Dissecting histidine interactions of ribonuclease T1 with asparagine and glutamine replacements: analysis of double mutant cycles at one position.

TitleDissecting histidine interactions of ribonuclease T1 with asparagine and glutamine replacements: analysis of double mutant cycles at one position.
Publication TypeJournal Article
Year of Publication1998
AuthorsDe Vos, S., J. Doumen, U. Langhorst, and J. Steyaert
JournalJ Mol Biol
Volume275
Issue4
Pagination651-61
Date Published1998 Jan 30
ISSN0022-2836
KeywordsAmino Acid Substitution, Asparagine, Crystallography, X-Ray, Esterification, Exoribonucleases, Glutamine, Histidine, Kinetics, Models, Molecular, Mutagenesis, Site-Directed, Protein Conformation
Abstract

His92 of Ribonuclease T1 combines functional and structural features involving both imidazole nitrogens. To evaluate the use of Asn and Gln substitutions in dissecting the properties of histidines, we analysed the consequences of the His92Gln and His92Asn substitutions on the enzyme's structure, function, and conformational stability by protein engineering and X-ray crystallographic methods. In the X-ray structures of wild-type and His92Gln RNase T1 in complex with 2'-GMP the His92-N epsilon 2 and Gln92-N epsilon 2 atoms are isosterically equivalent. Similarly, the His92N delta 1H...OAsn99 hydrogen bond observed in wild-type is replaced by an equivalent Asn92N delta 2H...OAsn99 in the His92Asn mutant structure. Double mutant cycles at a single position were used to analyse the intermolecular and intramolecular interactions of the exchangeable proton and the individual histidine nitrogens. Urea denaturation measurements as a function of pH revealed that the exchangeable proton of His92, rather than its imidazole ring is contributing about 1 kcal/mol to the conformational stability of RNase T1. The stabilizing and the destabilizing effects of the (His-->Gln) and the (His-->Asn) mutations on urea denaturation of RNase T1 at pH 9.0 suggest that the unprotonated N delta 1 and N epsilon 2 atoms contribute in a compensating way to the conformational stability of RNase T1. A comparative study of the kinetics of all mutants suggests that the protonated His92 imidazole is a strictly co-operative catalytic device.

DOI10.1006/jmbi.1997.1480
Alternate JournalJ. Mol. Biol.
PubMed ID9466938