Activity-Stability Relationships in Extremophilic Enzymes

Abstract

Psychrophilic, mesophilic, and thermophilic ␣-amylases have been studied as regards their conformational stability, heat inactivation, irreversible unfolding, activation parameters of the reaction, properties of the enzyme in complex with a transition state analog, and structural permeability. These data allowed us to propose an energy landscape for a family of extremophilic enzymes based on the folding funnel model, integrating the main differences in conformational energy, cooperativity of protein unfolding, and temperature dependence of the activity. In particular, the shape of the funnel bottom, which depicts the stability of the native state ensemble, also accounts for the thermodynamic parameters of activation that characterize these extremophilic enzymes, therefore providing a rational basis for stability-activity relationships in protein adaptation to extreme temperatures.Our planet harbors a huge number of harsh environments that are considered as "extreme" from an anthropocentric point of view, as far as temperature, pH, osmolarity, free water, or pressure are concerned. Nevertheless, these peculiar biotopes have been successfully colonized by numerous organisms, mainly extremophilic bacteria and archaea. As the curiosity of scientists stimulates the exploration of new environments, it seems that there is no "empty space" for life on Earth and, for instance, even the supercooled cloud droplets contain actively growing bacteria (1). Among the extremophilic microorganisms, those living at extreme temperatures have attracted much attention. Thermophiles have revealed the unsuspected upper temperature for life at about 113°C (2, 3). Their enzymes have also demonstrated a considerable biotechnological potential such as the various thermostable DNA polymerases used in PCR that have boosted many laboratory techniques. At the other end of the temperature scale, metabolically active psychrophilic bacteria have been detected in liquid brine veins of sea ice at Ϫ20°C (4). These cold-loving microorganisms face the thermodynamic challenge to maintain enzyme-catalyzed reactions and metabolic rates compatible with sustained growth near or below the freezing point of pure water (5, 6). Directed evolution experiments have highlighted that, in theory, cold activity of enzymes can be gained by several subtle adjustments of the protein structure (7). However, in natural cold environments, the consensus for the adaptive strategy is to take advantage of the lack of selective pressure for stable proteins for losing stability, therefore making the enzyme more mobile or flexible at temperatures that "freeze" molecular motions and reaction rates (8).The crystal structures of extremophilic enzymes unambiguously indicate a continuum in the molecular adaptations to temperature. There is indeed a clear increase in the number and strength of all known weak interactions and structural factors involved in protein stability from psychrophiles to mesophiles (living at intermediate temperatures close to 37°C) and to thermophiles (2, 9 -1...

Stepwise Adaptations to Low Temperature as Revealed by Multiple Mutants of Psychrophilic α-Amylase from Antarctic Bacterium

“…It has been shown that stabilization induced by acarbose binding closely follows the activation parameters of the amylolytic reaction (22) between the ground state (free enzyme) and the transition state intermediate (enzyme-acarbose complex). The thermograms of AHA and of its mutants were therefore recorded by DSC in the presence of acarbose (supplemental Fig.…”

“…GdmCl-induced unfolding was monitored at 20°C after overnight incubation of the samples at this temperature in 30 mM MOPS, 50 mM NaCl, 1 mM CaCl 2 , pH 7.2, on an LS50B spectrofluorometer (PerkinElmer Life Sciences) (22). The equilibrium condition was ascertained by recording unfolding as a function of time.…”

“…Unfolding Recorded by Intrinsic Fluorescence-Heat-induced unfolding was recorded using an SML-AMINCO model 8100 spectrofluorometer (Spectronic Instruments) at an excitation wavelength of 280 nm and at an emission wavelength of 350 nm (22). GdmCl-induced unfolding was monitored at 20°C after overnight incubation of the samples at this temperature in 30 mM MOPS, 50 mM NaCl, 1 mM CaCl 2 , pH 7.2, on an LS50B spectrofluorometer (PerkinElmer Life Sciences) (22).…”

“…Dynamic Quenching of Fluorescence-The acrylamide-dependent quenching of intrinsic protein fluorescence was monitored as described previously (22). The Stern-Volmer quenching constants K SV were calculated according to Equation 3,…”

“…It displays striking sequence and structure similarities with its mesophilic homologue from pig pancreas (PPA) (11,(21)(22)(23). For instance, all 24 residues forming the catalytic cleft (supplemental Fig.…”