Molten globules secondary structure

UV resonance Raman study of betanova, while consistent with a (3-sheet secondary structure, provides no evidence for a cooperative thermal transition 223 The CD spectra reported by Boyden and Asher 223 showed no indication of (3-sheet features, even at 0 °C, and were indicative of an unordered conformation. It has been suggested 115,223 that the CD in the 220 nm region is dominated by aromatic side-chain contributions, yet the resonance Raman data suggest a molten globule-like structure, which appears inconsistent with substantial CD contributions from aromatic side chains 224 ... 

It has been observed for several proteins that the intermediate structures are formed as the protein unfolds from N state to D state [26]. As the protein unfolds, protein loses tertiary structure and, frequently, secondary structure. In some instances, the secondary structure remains intact while the tertiary structure is lost [12], which is clear from spectral studies that measure loss of secondary and tertiary structural changes. One spectroscopic technique that is sensitive to tertiary structure (e.g., fluorescence) would detect changes, whereas other techniques that are sensitive to secondary structures (e.g., far UV CD) do not show any spectral changes. This molecular property is defined as molten globule or structured intermediate [12]. These intermediates expose hydrophobic domains, and thus promote aggregation or surface adsorption. 

The resulting apo-structure reproduced essentially all of the experimental data presented for the proposed molten globule, solution structure. The molten globular character of the apo-form was shown not to be directly related to loss of secondary structure, as some had incorrectly concluded, but rather to the increase in side-chain mobility (Figure 2). This... 

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.

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 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. 

An example of the use of chemical shifts to delineate residual secondary structure is given in Figure 3 for the molten globule state of apomyo-globin (Eliezer et al., 1998 Eliezer et al., 2000). Combined use of 13C , H , 13C, and 13CO secondary shifts gives a more precise definition of secondary structure boundaries than use of 13C shifts alone (Eliezer et al., 2000). 

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. 

Fig. 5. The effect of protein-protein interactions on Nephila edulis major ampullate circular dichroism spectra in solution. A change in secondary structure with increasing concentration is observed. At low concentration (minimal protein-protein interactions) silk proteins appear partially unfolded in solution. At higher concentration (higher protein-protein interactions) silk proteins refold into a helix-like structure, most likely a molten-like globule (from Dicko et al., 2004c). This final molten structure would facilitate local chain rearrangement while preserving the global structure for protein storage and transport. (Copyright 2004 American Chemical Society.)...

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 ... 

The calcium ion in a-LA plays a structural role in stabilizing the protein. The thermal stability of the calcium-bound form of a-LA increases more than 40 °C compared to that of the apo-form. At low pH (e.g. pH 2), a-LA releases the calcium ion and becomes partially unfolded (molten globule state). This partially unfolded protein loses its tertiary structure but retains its secondary structure. Other metals, such as manganese or magnesium, are able to compete with calcium at the same site with a similar stabilizing effect. However, the binding of zinc, which is proposed to bind at different locations, decreases a-LA stability. ... 

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