Flavin semiquinones

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Kurfuerst, M., Ghisla, S., and Hastings, J. W. (1986). Bacterial luciferase intermediates the neutral flavin semiquinone, its reaction with superoxide, and the flavin 4a-hydroxide. Method. Enzymol. 133 140-149. 

The addition of sulfite to APS reductase results in changes of the flavin visible spectrum that are explained by the formation of an adduct between the sulfite and the FAD group (135). Addition of AMP to the as-isolated enzyme causes no change in the spectroscopic properties. Addition of AMP to the sulfite-reacted enzyme causes the reduction of center I. However, the formation of a semiquinone signal has never been observed either by EPR or visible spectroscopies. Also, Mossbauer and EPR data indicate that AMP closely interacts with center I (139). 

Most likely the flavin triplet is the photochemically active species and the flavin semiquinone is the reducing agent for the b-type cytochrome. 

Riboflavin is the redox component of flavin adenine dinucleotide FAD. It is derived from FAD by hydrolysis of a phosphate ester link. The fully oxidised form of FAD is involved in many dehydrogenaze reactions during which it is converted to the fully reduced form. The fully oxidised state is restored either by another redox enzyme or by interaction with oxygen and hydrogen peroxide is liberated. The one-electron reduced, semiquinone form of FAD, is involved in some electron transfer steps. 

How the NOS isoforms compare to these related dual flavin enzymes is a matter of ongoing investigation. Characterization of the neuronal NOS revealed that it normally exists in its one-electron reduced form and maintains an air-stable, flavin semiquinone radical (Stuehr and Ikeda-Saito, 1992), as seen for NADPH-cytochrome P450 reductase. It is unknown which flavin in NOS contains the odd electron, although precedent argues that it probably resides on... 

Flavin redox states in a dual flavin enzyme. (Left) Single-electron reduction of the isoalloxazine ring generates the semiquinone radical, while reduction by two electrons generates the fully reduced species. (Right) Five possible oxidation levels of a dual flavin enzyme, where the FMN reduction potential is held at a more positive value relative U) FAD. The flavins can theoretically accept a maximum of four electrons obtained from two NADPH. However, in NADPH-cytochrome P450, reductase, full reduction of the flavins is not normally reached when NADPH serves as the reductant. 

Otvos, J. D., Krum, D. P., and Masters, B. S. (1986). Localization of the free radical on the flavin mononucleotide of the air-stable semiquinone state of NADPH-cytochrome P-450 reductase using 3IP NMR spectroscopy. Biochemistry 25, 7220-7228. 

Stuehr, D. ]., and Ikeda-Saito, M. (1992). Spectral characterization of brain and macrophage nitric oxide synthases. Cytochrome P-450-like heme proteins that contain a flavin semiquinone radical. J. Biol. Chem. 267, 20547-20550. 

Enzyme-Bound Flavin Semiquinones which are not Catalytic... 

The ongoing research into the structure and mechanism of flavoenzymes has been the subject of several recent excellent reviews The proceedings of six symposia held at intervals over the past 16 years provide an overall perspective on the progress of flavoenzyme research over this time period. The intent of this article will be to focus directly on the chemical and physical properties of the semiquinone form of flavin coenzymes to the extent that current knowledge permits, from the point of view of both model system studies and from existing knowledge of their properties in flavoenzyme systems. For an in-depth treatment of flavin and flavoenzyme redox properties in which the oxidized and hydroquinone forme as well as the semiquinone form are discussed as related to their biological function, the reader is refered to the article by F. Muller in this volume. 

II Properties of Neutral and Anionic Flavin Semiquinones in Model Systems... 

Two ionic forms of flavin semiquinones have been shown to exist in flavoenzymes the neutral form and the anionic form... 

Cationic flavin semiquinones are formed below pH 3.0 Since this form is not found in biological systems, it will not be considered further. 

Fig. 1. Absorption spectra of neutral and anionic flavin semiquinones. (-) Anionic riboflavin...

In contrast to their instability free in solution, flavin semiquinones bound to proteins are generally (although there are exceptions) quite stable and can be generated in nearly quantitative yields. This stability results from thermodynamic considerations in which the oxidized/semiquinone oxidation-reduction couple is usually more positive and well separated from the semiquinone/hydroquinone couple. In addition to thermodynamic stabilization, kinetic stabilization of protein-bound semiquinones has also been observed and will be discussed in more detail in subsequent portions of this article. 

Since flavin semiquinones are tricyclic heteronuclear aromatic systems, the spin density due to the unpaired electron can be distributed at a number of sites on the isoalloxazine ring. Knowledge of the location and the extent of spin density at these locations on the ring system is important for the determination of sites that may participate... 

Table 1. Isotropic hyperfine coupling constants of flavin semiquinones...

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