Origin and Role in Enzymatic Reactions
The structural foundation of flavin coenzymes is their precursor vitamin, riboflavin (vitamin $B_2$). The body synthesizes two active forms from this precursor: flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN).
These coenzymes are integral parts of enzymes belonging to the class of oxidoreductases. Specifically, they function within the subclass of dehydrogenases—enzymes that specialize in removing hydrogen atoms from various chemical substrates. Without FAD and FMN, biological oxidation processes in many metabolic pathways would be impossible.
Mechanism of Action: Substrate Oxidation
The primary role of flavin coenzymes is direct participation in substrate oxidation. The process follows a strict sequence:
- The dehydrogenase enzyme binds to the substrate to be oxidized.
- During the chemical reaction, exactly two protons ($2H^+$) and two electrons ($2e^-$) are removed from the substrate molecule.
- A key feature of FAD and FMN is that all of these removed particles (both protons and both electrons) attach directly to the coenzyme itself.
Thus, the coenzyme acts as a universal acceptor, taking up the hydrogen equivalents and converting the substrate into its oxidized state.
Chemical Structure: Oxidized and Reduced Forms
The functional or "active" part of flavin coenzyme molecules is the isoalloxazine ring. This is the site where key chemical events during proton and electron transfer take place.
- Oxidized Form: In its initial state (designated as FAD or FMN), the central ring of the isoalloxazine structure contains double bonds at the nitrogen atoms. This state is ready to accept hydrogen.
- Reduced Form: When the coenzyme accepts $2H^+$ and $2e^-$ from the substrate, the ring is reduced. Hydrogen atoms attach to the two nitrogen atoms in the central part of the isoalloxazine ring. As a result of this attachment, the double bonds at the nitrogen atoms break, forming the reduced molecules designated as $FADH_2$ and $FMNH_2$.