Riboavin (Vitamin B2)
Although riboavin can be synthesized de novo by plant, yeast, and prokaryotic cells, mammals need to obtain riboavin by consuming plant-based foods or, if necessary, supplemental sources. Riboavin is the precursor to the coenzymes, avin mononucleotide (FMN) and avin adenine dinucleotide (FAD). Historically, both FMN and FAD are referred to as nucleotides. Since the phosphate group of FMN is not esteried to a ribose moiety but rather to the N-10 ribitol side chain of riboavin, a more appropriate term is riboavin-5′-monophosphate. In similar fashion, FAD has its adenosine monophosphate bound in a pyrophosphate arrangement with riboavin-5′-phosphate and thus could be viewed as adenosine riboavin pyrophosphate. For convenience, this chapter will provide the more widely utilized and familiar terms FMN and FAD as the standard abbreviations for the avin coenzymes avin mononucleotide and avin adenine dinucleotide, respectively.