Ascorbic acid conversion to erythroascorbic acid, mediated by ubiquitin

We recently identified a microbial conversion of l-ascorbic acid (AsA) to l-erythroascorbic acid (eAsA), a five-carbon analog of AsA. In this paper, we show that ubiquitin plays a crucial role in this process. Based on an assay that determined AsA decomposition, we purified proteins that had N-terminal amino acid sequences identical to that of yeast ubiquitin. Purified ubiquitin facilitated decompositions of AsA and dehydro-AsA, accompanying a partial conversion to eAsA through C1-elimination. Acetylation or limited hydrolysis of ubiquitin abolished its activity. A mutant ubiquitin, with Lys6 replaced by Arg, completely lost activity, whereas a mutant, with six other Lys residues (positions at 11, 27, 29, 33, 48 and 63) substituted by Arg, retained activity. Thus, Lys6, which locates in close proximity to His68, is crucial for ubiquitin activity in the AsA conversion to eAsA.