; Pergamon Press: Oxford, U.K., 1977; pp 27-51. (3) He, X.-M.; Carter, D. C. Nature 1992, 358, 209. (4) Shirahama, K.; Tsujii, K.; Takagi, T. J. Biochem. 1974, 75, 309. (5) Reynolds, J. A.; Tanford, C. J. Biol. Chem. 1970, 245, 5161. (6) Takeda, K.; Shigeta, M.; Aoki, K. J. Colloid Interface Sci. 1987, 117, 120. (7) Takeda, K.; Sasa, K.; Kawamoto, K.; Wada, A.; Aoki, K. J. Colloid Interface Sci. 1988, 124, 284. (8) Takeda, K.; Moriyama, Y.; Hachiya, K. In Encyclopedia of Surfactant and Colloid Science; Hubbard, A. T., Ed.; Marcel Dekker: New York, 2002; pp 2558-2574.
10.1021/jp066807g CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
