Riboflavin (Vitamin B2), FMN and FAD

 
Monday's Molecule was Flavin Adenine Dinucleotide or FAD [Monday's Molecule #23]. The flavin moiety is the three ring structure at the top of the figure. It's attached to a sugar called ribitol drawn in an open chain conformation. The ribitol, in turn, is attached to a single phosphate group at the other end.

The structure shown in black is called flavin mononucleotide or FMN. The blue structure is an AMP group so the complete FAD molecule (black + blue) called a dinucleotide. FMN and FAD are important coenzymes that carry electrons from one reaction to another. We've already encountered FAD last week when we described the pyruvate dehdrogenase reaction. In that reaction the FAD molecule picked up two electrons from the lipoamide swinging arm and passed them on to NAD⁺.

FMN and FAD are required for important reactions in all species. They are made from riboflavin (right). Riboflavin can be synthesized in bacteria, protists, fungi, plants and some animals but mammals have lost the ability to make it. Instead, we have to obtain riboflavin from our food and that's why it's a vitamin in humans (vitamin B₂). (It's not a "vitamin" in other species since they can make it themselves.)

Riboflavin deficiency is quite rare because we can usually get enough from the bacteria that inhabit our intestines. The most common cases of riboflavin deficiency are seen in chronic alcoholics who often show deficiencies in many other vitamins as well.

FMN and FAD are tightly bound to the enzymes that require them as cofactors. These enzymes often have a characteristic yellow color because of the flavin. One of the most famous enzymes in biochemistry is a flavoprotein called "Old Yellow Enzyme," which turned out to be an NADPH oxidase.

FMN and FAD are cofactors that can carry one or two electrons as shown below. This makes them similar to ubiquinone. There are many reactions that exchange electrons between FMN/FAD and ubiquinone in short electron transport chains. The passage of electrons from one cofactor/coenzyme to another is governed by well-defined chemical rules developed by chemists and biochemists at the beginning of the last century.