Riboflavin is required as a cofactor for many enzymatic reactions involved with metabolism of carbohydrate, fat and protein. Riboflavin in coenzyme form (FMN or FAD) is called flavoprotein, and acts as an intermediary in the transfer of electrons in biological oxidation-reduction reactions. If levels of riboflavin are low, the respiration process becomes less efficient, and 10% to 15% more feed is required to meet energy needs (Christensen, 1983). Enzymes that function aerobically are called oxidases, and those that function anaerobically are called dehydrogenases. Their general function is in oxidation of substrate and generation of energy (i.e., ATP). By involvement in the electron transport system (cytochrome system), flavoproteins function by transferring electrons. Flavoproteins assist in the generation of ATP.
Flavoproteins may accept reduced hydrogen directly from a substrate, thus catalyzing oxidation of the substrate, or may catalyze the oxidation of a second cofactor by accepting a reduced hydrogen from it, for example, from the niacin-containing coenzymes, nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH). About 40 flavoprotein enzymes have been identified and arbitrarily classified into three groups:
- NADH dehydrogenases: Reduced pyridine nucleotide is the substrate, and the electron acceptor is either a member of the cytochrome system or an acceptor other than oxygen.
- Dehydrogenases: Accept electrons directly from the substrate and transfer directly to one of the cytochromes.
- Oxidases (true): Accept electrons from the substrate and transfer directly to oxygen (O2 reduced to CO2); these enzymes cannot directly reduce cytochromes.
Riboflavin functions in flavoprotein-enzyme systems to help regulate cellular metabolism, and is specifically involved in metabolism of carbohydrates. Riboflavin is also an essential factor in amino acid metabolism as part of amino acid oxidases. These enzymes oxidize amino acids to ammonia and a keto-acid. Distinct oxidized forms of the D-amino acids (prosthetic group FAD) and L-amino acids (prosthetic group FMN) are produced. In addition, riboflavin plays a role in fat metabolism (Cooperman and Lopez, 1991). FAD flavoprotein plays an important role in fatty acid oxidation, in the acetyl-coenzyme A dehydrogenases, which are necessary for the stepwise degradation of fatty acids. A FMN flavoprotein is required for synthesis of fatty acids from acetate. Thus, flavoproteins are necessary for both degradation and synthesis of fatty acids in vivo.