Myoglobin is an ␣-helical globular protein that contains two highly conserved tryptophan residues located at positions 7 and 14 in the N-terminal region of the protein. Replacement of both indole residues with phenylalanine residues, i.e. W7F/W14F, results in the expression of an unstable, not correctly folded protein that does not bind the prosthetic group. Here we report data (Congo red and thioflavine T binding assay, birefringence, and electron microscopy) showing that the double Trp/Phe replacements render apomyoglobin molecules highly susceptible to aggregation and amyloid-like fibril formation under physiological conditions in which most of the wild-type protein is in the native state. In refolding experiments, like the wild-type protein, the W7F/W14F apomyoglobin mutant formed a soluble, partially folded helical state between pH 2.0 and pH 4.0. A pH increase from 4.0 to 7.0 restored the native structure only in the case of the wild-type protein and determined aggregation of W7F/W14F. The circular dichroism spectrum recorded immediately after neutralization showed that the polypeptide consists mainly of -structures. In conclusion, under physiological pH conditions, some mutations that affect folding may cause protein aggregation and the formation of amyloid-like fibrils.Such chronic disorders as Alzheimer's disease, senile systemic amyloidosis, transmissible spongiform encephalopathies, and dialysis-related amyloidosis are characterized by the extracellular deposition of insoluble protein aggregates known as amyloid fibrils (1-6). About 20 proteins are now known to be involved in the generation of amyloid in vivo. Fibril formation is initiated in vitro under conditions that stabilize partially unfolded soluble intermediates of the native proteins either after the partial destabilization of physiologically folded proteins in the case of globular proteins (7) or after the partial stabilization (i.e. folding) of random coil polypeptide chains in the case of natively unfolded proteins (8). Despite substantial differences in both sequence and length (from 40 to 250 residues), all the proteins responsible for amyloid deposition form fibrils composed of -strands oriented perpendicularly to the long axis of the fibril (9). Electron microscopy shows that the fibrils are straight and unbranched and are 40 -120 Å in diameter (9, 10). Also proteins not known to be associated with amyloid disease may form amyloid fibrils under in vitro conditions that favor partially folded states (11-15). These states are more prone to aggregation than the native state because hydrophobic residues, which are largely buried within the core of the native protein, become more exposed upon partial unfolding. The way in which proteins aggregate in the test tube is remarkably similar to how proteins form the so-called "amyloid" deposits. Even myoglobin, an ordinary all-␣ globular protein, can form fibrils containing -strands under experimental conditions that favor the formation of partially folded states (15). Thus, amyloid formation d...
The conformational properties of partially folded states of apomyoglobin have been investigated using an integrated approach based on fluorescence spectroscopy and hydrogen/deuterium exchange followed by mass spectrometry. The examined states were those obtained: (i) by adding 4% v/v hexafluoroisopropanol to native myoglobin, HFIP-MG(N); (ii) by adding 4% v/v hexafluoroisopropanol to acid unfolded myoglobin, HFIP-MG(U); (iii) at pH 3.8, I-1 state; and (iv) at pH 2.0-0.2 M NaCl, A state. Proteolytic digestion of the hydrogen/deuterium exchanged proteins showed that, in I-1 state, the helices C, D, E, and F incorporate more deuterium, whereas in HFIP-MG(N) the exchange rate is similar for all protein regions. These results suggest that I-1 contains the ABGH domain in a native-like organization, whereas HFIP-MG(N) loses a large number of tertiary interactions, thus acquiring a more flexible structure. The fluorescence data are consistent with the above picture. In fact, the tryptophan/ANS energy transfer is much less efficient for the ANS-HFIP-MG(N) complex than for the other complexes, thus suggesting that the distances between the fluorophores might be increased. Moreover, fluorescence polarization measurements indicated that the rotational motion of HFIP-MG(N) occurs on a longer time scale than the other partially folded states, thus suggesting that the volume of this state could be larger. The overall results indicate that addition of hexafluoroisopropanol to native myoglobin results in the formation of a true molten globule where tertiary interactions are reduced, while the secondary structure and the globular compactness are conserved.
Resonance energy transfer between tryptophanyl residues and the apolar fluorescent dye 1-anilino-8-naphthalene sulfonate (ANS) occurs when the fluorophore is bound to native folded sperm whale apomyoglobin. The individual transfer contribution of the two tryptophanyl residues (W7 and W14, both located on the A-helix of the protein) was resolved by measuring the tryptophan-ANS transfer efficiency for the ANS-apomyoglobin complexes formed by wild-type protein and protein mutants containing one or no tryptophanyl residues, i.e. W7F, W14F and W7YW14F. The transfer efficiency of W14 residue was found to be higher than that of W7, thus indicating that W14 acts as the main energy donor in the ANS-apomyoglobin complex. This suggests that the plane containing the anilinonaphthalene ring of the extrinsic fluorophore has a spatial orientation similar to that of W14 and, hence, to the heme group in the holoprotein.
Resonance energy transfer between tryptophanyl residues and the apolar fluorescent dye 1‐anilino‐8‐naphthalene sulfonate (ANS) occurs when the fluorophore is bound to native folded sperm whale apomyoglobin. The individual transfer contribution of the two tryptophanyl residues (W7 and W14, both located on the A‐helix of the protein) was resolved by measuring the tryptophan–ANS transfer efficiency for the ANS–apomyoglobin complexes formed by wild‐type protein and protein mutants containing one or no tryptophanyl residues, i.e. W7F, W14F and W7YW14F. The transfer efficiency of W14 residue was found to be higher than that of W7, thus indicating that W14 acts as the main energy donor in the ANS–apomyoglobin complex. This suggests that the plane containing the anilinonaphthalene ring of the extrinsic fluorophore has a spatial orientation similar to that of W14 and, hence, to the heme group in the holoprotein.
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