Reversibility of thermal transitions in proteins: Racemization and aggregation as factors in the reversible denaturation of a soluble keratin derivative (SCMKA)
✍ Scribed by B. S. Harrap
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 1969
- Tongue
- English
- Weight
- 749 KB
- Volume
- 8
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
The reversibility of the thermal denaturation of a low-sulfur fraction of solubilized wool keratin (SCMKA) has been studied under a variety of conditions of time, protein concentration, and pH. Two types of irreversibility for the transition have been encountered. One of these is associated with an aggregation of the protein on denaturation to give a product which may contain elements of a conformation. This type of irreversibility is favored by high protein concentration, and the original conformation may in fact be regained if the aggregated structure is broken down by a solvent such as 8M urea and the urea subsequently removed by dialysis. The other type of irreversibility appears to be due to racemixation of the protein. It does not seem to be dependent on protein concentration and is apparent only at temperatures beyond the actual transition range (-40-65°C) at p1-I values below 11. At pH 12, however, racemization appears to proceed slowly even at 4°C. The thermal transition a t pH 9 and pH 10 has been shown to be multistage in nature. Over the pH range 9-12 there is a progressive decrease in thermal stability with increase of pH. Addition of NaCl a t p1-I 10 leads to an increase in thermal stability of the molecule.
Methods
Optical rotatory dispersion measurements were made 578, 546,436,405, and 365 mp with a Perkin-Elmer model 141 spectropolarimeter. The wave length scan could be completed in less than 1 min which was useful in temperature studies as indicated below. Solutions were clarified by filtration through Millipore filters (0.45 p porosity). Jacketed cells of