Catalytic Mechanism of the Cyclohydrolase Activity of Human Aminoimidazole Carboxamide Ribonucleotide Formyltransferase/Inosine Monophosphate Cyclohydrolase

The bifunctional enzyme aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) is responsible for catalysis of the last two steps in the de novo purine pathway. Using recently determined crystal structures of ATIC as a guide, four candidate residues, Lys...

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Bibliographic Details
Published in:Biochemistry (Easton) 2004-02, Vol.43 (5), p.1184-1192
Main Authors: Vergis, James M, Beardsley, G. Peter
Format: Article
Language:English
Subjects:
Alanine - genetics
Animals
Arginine - genetics
Aspartic Acid - genetics
Binding Sites
Birds
Catalysis
Deuterium Exchange Measurement
Humans
Hydroxymethyl and Formyl Transferases - chemistry
Hydroxymethyl and Formyl Transferases - genetics
Kinetics
Lysine - genetics
Multienzyme Complexes - chemistry
Multienzyme Complexes - genetics
Mutagenesis, Site-Directed
Nucleotide Deaminases - chemistry
Nucleotide Deaminases - genetics
Phosphoribosylaminoimidazolecarboxamide Formyltransferase
Protein Structure, Tertiary - genetics
Recombinant Proteins - chemistry
Tyrosine - genetics
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Summary:The bifunctional enzyme aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) is responsible for catalysis of the last two steps in the de novo purine pathway. Using recently determined crystal structures of ATIC as a guide, four candidate residues, Lys66, Tyr104, Asp125, and Lys137, were identified for site-directed mutagenesis to study the cyclohydrolase activity of this bifunctional enzyme. Steady-state kinetic experiments on these mutants have shown that none of these residues are absolutely required for catalytic activity; however, they strongly influence the efficiency of the reaction. Since the FAICAR binding site is made up mostly of backbone interactions with highly conserved residues, we postulate that these conserved interactions orient FAICAR in the active site to favor the intramolecular ring closure reaction and that this reaction may be catalyzed by an orbital steering mechanism. Furthermore, it was shown that Lys137 is responsible for the increase in cyclohydrolase activity for dimeric ATIC, which was reported previously by our laboratory. From the experiments presented here, a catalytic mechanism for the cyclohydrolase activity is postulated.
ISSN:0006-2960
1520-4995
DOI:10.1021/bi035139b