Linked Life Data

12578395

Source:http://linkedlifedata.com/resource/pubmed/id/12578395

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
6
pubmed:dateCreated
2003-2-11
pubmed:abstractText
Trp-999 is a key residue for the action of beta-galactosidases (Escherichia coli). Several site specific substitutions (Phe, Gly, Tyr, Leu) for Trp-999 were made. Each substitution caused greatly decreased affinities for substrates and inhibitors that bind in the "shallow" mode, while the affinities of inhibitors that bind in the "deep" mode were not decreased nearly as much. This shows that Trp-999 is important for binding in the shallow mode. The residue is also very important for binding glucose to galactosyl-beta-galactosidase (as a transgalactosidic acceptor). Substitution greatly diminished the affinity for glucose. Substitutions also changed the activation thermodynamics and, subsequently, the rates of the catalytic reactions. The enthalpies of activation of the glycolytic bond cleavage step (galactosylation, k(2)) became less favorable while the entropies of activation of that step became more favorable as a result of the substitutions. Differing magnitudes of these enthalpic and entropic effects with ONPG as compared to PNPG caused the k(2) values for ONPG to decrease but to increase for PNPG. The enthalpies of activation for the common hydrolytic step (degalactosylation, k(3)) increased while the entropies of activation for this step did not change much. As a result, k(3) became small and rate determining for each substituted enzyme. The substitutions caused the rate constant (k(4)) of the transgalactosidic acceptor reactions with glucose (for the formation of allolactose) to become much larger and of the same order of magnitude as the normally large rate constants for transgalactosidic acceptor reactions with small alcohols. This is probably because glucose can approach with less restriction in the absence of Trp-999. However, since glucose binds very poorly to the galactosyl-beta-galactosidases with substitutions for Trp-999, the proportion of lactose molecules converted to allolactose is small. Thus, Trp-999 is also important for ensuring that an appropriate proportion of lactose is converted to allolactose.
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
Feb
pubmed:issn
0006-2960
pubmed:author
pubmed:issnType
Print
pubmed:day
18
pubmed:volume
42
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
1796-803
pubmed:dateRevised
2006-11-15
pubmed:meshHeading
pubmed:year
2003
pubmed:articleTitle
Trp-999 of beta-galactosidase (Escherichia coli) is a key residue for binding, catalysis, and synthesis of allolactose, the natural lac operon inducer.
pubmed:affiliation
Division of Biochemistry, Faculty of Science, University of Calgary, Calgary, Alberta, Canada T2N 1N4. huber@ucalgary.ca
pubmed:publicationType
Journal Article, Research Support, Non-U.S. Gov't