Native globule

Fig. 4 a,b The sequence distribution for rather large sections of a synthetic random copolymer is practically identical to that of the whole polymer. c,d If a protein sequence is cut into two pieces, those pieces neither correspond to a native globule nor have any biological function... 

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. 

The principal defining properties of the molten globule are as follows (Arai and Kuwajima, 2000) (1) substantial secondary structure (2) no significant tertiary structure (3) structure only slightly expanded from the native state (10—30% increase in radius of gyration) (4) a loosely packed hydrophobic core with increased solvent accessibility. The first two criteria are readily assessed by far- and near-UV CD, respectively. Therefore, CD has been extensively applied to the detection and characterization of molten globules. 

Fig. 37. Difference CD spectrum (native-molten globule) in bovine a-lactalbumin ( ). Derivedfrom data of Kuwajima et al (1985) for native (—) and molten globule ( ) forms. 

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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