Linked Life Data

8663036

Source:http://linkedlifedata.com/resource/pubmed/id/8663036

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
25
pubmed:dateCreated
1996-8-13
pubmed:abstractText
His-42 plays a critical role in the H2O2-dependent catalytic turnover of horseradish peroxidase (HRP). This is clearly illustrated by the finding that an H42A mutation decreases the rate of Compound I formation by a factor of approximately10(6). As shown here, the addition of 2-substituted imidazoles partially rescues both the rate of formation of Compound I and the peroxidase activity of the H42A mutant. 2-Substituted imidazoles are the most effective because they do not coordinate to the iron. In contrast to native HRP, which exhibits a parabolic pH profile, and the H42A mutant, for which the activity increases linearly with increasing pH, the activity of the H42A mutant in the presence of 1,2-dimethylimidazole (pKa = 8.0) exhibits a sigmoidal pH dependence with a midpoint at pH 8.0 +/- 0.2. Similar results are obtained with 2-methylimidazole. These results establish that the free base forms of these imidazoles facilitate HRP turnover. The spectroscopic binding constants for 1, 2-dimethylimidazole and 2-methylimidazole are Kd = 2.9 +/- 1.3 and 2. 5 +/- 0.2 M, respectively. When cyanide is bound to the heme, the Kd for 1,2-dimethylimidazole is 0.17 M. This >10-fold decrease in Kd may reflect hydrogen bonding of the protonated imidazole to the iron-coordinated cyanide. The log of the rate of Compound I formation exhibits a linear dependence on the molecular volume of the imidazoles used to rescue the activity. If the rates are corrected for differences in the size of the imidazoles, the log of the rates is linearly related to the pKa of the imidazoles. This Brönsted analysis predicts that approximately 60% of a positive charge develops on the imidazole in the transition state of Compound I formation. The results confirm the acid-base role of the distal histidine, demonstrate that exogenous histidines can function as surrogates for the missing histidine in the H42A mutant, and provide a transition state model of relevance to the formation of Compound I in the native protein.
pubmed:grant
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
Jun
pubmed:issn
0021-9258
pubmed:author
pubmed:issnType
Print
pubmed:day
21
pubmed:volume
271
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
14891-6
pubmed:dateRevised
2007-11-14
pubmed:meshHeading
pubmed:year
1996
pubmed:articleTitle
Rescue of the catalytic activity of an H42A mutant of horseradish peroxidase by exogenous imidazoles.
pubmed:affiliation
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco, California 94143-0446, USA.
pubmed:publicationType
Journal Article, Comparative Study, Research Support, U.S. Gov't, P.H.S.