Structural and functional coupling in cross-linking uracil-DNA glycosylase UDGX.

Enzymes in uracil-DNA glycosylase (UDG) superfamily are involved in removal of deaminated nucleobases such as uracil, methylcytosine derivatives such as formylcytosine and carboxylcytosine, and other base damage in DNA repair. UDGX is the latest addition of a new class to the UDG superfamily with a sporadic distribution in bacteria. UDGX type enzymes have a distinct biochemical property of cross-linking itself to the resulting AP site after uracil removal. Built on previous biochemical and structural analyses, this work comprehensively investigated the kinetic and enzymatic properties of Mycobacterium smegmatis UDGX. Kinetics and mutational analyses, coupled with structural information, defined the roles of E52, D56, D59, F65 of motif 1, H178 of motif 2 and N91, K94, R107 and H109 of motif 3 play in uracil excision and cross-linking. More importantly, a series of quantitative analyses underscored the structural coupling through inter-motif and intra-motif interactions and subsequent functional coupling of the uracil excision and cross-linking reactions. A catalytic model is proposed, which underlies this catalytic feature unique to UDGX type enzymes. This study offers new insight on the catalytic mechanism of UDGX and provides a unique example of enzyme evolution.