Conformational studies on the inactivation of glyceraldehyde-3-phosphate dehydrogenase from the facultative...

Among the various proposals that have been made in attempting to explain the ability of thermophiles to reproduce at high temperatures, there is no doubt that obligate and extreme thermophiles synthesize proteins (and other molecules) that have sufficient intrinsic molecular stability to withstand increased thermal stress. In contrast, the glyceraldehyde-3-phosphate dehydrogenase from the facultative thermophile Bacillus coagulans KU has been shown to be quite thermolabile in vitro. Thermal inactivation is not due to loss of bound NAD+. It has also been shown that the enzymatic activity can be thermostabilized in vitro by increased ionic strength. As previously reported [J. W. Crabb, A. L. Murdock, and R. E. Amelunxen (1975) Biochem. Biophys. Res. Commun. 62, 627; (1977) Biochemistry 16, 4840], the enzyme loses 94-97% of enzymatic activity after heat treatment at 55 degrees C for 5 min in 0.05 M sodium phosphate buffer (pH 7.1); however, by increasing the ionic strength to 1.8, complete protection was conferred at this temperature. Gel-filtration chromatography has been used to study the initial dissociation and subsequent aggregation of the glyceraldehyde-3-phosphate dehydrogenase after thermal inactivation. Aggregation occurs when the enzyme is heated at 50 degrees or 55 degrees C. Loss of enzymatic activity is correlated with changes in the tertiary structure as measured by the near-uv CD spectrum of the enzyme following heat inactivation, with essential disappearance of the peaks at 263 and 296 nm, and a blue shift of the far-uv spectrum, which is a measure of secondary structure. Estimation of secondary structure of the unheated protein from the far-uv CD data showed the enzyme contains approximately 26% alpha-helix, approximately 21% beta-structure, and approximately 53% disordered structure. Heat treatment at various temperatures resulted in only slight changes of the estimated secondary structure. Increased ionic strength prevents thermal alteration of the CD spectrum in both near- and far-uv regions. The data support the previous proposal that thermolabile enzymes such as the glyceraldehyde-3-phosphate dehydrogenase from the facultative thermophile B. coagulans are thermostabilized in vivo mainly by the intracellular charged macromolecular environment.