Proteins molten globule

These spectra are similar to that of native RNase A at 95°C (not shown). The far-UV spectrum at 95°C indicates a retention of substantial p-sheet secondary structure, but a significant loss of the a-helix conformation as indicated by the decrease of intensity at 222 nm.48 The near-UV spectrum at 95°C indicates a complete collapse of tertiary structure as is seen in molten globule proteins.49 Trace 3 is the sample from trace 2 after cooling the protein to 23°C. Both spectra reveal little recovery of either secondary or tertiary protein structure. 

Denisov DP, Jonsson BH, Halle B Hydration of denatured and molten globule proteins. Nat. Struct. Biol. 1999, 6 253-260. 

A number of proteins are known to pass through a transient intermediate state, the so-called molten globule state. The precise stmctural features of this state are not known, but appear to be compact, and contain most of the regular stmcture of the folded protein, yet have a large side-chain disorder (9). 

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 ... 

A gene encoding this sequence was synthesized and the corresponding protein, called Janus, was expressed, purified, and characterized. The atomic structure of this protein has not been determined at the time of writing but circular dichroic and NMR spectra show very clear differences from B1 and equally clear similarities to Rop. The protein is a dimer in solution like Rop and thermodynamic data indicate that it is a stably folded protein and not a molten globule fold like several other designed proteins. 

Creighton, T. E., 1997. How important i.s the molten globule for correct protein folding Trends in Biochemical Sciences 22 6-11. 

Proteins that assist folding include protein disulfide isomerase, protine- V,rn2 j,-isomerase, and the chaperones that participate in the folding of over half of mammalian proteins. Chaperones shield newly synthesized polypeptides from solvent and provide an environment for elements of secondary stmcture to emerge and coalesce into molten globules. 

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... 

The near-UV CD of bovine o -lactalbumin is shown in Figure 35b. The strong CD of the native protein contrasts with the weak CD of the molten globule, which is comparable to that of the heat- and Gdm-HCl-denatured protein. The weakness of the aromatic CD bands in the molten globule is attributable to the absence of a well-defined conformation and environment for the aromatic side chains, which leads to averaging of the aromatic CD contributions over many conformations and thus to extensive cancellation. 

Carbonic anhydrase is another protein that forms a compact A-state at low pH (Wong and Hamlin, 1974). In this case, the far-UV CD changes on going from native protein to molten globule are quite spectacular, as illustrated in Figure 38. At neutral pH the protein has a rather weak... 

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