UBA/THIF-type NAD/FAD binding fold Ubiquitin-activating enzyme (E1 enzyme) [<cite idref="PUB00000623"/>, <cite idref="PUB00005373"/>] activates ubiquitin by firstadenylating with ATP its C-terminal glycine residue and thereafter linkingthis residue to the side chain of a cysteine residue in E1, yielding anubiquitin-E1 thiolester and free AMP. Later the ubiquitin moiety istransferred to a cysteine residue on one of the many forms of ubiquitin-conjugating enzymes (E2).<p>The family of ubiquitin-activating enzymes shares in its catalytic domain significant similarity with a largefamily of NAD/FAD-binding proteins. This domain is based on the common NAD/FAD-binding fold andfinds members of several families, including UBA ubiquitin activating enzymes; the hesA/moeB/thiF family;NADH peroxidases; the LDH family; sarcosin oxidase; phytoene dehydrogenases; alanine dehydrogenases;hydroxyacyl-CoA dehydrogenases and many other NAD/FAD dependent dehydrogenases and oxidases.</p>