Abstract
A substantial challenge for genomic enzymology is the reliable annotation for proteins of unknown function. Described here is an interrogation of uncharacterized enzymes from the amidohydrolase superfamily using a structure-guided approach that integrates bioinformatics, computational biology, and molecular enzymology. Previously, Tm0936 from Thermotoga maritima was shown to catalyze the deamination of S-adenosylhomocysteine (SAH) to S-inosylhomocysteine (SIH). Homologues of Tm0936 homologues were identified, and substrate profiles were proposed by docking metabolites to modeled enzyme structures. These enzymes were predicted to deaminate analogues of adenosine including SAH, 5'-methylthioadenosine (MTA), adenosine (Ado), and 5'-deoxyadenosine (5'-dAdo). Fifteen of these proteins were purified to homogeneity, and the three-dimensional structures of three proteins were determined by X-ray diffraction methods. Enzyme assays supported the structure-based predictions and identified subgroups of enzymes with the capacity to deaminate various combinations of the adenosine analogues, including the first enzyme (Dvu1825) capable of deaminating 5'-dAdo. One subgroup of proteins, exemplified by Moth1224 from Moorella thermoacetica, deaminates guanine to xanthine, and another subgroup, exemplified by Avi5431 from Agrobacterium vitis S4, deaminates two oxidatively damaged forms of adenine: 2-oxoadenine and 8-oxoadenine. The sequence and structural basis of the observed substrate specificities were proposed, and the substrate profiles for 834 protein sequences were provisionally annotated. The results highlight the power of a multidisciplinary approach for annotating enzymes of unknown function.