Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0847097 (acidity)
 15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In order to clarify the function of the DXDDTA motif in squalene-hopene cyclase and to identify the acidic amino acid residues crucial for the catalysis, site-directed mutagenesis experiments were carried out. The following results were found: (1) residues D374 and D376 work for the initiation of polyolefin cyclization which arises from the proton attack on the terminal double bond; (2) residue D377 stabilizes C-10 carbocation of the initially cyclized A-ring intermediate, leading to subsequent B-ring closure, which was further verified by isolating the partially cyclized monocyclic product; (3) residues D313 and D447 outside the DXDDTA motif were identified as new active sites; (4) the H451 residue is likely to work in the protonated form to enhance the acidity of the carboxyl groups of D374 and/or D376.
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PMID:Functional analysis of the DXDDTA motif in squalene-hopene cyclase by site-directed mutagenesis experiments: initiation site of the polycyclization reaction and stabilization site of the carbocation intermediate of the initially cyclized A-ring. 1066 52

Site-directed mutagenesis experiments on all the conserved residues of Phe and Tyr in all the known squalene-hopene cyclases (SHCs) were carried out to identify the active site residues of thermophilic Alicyclobacillus acidocaldarius SHC. The following functions are proposed on the basis of kinetic data and trapping of the prematurely cyclized products: (1) The Y495 residue probably amplifies the D376 acidity, which is assumed to work as a proton donor for initiating the polycyclization cascade, but its role is moderate. (2) Y609 possibly assists the function of F365, which has previously been assigned to exclusively stabilize the C-8 carbocation intermediate through cation-pi interaction. The Y609A mutant produced a partially cyclized bicyclic triterpene. (3) Y612 works to stabilize both the C10 and C8 carbocations, this being verified by the finding that mono- and bicyclic products were formed with the Y612A mutant. (4) F129 was first identified to play a crucial role in catalysis. (5) The three residues, Y372, Y474 and Y540, are responsible for reinforcing the protein structure against thermal denaturation, Y474 being located inside QW motif 3.
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PMID:Catalytic function of the residues of phenylalanine and tyrosine conserved in squalene-hopene cyclases. 1175 15