Flavoproteins are ubiquitous proteins that use flavins as prosthetic groups. Flavorproteins require the ribo-flavin-derived redox cofactors FAD and FMN. The active isoalloxazine ring of flavin cofactors may undergo one or two electron redox transitions in a broad range of biochemical events relevant for cell bioenergetics, protein folding, and redox homeostasis. In most flavoproteins, the flavin cofactor is tightly, but noncovalently bound.

Function

Recent studies indicate that, in addition to catalysis, flavin cofactors may play regulatory roles, affecting protein expression and stability. Riboflavin deficiency in humans has been linked to pathological conditions such as anemia, cancer, cardiovascular diseases and neurological disorders.

Synthesis of Flavin Cofactors

In mammals, flavin cofactors are obtained from dietary riboflavin (vitamin B2), which is actively absorbed by intestinal cells and delivered to peripheral districts in tight association with plasma albumin. The circulatingvitamin is efficiently imported into the peripheral cells via specific plasma membrane transporters. Once inside the cells riboflavin is rapidly converted into catalytically active cofactors via the sequential actions ofriboflavin kinase (AT: riboflavin 5’phos-photransferase) and FAD synthase (FADS; FMN: ATP adenylyltransferase).

FAD Synethase (FADS):

The FAD (Flavin Adenin Dinucleotide) protein synthetase (FAD synethetase of ATP:FMN adenylyl transferase or FADS or FMNAT or EC 2.7.7.2) is an enzyme which catalyses an obligatory and ubiquitous phase of the energetic metabolism, namely the ATP adenylazation depending on flavin mononucleotide or FMN, which in turn derives from the B2 vitamin or riboflavin. Proacaryotic and eucaryotic cells use FAD as co-maker of hundreds of different flavoenzymes with dehydrogenasic and oxydasic activities. FADS converts riboflavin into the redox cofactor FAD. These are involved in the terminal metabolism, in the processes of energetic transduction, in protection from cellular stress, in folding of the secretion protein, in the independent caspase apoptosis, and in chromatin remodeling processes.

In yeast and animals, distinct monofunctional enzymes exist with either riboflavin kinase or FADS activity.The first eukaryoic gene coding for FADS, namely FAD1, has been identified and cloned in Saccharamyces cerevisiae. The encoded protein (Fad1p) belongs to the phospho-adenosine phosphosulfate reductase family and has little or no sequence similarity to the bacterial enzymes. Using homology searches with Fad1p as the query sequence, the human gene coding for FADS, namely FLAD1, has been identified. Human FADS is organized into 2 domains: (a) a phosphoadenosine phosposulfate reductase domain localized at the C-terminus, which shares 60% similarity and 34% identity with the corresonding Fad1p domain, and (b) an additional N-terminus domain, resimbling a molybdo-prterin-binding domain, whose function is still unknown. Because the mammalian FAD-forming enzyme is unrelated to the bacterial enzyme, and the bacterial enzyme is required for bacterial viability, FADS is particularly interesting as a potential target for the devleopment of  novel antimicrobial drugs.

Searches for FLAD1 gene products have shown at least 4 predicted proteins, encoded by different transcript variants. Cloning, overexpression and partial functional characterization of FADS isoforms 1 and 2 (hFADS1 and HFADS2), which are the products, respectively, of FLAD1 transcript variant 1 and transcript variant 2 has been performed. It has been shown that hFADS1 is localized in mitochondra, whereas hFADS2 is localized in the cytosol. Tissue distribution and subcellular localization of the other isoforms are still uncharacterized and enignatic, and no structural data are available for any of the FADS isoforms (Torchetti et al., “Human FAD synthase (isoform 2): a component of the machinery that delivers FAD to apo-flavoproteins” FEBS Journal, 278(22), Oct 2011, pp. 4435-4449