{'fraction_non_alphanumeric': 0.056639395846444306, 'fraction_numerical': 0.032724984266834484, 'mean_word_length': 4.910780669144981, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Identification of the Active Site Nucleophile in Nucleoside 2-Deoxyribosyltransferase as Glutamic Acid 98 (*)

2′-Fluoro-2′-deoxyarabinonucleosides are time-dependent inhibitors of nucleoside 2-deoxyribosyltrans-ferase. 2,6-Diamino-9-(2′-deoxy-2′-fluoro-β-D-arabino-furanosyl)-9H-purine (dFDAP) inhibited the enzyme by formation of a primary complex (K = 140 μM) that isomerized to a secondary complex with a first-order rate constant of 0.2 min. Inhibited enzyme contained stoichiometric amounts of covalently bound 2′-fluoro-2′-deoxyarabinosyl moiety, recovered less than 5% of its activity after storage for a week at 5°C, but regained over 70% of the lost activity by treatment with 600 μM Ade. 6-Amino-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)-9H-purine (dFAdo) was a product of the reactivation reaction. Proteolysis of inhibited enzyme identified a modified fragment that spanned residues 82-107 which could not be sequenced past Gly-96. dFDAP-inhibited enzyme and enzyme reacted with normal substrates (i.e. dThd and dAdo) were hydrolyzed between Met-97 and Glu-98 by 0.1 M NaOH. These findings and model studies on the base lability of peptides containing glutamyl esters suggested that the -carboxylate of Glu-98 was esterfied during catalysis. The role of Glu-98 was confirmed by changing this residue to alanine. The specific activity of wild-type enzyme was 3 orders of magnitude greater than that of the mutant enzyme. Collectively, chemical modification and mutagenesis studies have identified Glu-98 as the active site nucleophile of nucleoside 2-deoxyribosyltransferase.