Quasiharmonic Analysis of the Energy Landscapes of Dihydrofolate Reductase from Piezophiles and Mesophiles.

A quasiharmonic analysis (QHA) method is used to compare the potential energy landscapes of dihydrofolate reductase (DHFR) from a piezophile (pressure-loving organism), Moritella profunda (Mp), and a mesophile, Escherichia coli (Ec). The QHA method considers atomic fluctuations of the protein as motions of an atom in a local effective potential created by neighboring atoms and quantitates it in terms of effective force constants, isothermal compressibilities, and thermal expansivities. The analysis indicates that the underlying potential energy surface of MpDHFR is inherently softer than that of EcDHFR. In addition, on picosecond time scales, the energy surfaces become more similar under the growth conditions of Mp and Ec. On these time scales, DHFR behaves as expected; namely, increasing temperature makes the effective energy minimum less steep because thermal fluctuations increase the available volume, whereas increasing pressure steepens it because compression reduces the available volume. Our longer simulations show that, on nanosecond time scales, increasing temperature has a similar effect as on picosecond time scales because thermal fluctuations increase the volume even more on a longer time scale. However, these simulations also indicate that, on nanosecond time scales, pressure makes the local potential less steep, contrary to picosecond time scales. Further examination of the QHA indicates the nanosecond pressure response may originate at picosecond time scales at the exterior of the protein, which suggests that protein-water interactions may be involved. The results may lead to understanding adaptations in enzymes made by piezophiles that enable them to function at higher pressures.