A hyperthermophilic protein acquires function at the cost of stability.
Active-site residues are not often optimized for conformational stability (activity-stability trade-offs) in proteins from organisms that grow at moderate temperature. It is unknown if the activity-stability trade-offs can be applied to proteins from hyperthermophiles. Because enzymatic activity usually increases at higher temperature and hyperthermophilic proteins need high conformational stability, they might not sacrifice the stability for their activity. This study attempts to clarify the contribution of active-site residues to the conformational stability of a hyperthermophilic protein. We therefore examined the thermodynamic stability and enzymatic activity of wild-type and active-site mutant proteins (D7N, E8A, E8Q, D105A, and D135A) of ribonuclease HII from Thermococcus kodakaraensis (Tk-RNase HII). Guanidine hydrochloride (GdnHCl)-induced denaturation was measured with circular dichroism at 220 nm, and heat-induced denaturation was studied with differential scanning calorimetry. Both GdnHCl- and heat-induced denaturation were highly reversible in these proteins. All the mutations of these active-site residues, except that of Glu8 to Gln, reduced the enzymatic activity dramatically but increased the protein stability by 7.0 to 11.1 kJ mol(-1) at 50 degrees C. The mutation of Glu8 to Gln did not seriously affect the enzymatic activity and increased the stability only by 2.5 kJ mol(-1) at 50 degrees C. These results indicate that hyperthermophilic proteins also exhibit the activity-stability trade-offs. Therefore, the architectural mechanism for hyperthermophilic proteins is equivalent to that for proteins at normal temperature.
Mukaiyama
Atsushi
A
Department of Material and Life Science, Osaka University, Yamadaoka, Suita 565-0871, Japan.
Haruki
Mitsuru
M
Ota
Motonori
M
Koga
Yuichi
Y
Takano
Kazufumi
K
Kanaya
Shigenori
S
D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D002942:Circular Dichroism; D003460:Crystallization; D004795:Enzyme Stability; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016914:Ribonuclease H / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D019712:Thermococcus / Q000201:enzymology*; D013816:Thermodynamics
D001426:Bacterial Proteins; D011994:Recombinant Proteins; C091139:ribonuclease HII; D016914:Ribonuclease H