Sepiapterin reductases from Chlorobium tepidum and Chlorobium limicola catalyze the synthesis of l-threo-tetrahydrobiopterin from 6-pyruvoyltetrahydropterin

The ORF sequences of the gene encoding sepiapterin reductase were cloned from the genomic DNAs of Chlorobium tepidum and Chlorobium limicola, which are known to produce l-threo- and l-erythro-tetrahydrobiopterin (BH4)-N-acetylglucosamine, respectively. The deduced amino acid sequence of C. limicola consists of 241 residues, while C. tepidum SR has three residues more at the C-terminal. The overall protein sequence identity was 87.7%. Both recombinant proteins generated from Escherichia coli were identified to catalyze reduction of diketo compound 6-pyruvoyltetrahydropterin to l-threo-BH4. This result suggests that C. limicola needs an additional enzyme for l-erythro-BH4 synthesis to yield its glycoside. The catalytic activity of Chlorobium SRs also supports the previously proposed mechanism of two consecutive reductions of C1′ carbonyl group of 6-pyruvoyltetrahydropterin via isomerization reaction.