To examine the role of histidine residues in ribonuclease H from Escherichia coli, kinetic parameters for the enzymatic activity and conformational stabilities against guanidine hydrochloride denaturation of mutant enzymes, in which each of the five histidine residues was replaced with alanine, were determined and compared with the wild-type enzyme. The mutation of His83 resulted in a marked increase in Km along with an increase in kcat. The mutation of His114 caused a large reduction in both the free energy of unfolding in water, delta GH2O, and the mid-point of the unfolding curve, [D]1/2. These results indicate that His83, which is one of the four well-exposed histidine residues in the crystal structure, is located close to a substrate-binding site, and His114, which is buried inside the protein molecule, contributes to the conformational stability, probably through the formation of a hydrogen bond with a main-chain carbonyl group. None of the histidine residues is required for activity. Study holds ProTherm entries: 383, 384, 385, 386, 387, 388 Extra Details:
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:15 p.m.
|Number of data points||35|
|Proteins||Ribonuclease HI ; Ribonuclease HI|
|Assays/Quantities/Protocols||Experimental Assay: Cm ; Experimental Assay: activity_kcat ; Experimental Assay: m ; Experimental Assay: dG_H2O ; Experimental Assay: activity_km ; Derived Quantity: ddG_H2O|
|Libraries||Mutations for sequence MLKQVEIFTDGSCLGNPGPGGYGAILRYRGREKTFSAGYTRTTNNRMELMAAIVALEALKEHCEVILSTDSQYVRQGITQWIHNWKKRGWKTADKKPVKNVDLWQRLDAALGQHQIKWEWVKGHAGHPENERCDELARAAAMNPTLEDTGYQVEV|