Molten globules unfolded

Figure 6.2 The molten globule state is an important intermediate in the folding pathway when a polypeptide chain converts from an unfolded to a folded state. The molten globule has most of the secondary structure of the native state but it is less compact and the proper packing interactions in the interior of the protein have not been formed.

The collapse of the unfolded state to generate the molten globule embodies the main mystery of protein folding. What is the driving force behind the choice of native tertiary fold from a randomly oriented polypeptide chain ... 

Raschke, T. M., and Marqnsee, S., 1997. The kinetic folding intermediate of ribonnclease H resembles the acid molten globule and partially unfolded molecules detected under native conditions. Nature Structural Biology 4 298-304. 

The conformational plasticity supported by mobile regions within native proteins, partially denatured protein states such as molten globules, and natively unfolded proteins underlies many of the conformational (protein misfolding) diseases (Carrell and Lomas, 1997 Dobson et al., 2001). Many of these diseases involve amyloid fibril formation, as in amyloidosis from mutant human lysozymes, neurodegenerative diseases such as Parkinson s and Alzheimer s due to the hbrillogenic propensities of a -synuclein and tau, and the prion encephalopathies such as scrapie, BSE, and new variant Creutzfeldt-Jacob disease (CJD) where amyloid fibril formation is triggered by exposure to the amyloid form of the prion protein. In addition, aggregation of serine protease inhibitors such as a j-antitrypsin is responsible for diseases such as emphysema and cirrhosis. 

This chapter has reviewed the application of ROA to studies of unfolded proteins, an area of much current interest central to fundamental protein science and also to practical problems in areas as diverse as medicine and food science. Because the many discrete structure-sensitive bands present in protein ROA spectra, the technique provides a fresh perspective on the structure and behavior of unfolded proteins, and of unfolded sequences in proteins such as A-gliadin and prions which contain distinct structured and unstructured domains. It also provides new insight into the complexity of order in molten globule and reduced protein states, and of the more mobile sequences in fully folded proteins such as /1-lactoglobulin. With the promise of commercial ROA instruments becoming available in the near future, ROA should find many applications in protein science. Since many gene sequences code for natively unfolded proteins in addition to those coding for proteins with well-defined tertiary folds, both of which are equally accessible to ROA studies, ROA should find wide application in structural proteomics. 

Bovine a -lactalbumin (BLA) is a protein whose structure appears to be unusually malleable and, as such, has been the focus of many studies of what is termed the molten globule transition. At low pH, BLA expands and is said to lose tertiary structure, but it maintains substantial secondary structure in a partial unfolding transition (molten globule... 

We will limit ourselves to reviewing recent SAXS and SANS studies of putatively fully unfolded states formed at equilibrium. We direct readers interested in partially folded states (kinetic and equilibrium molten globules and their brethren) to a number of excellent recent articles and reviews (Kataoka and Goto, 1996 Kataoka et al., 1997 Uversky etal., 1998 Pollack et al., 1999 Doniach, 2001). Similarly, we will not discuss in detail the technical aspects of scattering studies or the precise interpretation of scattering profiles, but instead direct the reader to the appropriate resources (Glatter and Kratky, 1982 Doniach et al., 1995 Kataoka and Goto, 1996 Doniach, 2001). 

Kobashigawa Y, Sakurai M, Nitta K. Effect of hydrostatic pressure on unfolding of alpha-lactalbumin volumetric equivalence of the molten globule and unfolded state. Protein Sci. 1999 8 2765-2772. 

Dobson, C. M. (1992). Unfolded proteins, compact states and molten globules Current opinion in structural biology. Curr. Opin. Struct. Biol. 2, 6-12. 

Significant improvements in the appearance of the NMR spectrum of molten globule states are, however, observed upon addition of small amounts of trifluoroethanol (TFE), less than 10 vol%. Amide proton exchange rates are reduced and resonances are sharpened although the chemical shift dispersion is also decreased [51]. The net result is an increased resolution that simphfies considerably the spectral assignment. The increased spectral resolution is in fact due to the fact that the peptides become more denatured rather than more structured and that, in order to obtain more structural information about the folded state, the peptide is partially unfolded ... 

A puzzling problem was posed by Levinthal many years ago.329 We usually assume that the peptide chain folds into one of the most stable conformations possible. However, proteins fold very rapidly. Even today, no computer would be able, in our lifetime, to find by systematic examination the thermodynamically most stable conformation.328 It would likewise be impossible for a folding protein to "try out" more than a tiny fraction of all possible conformations. Yet folded and unfolded proteins often appear to be in a thermodynamic equilibrium Experimental results indicate that denatured proteins are frequently in equilibrium with a compact denatured state or "molten globule" in which hydrophobic groups have become clustered and some secondary structures exists.330-336 From this state the polypeptide may rearrange more slowly through other folding intermediates to the final "native conformation."3363 3361 ... 

Uversky, V.N., S. Winter, and G. Lober. 1996. Use of fluorescence decay times of 8-ANS-protein complexes to study the conformational transitions in proteins which unfold through the molten globule state. Biophys. Chem. 60(3) 79-88. 

Even though it was reported earlier (Dolgikh et al., 1985) that the molten globule state had a heat capacity similar to that of the unfolded state, implying that the molten globule was fully exposed to water, this view appears to be at odds with thermodynamic considerations and new experimental data. Also, if the molten globule is fully hydrated and its tertiary structure interactions are largely disrupted, why does it assume a compact conformation What are the forces... 

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