Fast compaction of alpha-lactalbumin during folding studied by stopped-flow X-ray scattering

To monitor the fast compaction process during protein folding, we have used a stopped-flow small-angle X-ray scattering technique combined with a two-dimensional charge-coupled device-based X-ray detector that makes it possible to improve the signal-to-noise ratio of data dramatically, and measured the kinetic refolding reaction of alpha-lactalbumin. The results clearly show that the radius of gyration and the overall shape of the kinetic folding intermediate of alpha-lactalbumin are the same as those of the molten globule state observed at equilibrium. Thus, the identity between the kinetic folding intermediate and the equilibrium molten globule state is firmly established. The present results also suggest that the folding intermediate is more hydrated than the native state and that the hydrated water molecules are dehydrated when specific side-chain packing is formed during the change from the molten globule to the native state.     

Effect of dextran on protein stability and conformation attributed to macromolecular crowding

Thermally induced transition curves of hen egg-white lysozyme were measured in the presence of several concentrations of dextran at pH 2.0 by near-UV and far-UV CD. The transition curves were fitted to a two-state model by a non-linear, least-squares method to obtain the transition temperature (T(m)), enthalpy change (deltaH(u)(T(m))), and free energy change (deltaG(u)(T)) of the unfolding transition. An increase in T(m) and almost constant deltaH(u)(T(m)) values were observed in the presence of added dextran at concentrations exceeding ca 100 g l(-1). In addition, dextran-induced conformational changes of fully unfolded protein were investigated by CD spectroscopy. Addition of high concentrations of dextran to solutions of acid-unfolded cytochrome c at pH 2.0 results in a shift of the CD spectrum from that characteristic of the fully unfolded polypeptide to that characteristic of the more compact, salt-induced molten globule state, a result suggesting that the molten globule-like state is stabilized relative to the fully unfolded form in crowded environments. Both observations are in qualitative accord with predictions of a previously proposed model for the effect of intermolecular excluded volume (macromolecular crowding) on protein stability and conformation.     

Models for protein folding and nature's choice of protein as catalyst

The study of protein folding and unfolding pathways lends a fascinating dimension to protein biochemistry. Several models for protein folding have been postulated. Two powerful probes used in protein folding study are far UV-CD monitored stopped flow kinetics and pulse hydrogen exchange in conjunction with NMR. The formation of molten globule, which is an intermediate possessing secondary structure but not a well packed tertiary structure, is now emerging as a common feature on the folding pathway of many proteins. The molten globule is recognized by a class of molecules called chaperones which act as accelerators of protein folding. This article ends by elucidating why proteins are Nature's choice as catalysts.     

Botulinum Neurotoxin Light Chain Refolds at Endosomal pH for its Translocation

Botulinum neurotoxins (BoNTs), the most poisonous member of class A biothreat agent, cause neuroparalysis by blocking neurotransmitter release at the neuromuscular junctions. In its mechanism of action, the catalytic domain (light chain (LC) of BoNT) is transported to the cytosol by the heavy chain (HC) in order to reach its proteolytic substrates. The BoNT HC forms a membrane channel under acidic conditions encountered in endosomes to serve as a passageway for LC to enter into cytosol. We demonstrate here that BoNT/A LC undergoes unique structural changes under the low pH conditions, and adopts a molten globule state, exposing substantial number of hydrophobic groups. The flexibility of the molten globular structure combined with retention of the secondary structure and exposure of specific residues of LC for interaction with the HC, allows its translocation through the narrow endosomal membrane channel.     

One-state downhill versus conventional protein folding

Classical protein folding invokes a cooperative transition between distinct thermodynamic states that are individually populated at equilibrium and separated by an energy barrier. It has been proposed, however, that the small protein, BBL, undergoes one-step downhill folding whereby it folds non-cooperatively to its native state without encountering an appreciable energy barrier. Only a single conformational ensemble is populated under given conditions, and so the denatured state ensemble progressively changes into the native structure. A wide dispersion of thermal denaturation midpoints that was observed for an extrinsically labelled fragment of BBL is proposed to be evidence for its one-state, downhill folding, a phenomenon that is also suggested to be functionally important for BBL and its homologues. We found, however, that thermal denaturation of unlabelled wild-type BBL was highly cooperative, with very similar transition midpoints for the melting of secondary and tertiary interactions, as well as for individual residues when monitored by NMR. Similar results were also observed for two other homologues, E3BD and POB. Further, the extrinsic fluorophores perturbed the unfolding energetics of labelled BBL, and complicated its equilibrium behaviour. One-step downhill folding may well occur for some proteins that do not have distinct folded states but not for BBL and its well-folded homologues.     

Folding of bovine growth hormone is consistent with the molten globule hypothesis

Previous results from equilibrium and kinetic studies of the folding of bovine growth hormone (bGH) have demonstrated that bGH does not follow a simple two-step folding mechanism. These results are summarized and interpreted according to the "molten globule" model. The molten globule state of bGH is characterized as a folding intermediate which is largely alpha-helical, retains a compact hydrodynamic radius, has packing of the aromatic side chains that is similar to the unfolded state, and possesses a solvent-exposed hydrophobic surface along helix 106-127 that readily leads to association.