STUDIES ON THE FORMATION OF BIOTIN FROM DESTHIOBIOTIN AND SULFATE IN SACCHAROMYCES CEREVISIAE. 3. ISOTOPIC STUDIES ON BIOTIN FORMATION FROM DESTHIOBIOTIN AND S35-LABELLED SULFATE BY WASHED CELL SUSPENSION OF YEAST

Overexpression of biotin synthase and biotin ligase is required for efficient generation of sulfur-35 labeled biotin in E. coli

Background

Biotin is an essential enzyme cofactor that acts as a CO₂ carrier in carboxylation and decarboxylation reactions. The E. coli genome encodes a biosynthetic pathway that produces biotin from pimeloyl-CoA in four enzymatic steps. The final step, insertion of sulfur into desthiobiotin to form biotin, is catalyzed by the biotin synthase, BioB. A dedicated biotin ligase (BirA) catalyzes the covalent attachment of biotin to biotin-dependent enzymes. Isotopic labeling has been a valuable tool for probing the details of the biosynthetic process and assaying the activity of biotin-dependent enzymes, however there is currently no established method for ³⁵S labeling of biotin.

Results

In this study, we produced [³⁵S]-biotin from Na³⁵SO₄ and desthiobiotin with a specific activity of 30.7 Ci/mmol, two orders of magnitude higher than previously published methods. The biotinylation domain (PfBCCP-79) from the Plasmodium falciparum acetyl-CoA carboxylase (ACC) was expressed in E. coli as a biotinylation substrate. We found that overexpression of the E. coli biotin synthase, BioB, and biotin ligase, BirA, increased PfBCCP-79 biotinylation 160-fold over basal levels. Biotinylated PfBCCP-79 was purified by affinity chromatography, and free biotin was liberated using acid hydrolysis. We verified that we had produced radiolabeled biologically active [D]-biotin that specifically labels biotinylated proteins through reuptake in E. coli.

Conclusions

The strategy described in our report provides a simple and effective method for the production of [³⁵S]-biotin in E. coli based on affinity chromatography.

The origin of sulfur in biotin

The comparative ability of Escherichia coli K-12 hpb lambda-, a biotin overproducing strain, to incorporate 35S from isotopically labeled L-methionine, L-cystine, and the sulfane sulfur of thiocystine was determined. Comparison of the specific activity of sulfur in biotin produced by the organism with that of the 35S-labeled amino acids demonstrates that the sulfur of cystine is transferred to biotin with an efficiency of at least 75%, that the sulfur of methionine does not contribute to biotin significantly, and that the sulfane sulfur of thiocystine contributes approximately one-third of the sulfur to newly synthesized biotin and is essentially equivalent in utilization to that of the other two sulfur atoms in the molecule.