Related Experiment Videos
Biosynthesis of vitamin B2
Johannes Kaiser1, Nicholas Schramek, Sabine Eberhardt
1Lehrstuhl für Organische Chemie und Biochemie, Technische Universität München, Garching, Germany.
European Journal of Biochemistry
|October 24, 2002
Summary
GTP cyclohydrolase II uses a zinc ion to release pyrophosphate and then formate from GTP, a key step in riboflavin biosynthesis. Mutating cysteine residues prevents zinc binding and formate release, confirming zinc
Area of Science:
- Biochemistry
- Enzymology
- Metabolic Pathways
Background:
- GTP cyclohydrolase II is crucial for riboflavin biosynthesis.
- The enzyme catalyzes the conversion of GTP to 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate.
- The enzyme contains a zinc ion per subunit.
Purpose of the Study:
- To investigate the role of zinc in GTP cyclohydrolase II activity.
- To elucidate the reaction mechanism of GTP cyclohydrolase II.
- To identify key residues involved in zinc binding and catalysis.
Main Methods:
- Site-directed mutagenesis of cysteine residues (54, 65, 67) to serine.
- Enzyme activity assays measuring formate and pyrophosphate release.
- Analysis of zinc content in wild-type and mutant enzymes.
Main Results:
- Mutant proteins lacking zinc were unable to release formate from GTP or an analog.
- Mutant proteins retained pyrophosphate release activity.
- The data suggest an ordered reaction mechanism with pyrophosphate release preceding imidazole ring attack.
Conclusions:
- Zinc is essential for the hydrolytic release of formate, likely acting as a Lewis acid.
- The reaction proceeds via sequential hydrolysis, with pyrophosphate release occurring before ring opening and formate release.
- Cysteine residues 54, 65, and 67 are critical for zinc coordination and catalysis.