Inactivation of Lactobacillus leichmannii ribonucleotide reductase by F2CTP: covalent modification

Abstract

Ribonucleotide reductase (RNR) from Lactobacillus leichmannii, a 76 kDa monomer using adenosylcobalamin (AdoCbl) as a cofactor, catalyzes the conversion of nucleoside triphosphates to deoxynucleotides and is rapidly (<30 s) inactivated by 1 equiv of 2′,2′-difluoro-2′-deoxycytidine 5′-triphosphate (F[subscript 2]CTP). [1′-[superscript 3]H]- and [5-[superscript 3]H]F[subscript 2]CTP were synthesized and used independently to inactivate RNR. Sephadex G-50 chromatography of the inactivation mixture revealed that 0.47 equiv of a sugar was covalently bound to RNR and that 0.71 equiv of cytosine was released. Alternatively, analysis of the inactivated RNR by SDS−PAGE without boiling resulted in 33% of RNR migrating as a 110 kDa protein. Inactivation of RNR with a mixture of [1′-[superscript 3]H]F[subscript 2]CTP and [1′-[superscript 2]H]F[subscript 2]CTP followed by reduction with NaBH[subscript 4], alkylation with iodoacetamide, trypsin digestion, and HPLC separation of the resulting peptides allowed isolation and identification by MALDI-TOF mass spectrometry (MS) of a 3H/2H-labeled peptide containing C[subscript 731] and C[subscript 736] from the C-terminus of RNR accounting for 10% of the labeled protein. The MS analysis also revealed that the two cysteines were cross-linked to a furanone species derived from the sugar of F[subscript 2]CTP. Incubation of [1′-[superscript 3]H]F[subscript 2]CTP with C119S-RNR resulted in 0.3 equiv of sugar being covalently bound to the protein, and incubation with NaBH[subscript 4] subsequent to inactivation resulted in trapping of 2′-fluoro-2′-deoxycytidine. These studies and the ones in the preceding paper (DOI: 10.1021/bi9021318) allow proposal of a mechanism of inactivation of RNR by F[subscript 2]CTP involving multiple reaction pathways. The proposed mechanisms share many common features with F[subscript 2]CDP inactivation of the class I RNRs.