PubMed · 7176920
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R E Viola, W W Cleland. 1982. Initial velocity analysis for terreactant mechanisms.. https://doi.org/10.1016/s0076-6879(82)87021-3
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We have used citrate synthase from Thermoplasma (Tp.) acidophilum as a thermostable model system to investigate the role of hydrophobic interactions in dimer interface for maintaining high temperature stability. Three mutant enzymes were constructed by single amino acid substitutions in the interface helices: Ala97-->Ser, Ala104-->Thr, and Gly209-->Ala. All of the mutations enhanced the thermostability of Tp. citrate synthase, while improving its catalytic properties (Km, Vmax, and specific activity). The highest thermostability was achieved by the Gly209-->Ala substitution. The half-life of irreversible inactivation of the G209A mutant enzyme at 85 degreesC was about 57 min, and the midpoint of guanidinium chloride (GdmCl) induced irreversible denaturation was at 2.0 M GdmCl. Our results showed that amino acid substitutions increasing or decreasing interface hydrophobicity could further increase the thermostability of the Tp. citrate synthase. Thus, interface substitutions affecting the entropy of the unfolded state did not prove to be so critical in protein thermostabilization at higher temperatures.
The effects of alkali chlorides on phosphatidylcholine-water bilayer systems in the Lalpha-phase were investigated by using small- and wide-angle X-ray scattering. The ternary system LiCl-POPC-H2O under isothermal conditions has shown that above Li+/POPC molar ratios of 0.1 and a lipid concentration above 5% (w/w), a splitting of the lamellar Bragg diffraction peaks into discrete components indicates a phase separation into different lamellar liquid crystalline (smectic A) phases. It is also shown that in saturated distearoyl phosphatidylcholine and in egg phosphatidylcholine, alkali chlorides induce Lalpha-phase separation. The number and repeat distance of the coexisting lamellar phases depend on the nature and concentration of the alkali chloride, the concentration of the phosphatidylcholine, and the degree of the acyl chain unsaturation.
A theory is presented of the phase separation of supercoiled DNA into a nucleoid in a bacterial cell. The suspension consists of DNA interacting with globular proteins in excess salt. A cross virial between DNA and a protein is computed as well as the DNA self-energy arising from excluded volume. The cellular parameters of Escherichia coli would appear to be compatible with the thermodynamic equilibrium derived theoretically. The state of superhelical DNA in the nucleoid could be liquid crystalline and rippled.