Linked Life Data

8204600

Source:http://linkedlifedata.com/resource/pubmed/id/8204600

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
21
pubmed:dateCreated
1994-7-8
pubmed:abstractText
The substrate-bound form of the enzyme heme oxygenase (HO), which catalyzed the stereospecific alpha-meso bridge cleavage of hemin to yield biliverdin IX alpha, has been investigated by 1H NMR in both its primarily high-spin and its cyanide-inhibited low-spin forms. Both derivatives yield 1H NMR spectra indicative of extensive heterogeneity that is largely resolved when a 2-fold-symmetric hemin substrate is bound. The structural origin of the heterogeneity is shown to result from approximately 1:1 isomeric binding of the native hemin substrate in the binding pocket. The substrate orientational disorder is about the alpha,gamma-meso axis, as established on the basis of 2D NMR experiments that identify characteristic aromatic van der Waals contact in the substrate binding pocket. The isomeric substrate-HO complexes exhibit differential cyanide affinity, and the ratio of isomers is sensitive to the hemin 2,4-substituents. The assignment of hemin signals by isotopic labeling and 2D NMR methods reveals a contact shift pattern that reflects an unusual hemin electronic structure that is characterized by large differences in delocalized spin density for the two positions within a given pyrrole, rather than the more conventional large differences between adjacent pyrroles. This pattern of spin density delocalized primarily to the pyrrole positions adjacent to the alpha,gamma-meso axis can be rationalized by postulating a direct electronic perturbation of the hemin by the protein matrix in the form of an anionic side chain close to the alpha-meso carbon. Similar influences on hemin electronic structure, in the form of chemical substitution of the meso positions, have been observed in iron porphyrin compounds and successfully modeled by simple molecular orbital theory (Tan et al., 1994). This is interpreted as evidence for a direct electronic effect by HO to activate the alpha-meso position for electrophilic rather than nucleophilic attack. The unique contact shift pattern is present to different degrees for the two hemin orientations, is strongly pH dependent, and is largely abolished at acidic pH. Portions of several heme pocket residues are located and it is shown that the pattern of the dipolar shifts for these residues, which likely reflects the distal steric influence on the tilt of the coordinated cyanide, differs significantly for the two substrate orientations.(ABSTRACT TRUNCATED AT 400 WORDS)
pubmed:grant
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
May
pubmed:issn
0006-2960
pubmed:author
pubmed:issnType
Print
pubmed:day
31
pubmed:volume
33
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
6631-41
pubmed:dateRevised
2007-11-14
pubmed:meshHeading
pubmed:year
1994
pubmed:articleTitle
Proton NMR investigation of substrate-bound heme oxygenase: evidence for electronic and steric contributions to stereoselective heme cleavage.
pubmed:affiliation
Department of Chemistry, University of California, Davis 95616.
pubmed:publicationType
Journal Article, Research Support, U.S. Gov't, P.H.S., Research Support, Non-U.S. Gov't