Folding or unfolding of proteins on hydrophobic supports

The mainly ^-sheeted serine proteinase such as a-chymotrypsin has a high structural stability like lysozyme, but undergoes a large unfolding of its ^-sheets upon adsorption on hydrophobic support (Baron et al. 1999; Zoungrana et al. 1997). The released peptide carbonyls form mainly intermolecular hydrogen bonds with neighbouring peptide segments, increasing the self-associated domains in the adsorbed layer of a-chymotrypsin (Noinville et al. 2002). Adsorption on hydrophobic surfaces may result in an increased or decreased order in protein structure, whether the balance between the energetically favourable interaction and the conformational entropy is positive or not. For instance another serine enzyme such as Humicola lanuginosa lipase, with a higher overall hydrophobicity than the above proteinase, adsorbs onto the hydrophobic support with a gain in its stereoregular a/p fold (Noinville et al. 2002). Some antimicrobial peptides known to be randomly coiled in solution adsorb onto hydrophobic supports by forming amphipathic helices with the hydrophobic external side of the helix oriented towards the support and the polar side to the interfacial solution (Noinville et al. 2003). Surface-induced conformational changes of the amyloid p-peptide have been studied using CD spectroscopy on Teflon particles (Giacomelli and Norde 2003). The authors show that the hydrophobic support promotes a-helix formation at low surface coverage, but leads to a more enriched p-sheeted structure at high surface coverage, probably initiating self-aggregation. Even if the extent and the pathways of structural conversion in the case of very flexible polypeptides such as amyloid peptides or prions are still complex to tackle, the effect of the contact with a solid phase is to stabilise adsorbed conformers, which could inhibit protein aggregation by the complex interplay of electrostatic and hydrophobic interactions (Revault et al. 2005).

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